It seems that if everything in the world were just as it is now, but transparent, we would be able to see what is beyond the world. — Amalac

Everything is pretty much transparent. Once we get above our atmosphere there's nothing between us and the cosmic microwave background. Which we can "see" with appropriate instruments. In fact we can see it with land-based radio telescopes. Isn't that the answer to your question? In fact here is the answer:

Just as a burning coal (around 1500 K) glows red, and a hot bright star (around 6000 K) glows yellow or blue, the CMB glow with a characteristic colour associated with it’s temperature. However, because it is so cold, the light which was emitted by the glowing Universe now has a much longer wavelength than we can see with our eyes. The CMB is brightest at a wavelength of around 2 mm, which is around 4000 times longer than the wavelength of the visible light we see with our eyes.

https://plancksatellite.org.uk/science/cmb/

Everything is pretty much transparent. — fishfry

Not in the sense I am speaking of. That's why I gave this definition:

Let's use this definition of “transparent”: such that through it some other object that is both not transparent and not translucent can be seen with clarity. — Amalac

And Wittgenstein's:

Transparency and reflection exist only in the depth of the dimension of a visual image.

The impression of the transparent medium is that something lies behind the medium. A completely monochromatic visual image cannot be transparent. — Ludwig Wittgenstein

But anyway, let me phrase it in another way: The window of your house looks like what can be seen through it, right? So then, how would it look like if the only objects that were in the world were other windows just like it (and where spaces were also transparent in the sense I described)?

But anyway, let me phrase it in another way: The window of your house looks like what can be seen through it, right? So then, how would it look like if the only objects that were in the world were other windows just like it (and where spaces were also transparent in the sense I described)? — Amalac

You'd see the CMB. Just as if you point a radio telescope to the sky where there aren't any galaxies in the way. I don't follow your point. Transparent is transparent. If everything in the world was transparent, you'd see the CMB. Because everything in the world IS transparent except where there's matter, and relatively speaking there's mostly empty space.

Maybe I'm being too literal, but Wittgy is full of it I think, at least on this matter.

If you aimed a radio telescope at your window with empty space in the background, you'd detect the CMB. How else can this question be understood? Perhaps whereof I can't speak, thereof I should put a sock in it.

If everything in the world was transparent, you'd see the CMB. — fishfry

But the CMB is part of the world/universe (in the widest, most all-encompassing sense), right? If so, when asking how it would look like if everything were transparent, that includes how it would look if the CMB were transparent.

The CMB is not transparent, but would you agree that it was logically possible for it to have been transparent?

The CMB is not transparent, but would you agree that it was logically possible for it to have been transparent? Or do you disagree with this? — Amalac

I do understand that you're asking what we'd see if the CMB were transparent.

It's not logically possible for the CMB to be transparent, because its existence and color temperature is a consequence of our best theories of physics AND it's been seen by experiment. So it's there and it's the way it is. I suppose in some other universe it's not there or something, but that's speculative physics bordering on metaphysics and theology.

But as a counterfactual, if the CMB wasn't there or was transparent (hence falsifying all of modern physics AND our actual physical observations) I suppose you could imagine it's any color you like. Rainbows and unicorns would be consistent with your idea because your premise is false. "If the sky weren't blue what color would it be?" What color would you like it to be?

You know I heard once that Wittenstein didn't believe Cantor's diagonal argument so I looked up what he wrote, and he did not understand a word of the argument. So Wittgy's not infallible by any means, nor apparently did he understand modern math and science, if this one example can be extrapolated.

It's not logically possible for the CMB to be transparent, because its existence is a consequence of our best theories of physics AND it's been seen by experiment. — fishfry

Ok, but we're talking about possible worlds. It may be physically impossible for the CMB to have been transparent, but if we say that it is logically impossible for it to have been transparent, that means that the idea of a transparent CMB would have to entail a logical contradiction (as in: there is no possible world in which the CMB is transparent). In that case, what is the contradiction?

As Wittgenstein pointed out in his “Remarks on color”, in the context of philosophy he doesn't use terms like “transparency” in the sense in which they are used by physicists, rather in the sense we use those terms in ordinary life. He adds later that he is not looking for a physical theory of color, but rather for the logic of color, or the logic of color concepts. And here I'm looking for the same thing.

Ok, but we're talking about possible worlds. It may be physically impossible for the CMB to have been transparent, but if we say that it is logically impossible for it to have been transparent, that means that the idea of a transparent CMB would have to entail a logical contradiction (as in: there is no possible world in which the CMB is transparent). In that case, what is the contradiction? — Amalac

Like I say, what color would you like it to be? If you're talking possible worlds, I suppose there's a world where there's a transparent CMB and behind it a background of rainbows and unicorns. Who can say otherwise?

In this universe to deny the CMB exactly as it is would entail massive destruction of most of physics. I mean you'd still have Newtonian physics but nothing from the twentieth century. Of course maybe the big bang theory is wrong, nobody really knows for sure. Maybe the CMB is something other than what we think it is. Anything's possible.

So tell me, is there a possible world where there's rainbows and unicorns at the limit of the observable universe? Maybe in some possible world light moves so slowly that the farthest you can see is across the street, and that's the observable universe. Possible worlds (you mean David Lewis or physics multiverse?) seem meaningless to me. I think David Lewis must be clinically insane, but a lot of smart people take his ideas seriously. Must just be me.

As Wittgenstein pointed out in his “Remarks on color”, in the context of philosophy he doesn't use terms like “transparency” in the sense in which they are used by physicists, rather in the sense we use those terms in ordinary life. He adds later that he is not looking for a physical theory of color, but rather on the logic of color, or the logic of color concepts. And here I'm looking for the same thing. — Amalac

Perhaps someone knowledgable about Wittgenstein would care to engage with this thread. I haven't any specific knowledge in that area, so I'm limited in how I can respond. I confess I don't know what the logic of color means, but I suppose that's a term of art in Wittgy's thinking that I'm ignorant of.

What I know about color is that photons of a given wavelength hit the retina, sent an electrochemical signal down the optic nerve, get processed by the visual cortex, and then get experienced as color by the mind. And nobody has the slightest idea how that last bit works.

ps -- I perused the brief Wiki article on Remarks on Colour and did not find any clues there at all. I have no idea what any of this is about. I'm sure there must be some Wittgenstein scholars here, maybe they'll chime in.

Possible worlds (you mean David Lewis or physics multiverse?) — fishfry

No, I mean Gottfried Leibniz's doctrine of many possible worlds, the one used in modal logic:

One of the most characteristic features of that (Leibniz's) philosophy is the doctrine of many possible worlds. A world is "possible" if it does not contradict the laws of logic. — Bertrand Russell

Like I say, what color would you like it to be? If you're talking possible worlds, I suppose there's a world where there's a transparent CMB and behind it a background of rainbows and unicorns. Who can say otherwise? — fishfry

It seems to me that you are missing the point of the experiment. In that possible world, absolutely everything would have to be transparent, and if there were something behind the CMB, then whatever was behind it, being a part of the world, would also be transparent, and if there were something beyond even that, it would also be transparent and so on ad infinitum.

Let's suppose that there some ultimate boundary of the universe, beyond which there is nothing, not even empty space (i.e. the universe is finite with regards space). Then I ask: Is it logically necessary that this boundary is not transparent? (Otherwise we would see “nothing” if we looked at it, no colors at all)

And let's suppose that the universe were infinite with regards space, how would it look like if absolutely everything was transparent?

It seems to me that you are missing the point of the experiment. — Amalac

I think I acknowledged that already.

In that possible world, absolutely everything would have to be transparent, and if there were something behind the CMB, then whatever was behind it, being a part of the world, would also be transparent, and if there something beyond it would also be transparent and so on ad infinitum.

Let's suppose that there some ultimate boundary of the universe, beyond which there is nothing, not even empty space (i.e. the universe is finite with regards space). Then I ask: Is it logically necessary that this boundary is not transparent? (Otherwise we would see “nothing” if we looked at it, no colors at all)

And let's suppose that the universe were infinite with regards space, how would it look like if absolutely everything was transparent? — Amalac

I hope someone else will take this up. I honestly don't understand the question. I can only express incredulity that others could think there is a definite answer to the question of what color lies beyond the boundary of a transparent universe in an alternate world. I'm going with rainbows and unicorns and who can say otherwise? And what if I say that rainbows and unicorns are logically necessary? Who could dispute me? On what grounds? But really I have nothing to contribute here.

And what if I say that rainbows and unicorns are logically necessary? — fishfry

If rainbows and unicorns were logically necessary, then their non-existence would imply a logical contradiction. But obviously that's false: no logical contradiction arises from denying their existence, since they are, in that sense, contingent (as Leibniz would put it: it would be logically possible for them not to exist).

When asking: Was it logically necessary that not everything in the world was transparent? I ask: Does the existence of such a world involve a logical (not physical) contradiction? If so, what is the contradiction?

If rainbows and unicorns were logically necessary, then their non-existence would imply a logical contradiction. But obviously that's false: no logical contradiction arises from denying their existence, since they are, in that sense, contingent (as Leibniz would put it: it would be logically possible for them not to exist).

When asking: Was it logically necessary that not everything in the world was transparent? I ask: Does the existence of such a world involve a logical (not physical) contradiction? If so, what is the contradiction? — Amalac

If 2 + 2 = 5 then I am the Pope. That's a true implication. Another true implication is that if 2 + 2 = 5 then I am NOT the Pope. Likewise, "If the CMB is transparent then what's on the other side is rainbows and unicorns" is true, and so is "If the CMB is transparent then what's on the other side is NOT rainbows and unicorns." Both are meaningless vacuous truths. So you're right that there is no logical contradiction, but there's no intellectual content either, except to demonstrate the counterintuitiveness of material implication.

If 2 + 2 = 5 — fishfry

There is no possible world in which 2+2=5

then I am the Pope — fishfry

p→q is equivalent to: not p or q, so “If 2+2=5 then I am the Pope” means: Either 2+2≠5 or I am the Pope, which is true because 2+2≠5. The paradoxes of material implication only arise because they contradict the way we use them in ordinary life, to logicians they don't pose any problems if we interpret them as not p or q. But at any rate, material implication has nothing to do with my question.

If 2 + 2 = 5
— fishfry

There is no possible world in which 2+2=5 — Amalac

How do you know?

In general, what propositions are logically true? Most people would agree that mathematical propositions are logically true. I myself am not convinced. After all, "I can not conceive" of a world in which 2 + 2 = 5, but making my own personal imagination the gold standard seems problematic. Perhaps there's some other world with different logic or different math. People do argue about that.

But I would like to ask you, besides mathematics, can you name a necessary truth? I mean one that's not trivial, such as that "if all men are mortal and Socrates is a man then Socrates is mortal." Can you name a meaningful proposition that is logically true, necessarily true in "all possible worlds," that doesn't rely on math?

p→q is equivalent to: not p or q, so “If 2+2=5 then I am the Pope” means: Either 2+2≠5 or I am the Pope, which is true because 2+2≠5. The paradoxes of material implication only arise because they contradict the way we use them in ordinary life, to logicians they don't pose any problems if we interpret them as not p or q. But at any rate, material implication has nothing to do with my question. — Amalac

I think it does. What I am thinking is that (1) The CMB has a color temperature of such and so. (2) Therefore IF its color temperature is otherwise, then that premise is false, hence I am the pope.

The claim that "the CMB is transparent" is factually false, so anything at all follows from it. That's my reasoning.

Now about these other worlds. For one thing, the CMB is the boundary of the observable universe. I imagine you might be inclined to grant that this is a necessary truth by definition. In which case, if it's transparent, we still can't see past it. So we'd see black. Necessarily, because by definition we can't possibly see past the CMB.

But if the CMB is transparent, then we're in a world with different laws if physics. In which case, why might there not be different laws of mathematics or logic?

Perhaps you can clarify these things for me. I'm not arguing so much as trying to understand reasoning about possible worlds. A possible world by definition is counterfactual to this world so anything is possible.

If 2 + 2 = 5
— fishfry

There is no possible world in which 2+2=5
— Amalac

How do you know? — fishfry

Technically I don't, but I thought you would just take this for granted. I mean, you can take the extreme sceptic route of doubting that 2+2=4 is true in all possible worlds, but that would just dispose of mathematics and logic altogether, and I doubt that's your aim.

A world where 2+2=5 is inconceivable. If you have 2 oranges and 2 other oranges, how could you possibly not altogether have 4 oranges? 2+2=4 is analytically true, just as “a bachelor is an unmarried man” is: it follows from the definition of 2+2. Perhaps you could doubt this, as Kant did, by saying that 2+2=4 is in fact synthetic. But even Kant did not doubt that 2+2=4 is a priori true.

But I would like to ask you, besides mathematics, can you name a necessary truth? I mean one that's not trivial, such as that "if all men are mortal and Socrates is a man then Socrates is mortal." Can you name a meaningful proposition that is logically true, necessarily true in "all possible worlds," that doesn't rely on math? — fishfry

All analytic propositions are necessarily true, because the predicate is contained in the notion of the subject. All analytic propositions are trivial because of that (and yet they are still meaningful).

Then there is of course the famous debate between empiricists and rationalists, as to whether we can have knowledge about the world that can be obtained by mere reasoning, without the aid of experience.

What I asked in the OP was if there could be statements which are true about the world, but which are known a priori (Such as: If the universe has a boundary, then such a boundary must not be transparent when seen by a human by logical necessity, since it is impossible for a human to see anything that does not have any color, in the sense in which black and white are also colors).

Immanuel Kant, for instance, held that the Law of causality was synthetic (not analytic), but known a priori. And the Law of Causality, if true, would give us knowledge about states of affairs that we have not yet experienced (a priori knowledge about the world).

The claim that "the CMB is transparent" is factually false, so anything at all follows from it. That's my reasoning. — fishfry

This here is maybe where you are going off the rails: I'm not claiming that this is the case, I'm asking if it is even possible for the boundary of the universe to be transparent, as in: could this be known by mere analysis of the concepts of “transparent”, “seeing”, “universe”,etc? Or is it a synthetic proposition that is nevertheless still a priori true, just as Kant held the Law of Causality to be?

Now about these other worlds. For one thing, the CMB is the boundary of the observable universe. I imagine you might be inclined to grant that this is a necessary truth. In which case, if it's transparent, we still can't see past it. So we'd see black. Necessarily, because by definition we can't possibly see past the CMB. — fishfry

In that case either: it looks black because there is something black beyond it, which contradicts the idea that nothing could be beyond that, or it looks black because it is black. But then it's no longer transparent (in the sense I gave in the OP), since as Wittgenstein pointed out something transparent cannot look monochromatic. This contradicts the definition of that possible world (as in: we would in that case no longer be talking about that possible world, but rather about some other possible world), so it too can't be the case.

It may in that case still be transparent as the word is used by physicists (in the sense that it let's X kinds of light to pass through, despite looking as if it were opaque), but not in the sense that you can see through it.

But even ignoring that, if by definition that is so, could we then say: “The CMB (or the boundary of the world) is not transparent (in Wittgenstein's sense)” is an analytically true proposition that nonetheless gives us knowledge about the world?

But if the CMB is transparent, then we're in a world with different laws if physics. In which case, why might there not be different laws of mathematics or logic? — fishfry

Because then we would not be talking about possible worlds, since a possible world in Leibniz's sense is one which does not contradict the Laws of logic. So, if that world is a possible world, then it can't possibly have different laws of logic, since otherwise it would not be a possible world. And if you accept that mathematics is a prolongation of logic, then the truths of mathematics also cannot be different (although this second claim about math being an extension of logic is far more controversial):

6.375 As there is only a logical necessity, so there is only a logical
impossibility — Ludwig Wittgenstein

Technically I don't, but I thought you would just take this for granted. I mean, you can take the extreme sceptic route of doubting that 2+2=4 is true in all possible worlds, but that would just dispose of mathematics and logic altogether, and I doubt that's your aim. — Amalac

As someone who finds the doctrine of possible worlds incoherent, I don't see why logic and math couldn't be different in some alternate world, just as physics is. I agree I can't conceive of it, but who made me the authority on such things?

A world where 2+2=5 is inconceivable. — Amalac

And a world where the CMB is transparent is inconceivable, by the very definition of the CMB. Or so one could argue. I don't argue the point because I don't understand how people take possible worlds seriously.

If you have 2 oranges and 2 other oranges, how could you possibly not altogether have 4 oranges? — Amalac

I could say, if the CMB is the remnant of the big bang, how could it be transparent? But we'd be arguing nonsense.

2+2=4 is analytically true, just as “a bachelor is an unmarried man” is: it follows from the definition of 2+2. Perhaps you could doubt this, as Kant did, by saying that 2+2=4 is in fact synthetic. But even Kant did not doubt that 2+2=4 is a priori true. — Amalac

I agree with you about that. But in some alternate world, who knows what logic and math are? I'm not trying to argue that seriously, I'm just saying that possible worlds make no sense to me.

All analytic propositions are necessarily true, because the predicate is contained in the notion of the subject. All analytic propositions are trivial because of that (and yet they are still meaningful). — Amalac

Well if the CMB is defined as the remnant of the big bang. and you drill that down to the physics, then the CMB can't be transparent because the remnant of the big bang has a color temperature. But again I'm not really arguing my side of this. I don't think possible worlds make sense.

Then there is of course the famous debate between empiricists and rationalists, as to whether we can have knowledge about the world that can be obtained by mere reasoning, without the aid of experience. — Amalac

Good question.

What I asked in the OP was if there could be statements which are true about the world, but which are known a priori (Such as: If the universe has a boundary, then such a boundary must not be transparent when seen by a human by logical necessity, since it is impossible for a human to see anything that does not have any color, in the sense in which black and white are also colors). — Amalac

Perhaps if you asked the question without encumbering it with cosmology this would get more ... clear, no pun intended. Can you "see" a perfectly transparent window, for example? That's the same question but without the extra complications of the CMB.

Immanuel Kant, for instance, held that the Law of causality was synthetic (not analytic), but known a priori. And the Law of Causality, if true, would give us knowledge about states of affairs that we have not yet experienced (a priori knowledge about the world). — Amalac

I've made many runs at the synthetic/analytic distinction over the years, with little luck. I didn't do well in undergrad philosophy. I remember the categorical imperative but that's about it.

This here is maybe where you are going off the rails: I'm not claiming that this is the case, I'm asking if it is even possible for the boundary of the universe to be transparent, as in: could this be known by mere analysis of the concepts of “transparent”, “seeing”, “universe”,etc? Or is it a synthetic proposition that is nevertheless still a priori true, just as Kant held the Law of Causality to be? — Amalac

I agree that I'm stretching a point. The CMB isn't transparent, therefore any proposition that assumes the CMB is transparent has a false antecedent. But in possible worlds theory, that's not valid reasoning. I agree I'm playing fast and loose with this point.

But if the CMB is leftover radiation by definition, how can it be transparent? It always has some small but nonzero color temperature.

In that case either: it looks black because there is something black beyond it, which contradicts the idea that nothing could be beyond that, or it looks black because it is black. But then it's no longer transparent (in the sense I gave in the OP), since as Wittgenstein pointed out something transparent cannot look monochromatic. This contradicts the definition of that possible world (as in: we would in that case no longer be talking about that possible world, but rather about some other possible world), so it too can't be the case. — Amalac

Sorry I got a little lost here. But if the point is that a perfect window has no color, I suppose I can agree with that.

It may in that case still be transparent as the word is used by physicists (in the sense that it let's X kinds of light to pass through, despite looking as if it were opaque), but not in the sense that you can see through it. — Amalac

I'll stipulate that a perfect window has no color of its own. Although physically I'm not really sure if that's true or not.

But even ignoring that, if by definition that is so, could we then say: “The CMB (or the boundary of the world) is not transparent (in Wittgenstein's sense)” is an analytically true proposition that nonetheless gives us knowledge about the world? — Amalac

What gave us knowledge of the world was Penzias and Wilson looking through their radio telescope and seeing the echo of the big bang; in conjunction with a purely theoretical prediction that it should be there to be seen. They got the Nobel prize for that. No amount of philosophical theorizing could possibly give us knowledge of the actual world. Is that the subject of the conversation? If you're locked in a sealed room without Internet (the horror!) and all you can do is think, what can you know about the world? On the other hand if you have a radio telescope, you can figure out quite a lot.

http://www.bell-labs.com/about/awards/1978-nobel-prize-physics/#gref

Because then we would not be talking about possible worlds, since a possible world in Leibniz's sense is one which does not contradict the Laws of logic. — Amalac

Yes I can see that. But consider Euclidean and non-Euclidean geometry. Neither contradict the laws of logic, but Einstein showed the latter was true. Would Leibniz say that there's a Newtonian world too? But Newtonian physics fails if you simply do the right experiment, like look at a star near the sun during an eclipse and measure the bending of its light. Do you believe there's a possible world in which Einstein's famous prediction failed and Newtonian physics reigns supreme? I find that hard to accept.

https://en.wikipedia.org/wiki/Eddington_experiment

So, if that world is a possible world, then it can't possibly have different laws of logic, since otherwise it would not be a possible world. And if you accept that mathematics is a prolongation of logic, then the truths of mathematics also cannot be different (although this second claim about math being an extension of logic is far more controversial): — Amalac

I don't want to die on the hill of saying math is contingent. But I am not convinced that physics is contingent either. I wonder if anything is contingent. In some logically possible other world, Socrates was a bricklayer and not a philosopher. But what other things would have had to change? You'd have to drill that down to his ancestry and environment and life experiences. I don't think I have enough imagination to believe in contingency at all. Today I'm wearing my determinist hat. Socrates was destined to be a philosopher from the moment of the big bang.

https://en.wikipedia.org/wiki/Eddington_experiment
6.375 As there is only a logical necessity, so there is only a logical
impossibility
— Ludwig Wittgenstein[/quote]

Out of context, it's hard to understand what he means. Then again with Wittgy, even the context doesn't always help.

I am afraid we must be far away from your actual original question. Which was what? Is a perfect window colorless? I suppose so. I can stipulate that. But you know just because a perfect window is transparent to visible frequencies, it still reflects heat. So you have to say what you mean by transparent. Is it transparent to all electromagnetic frequencies? Now you're into physics again.

As someone who finds the doctrine of possible worlds incoherent, I don't see why logic and math couldn't be different in some alternate world, just as physics is. I agree I can't conceive of it, but who made me the authority on such things? — fishfry

Well, the doctrine of many possible worlds is just taken for granted in modal logic, for example. When saying X is logically possible, all one means is that the existence of X does not violate any of the laws of logic. That's just the definition of a “possible world”, there's nothing incoherent about it. When asserting that there is some possible world in which X exists, one does not assert that such a world literally exists in some alternate reality (at least most people don't), rather one merely asserts that reality could have been that way, or could be that way, depending on what the assertion is.

I could say, if the CMB is the remnant of the big bang, how could it be transparent? But we'd be arguing nonsense. — fishfry

But then you are doing physics again, while I'm trying to do logic. This seems to me like a case where you are confusing one language game with another.

For example, Wittgenstein said this in his Remarks on Colour:

We are not doing physics here(...)

(...)The question is: how does our visual image have to be, if it is to show us a transparent medium? How, e.g., does the medium's color have to appear? Speaking in physical terms - although we are not directly concerned with the laws of physics here - everything seen through pure green glass should look more or less dark green.

(...) That is not a proposition of physics, but rather a rule for the spatial interpretation of our visual experience.

As you yourself pointed out before, a universe where the CMB were transparent would have different laws of physics, and there is at first no logical impossibility in it having laws which would allow it to be transparent, not until you start considering what that logically entails.

But if the CMB is leftover radiation by definition, how can it be transparent? It always has some small but nonzero color temperature. — fishfry

Again, is there some possible world in which the CMB had the necessary physical properties to be transparent, or were the Big Bang happened differently? Yes, so long as that does not involve a logical contradiction. That's all the doctrine of many possible worlds asserts. It does not claim that there are in fact alternate realities or multiverses, one in which that's the case (at least most interpreters of the doctrine don't, it seems to me).

But I am not convinced that physics is contingent either. I wonder if anything is contingent. In some logically possible other world, Socrates was a bricklayer and not a philosopher. But what other things would have had to change? You'd have to drill that down to his ancestry and environment and life experiences. I don't think I have enough imagination to believe in contingency at all. Today I'm wearing my determinist hat. Socrates was destined to be a philosopher from the moment of the big bang. — fishfry

I mean, it's quite simple in a sense: I can imagine/conceive that if I jumped of the roof of my house I started floating upwards instead of falling. I can imagine/ conceive of the sun not rising tomorrow, and these events imply that physics is contingent. And since I can conceive of them, they are possible, they involve no contradiction.

On the other hand, I can't conceive of me counting one orange, and then only another orange, and then somehow having altogether 50 oranges (1+1=50), because that contradicts the most fundamental and most self-evident axioms of mathematics. I can't conceive of seeing something that both was and was not a tree (at the same time and in the same sense), because that would violate the Law of Contradiction.

3.0321 We could present spatially an atomic fact which contradicted the laws of physics, but not one which contradicted the laws of geometry. — Ludwig Wittgenstein

However, supposing one takes the route of determinism, then I ask: Do you agree with Leibniz's Principle of Sufficient reason as interpreted in his esoteric system, according to which every true proposition is analytic, including empirical truths? If so, I suppose there's no sense in arguing with you about possible worlds in which the laws of physics were different.

Do you believe there's a possible world in which Einstein's famous prediction failed and Newtonian physics reigns supreme? — fishfry

To answer that we need to answer the question: Does such a state of affairs (a possible world in which Einstein's famous prediction failed and Newtonian physics reigns supreme) involve a logical contradiction? If it does, then there is no possible world in which that's the case. If it does not, then there is such a possible world.

It seems clear to me that such a scenario does not involve or imply a logical contradiction, but if you think otherwise, then tell me what the logical (not physical) contradiction that such a scenario implies would be.

No amount of philosophical theorizing could possibly give us knowledge of the actual world. — fishfry

Would you say this proposition does not give us knowledge about the world then?:

If the universe has a boundary, then such a boundary must not be transparent when seen by a human by logical necessity, since it is impossible for a human to see anything that does not have any color, in the sense in which black and white are also colors.

Notice that assuming that the universe were in fact finite with regards space, then we could have knowledge about the boundary from the truth of the proposition above, without the need of experience (such as the experience of observing the boundary), merely by logically analyzing the meanings of words. And although in that case it would be perhaps considered an analytic proposition, nonetheless it seems to gives us knowledge about the world.

Is a perfect window colorless? — fishfry

Well, that's not the question but it is related to it somewhat. I would like to confirm: if a clear glass window (which according to physics apparently does have a color: blue) was in a world were the only other objects in it were other transparent objects, then is it correct to say that they would all look monochromatically blue?

Some reddit user answered the question about what color is a window:

if you stack up enough windows you'll see that window glass is actually usually a blue/green color. It's just so translucent that with only one pane it's pretty much impossible to see

http://www.tucsontabletops.com/Images/seamed-v-polished/edge-color.png

And apparently the same is true for clear water:
https://www.usgs.gov/special-topic/water-science-school/science/water-color?qt-science_center_objects=0#qt-science_center_objects.

But that is relevant for the physics definition of transparent. I am using a different sense of that word, the one most commonly used in ordinary life by ordinary people.

So you have to say what you mean by transparent — fishfry

I did in my OP: such that through it some other object (at least one object) that is both not transparent (not transparent= such that you cannot see through it with clarity) and not translucent can be seen through it.

The important aspect of that definition is that it is not a physics definition of «transparent», but rather one which defines the word according to our visual experience.

If a window would indeed look monochromatically blue/green if it was surrounded only by other windows, then the average person who is not knowledgeable about physics would not call it transparent in the sense I gave in the OP.

They would not be wrong, they would in that case only use the word with a sense that is not the physicists' sense (the meaning of a word depends upon its context, and the way it's used in a particular language game, as Wittgenstein would put it).

But if the point is that a perfect window has no color, I suppose I can agree with that. — fishfry

My point was more like: If the universe has an edge, then such an edge must not be transparent by logical necessity (not merely physical necessity), because otherwise we would be able to see what is beyond the universe.

The argument would be: anything we see is part of the universe. Therefore, if we could see something through it, that thing would be both in the universe and beyond the universe, which is absurd. Therefore, it's impossible for there to be anything beyond the edge of the universe (in an all-encompassing sense, not in the sense some physicists speak of “multiverses”).

And yet, if nothing could be seen through the transparent object, then the transparent object would not have any color when we looked at it. But that's also absurd: since we impose color onto all that we perceive, it's impossible for us to see something that has no color.

Therefore, in view of the 2 previous conclusions, we conclude that it is logically necessary that such an edge is not transparent/translucent.

Everything is pretty much transparent — fishfry

You nailed it. It all depends on what kind of energy we're talking about. A masonry wall is transparent to sound though not to visible light. Walls are also transparent to radio-waves, X-rays, gamma-rays. You get the idea.

You nailed it. It all depends on what kind of energy we're talking about. A masonry wall is transparent to sound though not to visible light. Walls are also transparent to radio-waves, X-rays, gamma-rays. You get the idea. — TheMadFool

Great examples. I wonder if physics allows for anything that's totally transparent to all energy. Dark matter is transparent to everything except gravity, that's an interesting example. But of course nobody knows what it is or if it really exists.

My point was more like: If the universe has an edge, then such an edge must not be transparent by logical necessity (not merely physical necessity), because otherwise we would be able to see what is beyond the universe. — Amalac

I don't think I agree. The edge of the observable universe is as far as we can see. It doesn't matter what's beyond it. We can't see it in any event. It would appear black I assume.

The argument would be: anything we see is part of the universe. Therefore, if we could see something through it, that thing would be both in the universe and beyond the universe, which is absurd. Therefore, it's impossible for there to be anything beyond the edge of the universe (in an all-encompassing sense, not in the sense some physicists speak of “multiverses”). — Amalac

We already know there's part of the universe past the observable universe. We can't see it because light from there hasn't had time to get to us in the age of the universe.

And yet, if nothing could be seen through the transparent object, then the transparent object would not have any color when we looked at it. But that's also absurd: since we impose color onto all that we perceive, it's impossible for us to see something that has no color. — Amalac

A perfect window is transparent and has no color. But as @TheMadFool notes, it's not transparent to heat, sound, etc. What exactly do you mean by transparent? Bats can detect windows by echolocation. To a bat, a window is not transparent. So you need to "define your terms" as they say.

Therefore, in view of the 2 previous conclusions, we conclude that it is logically necessary that such an edge is not transparent/translucent. — Amalac

No, you haven't made that case at all. We simply can't see past the edge of the observable universe, period. Nor have you defined transparency. We see invisible (to visible light) things all the time via radar and sonar for example.

Well, the doctrine of many possible worlds is just taken for granted in modal logic, for example. ]/quote]

I'm saying the whole idea is incoherent to me, notwithstanding all the smart people to whom it's coherent. I'm just not one of those smart people.
— Amalac

When saying X is logically possible, all one means is that the existence of X does not violate any of the laws of logic. That's just the definition of a “possible world”, there's nothing incoherent about it. When asserting that there is some possible world in which X exists, one does not assert that such a world literally exists in some alternate reality (at least most people don't), rather one merely asserts that reality could have been that way, or could be that way, depending on what the assertion is. — Amalac

David Lewis claims possible worlds are real. That, I find clinically insane.

But you didn't address my question about non-Euclidean geometry. Is there a world where Euclidean geometry holds (Newton) and one where it doesn't (Einstein)? There are a lot of technical problems with that belief.

But then you are doing physics again, while I'm trying to do logic. This seems to me like a case where you are confusing one language game with another. — Amalac

I already said that I am not sure whether physics is contingent. Is there a possible Newtonian world?

For example, Wittgenstein said this in his Remarks on Colour:
We are not doing physics here(...) — Amalac

I can't comment on Wittgenstein.

As you yourself pointed out before, a universe where the CMB were transparent would have different laws of physics, and there is at first no logical impossibility in it having laws which would allow it to be transparent, not until you start considering what that logically entails. — Amalac

Is Newtonian physics logically possible? Yes, since Euclidean geometry is consistent. But there is no possible Newtonian world. In my opinion. I don't know what philosophers would say. It's a question I'd like to put to them.

Again, is there some possible world in which the CMB had the necessary physical properties to be transparent, or were the Big Bang happened differently? Yes, so long as that does not involve a logical contradiction. That's all the doctrine of many possible worlds asserts. It does not claim that there are in fact alternate realities or multiverses, one in which that's the case (at least most interpreters of the doctrine don't, it seems to me). — Amalac

Then they're wrong. In my opinion. Just like there is no possible Newtonian world. It would violate experiment. And Newtonian physics contains singularities, since two point masses can be arbitrarily close together, making their gravitational attraction unbounded.

I mean, it's quite simple in a sense: I can imagine/conceive that if I jumped of the roof of my house I started floating upwards instead of falling. I can imagine/ conceive of the sun not rising tomorrow, and these events imply that physics is contingent. And since I can conceive of them, they are possible, they involve no contradiction. — Amalac

Well you're making me more convinced of my belief that possible worlds that violate known physics are indeed incoherent. Sure we can imaging a world where bowling balls fall up, but if we take that fantasy seriously, that's the worst kind of philosophy.

On the other hand, I can't conceive of me counting one orange, and then only another orange, and then somehow having altogether 50 oranges (1+1=50), because that contradicts the most fundamental and most self-evident axioms of mathematics. I can't conceive of seeing something that both was and was not a tree (at the same time and in the same sense), because that would violate the Law of Contradiction. — Amalac

Maybe you just can't conceive well enough. I do understand the point you're making between logical and physical necessity, I just don't agree.

3.0321 We could present spatially an atomic fact which contradicted the laws of physics, but not one which contradicted the laws of geometry.
— Ludwig Wittgenstein — Amalac

Newtonian physics. Wittgenstein is wrong.

However, supposing one takes the route of determinism, then I ask: Do you agree with Leibniz's Principle of Sufficient reason as interpreted in his esoteric system, according to which every true proposition is analytic, including empirical truths? If so, I suppose there's no sense in arguing with you about possible worlds in which the laws of physics were different. — Amalac

I plead ignorance, so I can't answer the question. But there is no point arguing with me about this because I understand the point you're making and I disagree with it. There is no Newtonian possible world, except in our imagination. It would make a fun science fiction story, but NOT serious philosophy.

To answer that we need to answer the question: Does such a state of affairs (a possible world in which Einstein's famous prediction failed and Newtonian physics reigns supreme) involve a logical contradiction? If it does, then there is no possible world in which that's the case. If it does not, then there is such a possible world. — Amalac

I agree with your premise but not your conclusion. Newtonian physics is logically consistent but false in THIS world; and, I contend, false in ALL possible worlds. I'm not misunderstanding your conclusion, I'm simply disagreeing with it.

It seems clear to me that such a scenario does not involve or imply a logical contradiction, but if you think otherwise, then tell me what the logical (not physical) contradiction that such a scenario implies would be. — Amalac

I'm perfectly happy to agree that Euclidean geometry is logically consistent. It just happens to not be the right geometry for physics, in this or in any other world. We're flogging a deceased equine now. I already said many posts ago that I find possible worlds incoherent, and now you're just trying to get me to repeat myself over and over. I find logically consistent possible worlds an incoherent idea, as it posits the existence of a world in which bowling balls fall up. Helium-filled bowling balls maybe.

Would you say this proposition does not give us knowledge about the world then?:

If the universe has a boundary, then such a boundary must not be transparent when seen by a human by logical necessity, since it is impossible for a human to see anything that does not have any color, in the sense in which black and white are also colors. — Amalac

You're wrong about that. We can't by definition see beyond the boundary of the OBSERVABLE universe. We know that's not all there is, but it's all we can possible see under any circumstances.

Notice that assuming that the universe were in fact finite with regards space, then we could have knowledge about the boundary from the truth of the proposition above, without the need of experience (such as the experience of observing the boundary), merely by logically analyzing the meanings of words. — Amalac

Nonsense. We can see to the limit of the observable universe, and we can have NO KNOWLEDGE of what's past that. Multiverse theory, for example, is purely speculative. It's proponents well understand that point. We can not see or measure or observe anything past the limit of the observable universe. Information simply hasn't had enough time to get here since the universe started.

And although in that case it would be perhaps considered an analytic proposition, nonetheless it seems to gives us knowledge about the world. — Amalac

You haven't made your point. We can't see past the observable universe.

Well, that's not the question but it is related to it somewhat. I would like to confirm: if a clear glass window (which according to physics apparently does have a color: blue) was in a world were the only other objects in it were other transparent objects, then is it correct to say that they would all look monochromatically blue? — Amalac

To whom? To a bat? Bats perceive the solidity of a window because they rely on sound and not light. So you need to define transparency, which I should have challenged you on many posts ago.

Some reddit user answered the question about what color is a window:

if you stack up enough windows you'll see that window glass is actually usually a blue/green color. It's just so translucent that with only one pane it's pretty much impossible to see — Amalac

Those are of course physically made, imperfect window. I have for several posts already stipulated to a PERFECT window. You could have one ten miles thick and it would be perfectly transparent to visible light.

And apparently the same is true for clear water: — Amalac

You're conflating physical things with conceptual ones. I have for sake of discussion granted you purele transparent windows, water, and so forth.

But that is relevant for the physics definition of transparent. I am using a different sense of that word, the one most commonly used in ordinary life by ordinary people. — Amalac

I am granting you perfectly transparent glass and water, which does not exist in the world but serves as a limiting case for our discussion.

I did in my OP: such that through it some other object (at least one object) that is both not transparent (not transparent= such that you cannot see through it with clarity) and not translucent can be seen through it. — Amalac

Humans or bats? Human eyes or radio telescopes? Cameras or sonar? Cameras or radar? You haven't defined transparency at all.

The important aspec

t of that definition is that it is not a physics definition of «transparent», but rather one which defines the word according to our visual experience. — Amalac

What about bats? Radio telescopes? Sonar? Radar? Heat? Sound?

If a window would indeed look monochromatically blue/green if it was surrounded only by other windows, then the average person who is not knowledgeable about physics would not call it transparent in the sense I gave in the OP. — Amalac

You're going quite far afield now. What the average person would see of the real world is irrelevant to your point.

They would not be wrong, they would in that case only use the word with a sense that is not the physicists' sense (the meaning of a word depends upon its context, and the way it's used in a particular language game, as Wittgenstein would put it). — Amalac

Bats, radio telescopes, radar, sonar, heat. Define transparency. And try to stick to your original point. I'm willing to stipulate to theoretically perfect windows, perfectly transparent to visual frequencies of light. They're still not transparent to heat and sound. And in any event, you can't see past the observable universe no matter what.

I don't think I agree. The edge of the observable universe is as far as we can see. It doesn't matter what's beyond it. We can't see it in any event. It would appear black I assume. — fishfry

Ok, but I'm not talking merely about the observable universe, I'm talking about both the observable universe and the parts of the universe we have not observed yet (that's why I said I meant “universe” in an all-encompassing sense).

I use the term in the same sense as the one suggested by Bertrand Russell in this passage:

Space and time appear to be infinite in extent, and infinitely divisible. If we travel along a straight line in either direction, it is difficult to believe that we shall finally reach a last point, beyond which there is nothing, not even empty space.

But as TheMadFool notes, it's not transparent to heat, sound, etc. What exactly do you mean by transparent? Bats can detect windows by echolocation. To a bat, a window is not transparent. So you need to "define your terms" as they say. — fishfry

Again, I don't use the word “transparent” as it is used by physicists, but rather in the sense Wittgenstein uses it in my OP. You can substitute “see through” (meaning: such that you can see through it) every time I say “transparent” if you like. I guess it's a sense more akin to phenomenology than to physics.

At any rate, what is wrong with this definition of transparent object?:

An object such that when a human sees it, through it some other object that is both not transparent and not translucent can be seen with clarity.

David Lewis claims possible worlds are real. That, I find clinically insane. — fishfry

I already told you I'm talking about Leibniz's doctrine of possible worlds, not Lewis' strange claims (if he does in fact claim what you say he does, which I don't know). I don't agree with Lewis, I don't believe “possible worlds” (other than the actual world) exist.

But you didn't address my question about non-Euclidean geometry. Is there a world where Euclidean geometry holds (Newton) and one where it doesn't (Einstein)? There are a lot of technical problems with that belief. — fishfry

I did address it, I said if there is nothing logically contradictory about a world where Euclidean geometry holds or one where it doesn't, then by definition they are both possible worlds. If you are going to say that there is no possible world in which Euclidian geometry and newtonian physics are wholly true, then that must mean that their truth implies a logical contradiction, in which case you just have to tell me what that logical contradiction is.

Perhaps you could say that Euclidian geometry contradicts some basic axiom of mathematics or logic, in which case there are no possible worlds in which that's the case. If you are asking: Assuming that the current laws of physics don't change in the future, is there any possible world in which the whole of newtonian physics is true? Then the answer is no, since as you pointed out newtonian physics were, at least partially, falsified by observation.

But the thing is, logic is not limited by the laws of physics. When saying that there is some possible world in which newtonian physics is the case, what one means is that we can conceive of a world where newtonian physics wasn't ever falsified, and in which the behavior observed that falsified newtonian physics simply didn't happen. That's possible, since there is nothing logically selfcontradictory about such a state of affairs. But then you may still go back to determinism and deny this.

You say I'm just making you repeat yourself, yet you have asked me the same thing again about newtonian physics.

I'm saying the whole idea is incoherent to me, notwithstanding all the smart people to whom it's coherent. I'm just not one of those smart people. — Amalac

Well, I simply don't agree with you in the least about what you say here, so there's no point in discussing the matter of possible worlds further. There's no common ground. Perhaps the Stanford Enciclopedia of Philosophy can do a better job than I did: https://plato.stanford.edu/entries/possible-worlds/

Those are of course physically made, imperfect window. I have for several posts already stipulated to a PERFECT window. You could have one ten miles thick and it would be perfectly transparent to visible light. — fishfry

Ok then, if that's what you meant by “perfect window” then there are no problems. I accept that definition.

You're going quite far afield now. What the average person would see of the real world is irrelevant to your point. — fishfry

It's not irrelevant, I was trying to clarify what I meant by “transparent”: When the average person speaks about X object being transparent, they probably have in mind something very different from what physicists have in mind. When saying «a window is transparent», they usually don't mean that it lets X,Y or Z amount of light to pass through, they mean simply: you can see through a window.

To quote Wittgenstein again: The meaning of a word is its use in a particular language game. The language game of describing things in daily ordinary life is not the same as the language game that results from doing physics.

Humans or bats? Human eyes or radio telescopes? Cameras or sonar? Cameras or radar? You haven't defined transparency at all. — fishfry

Human eyes. Have I defined it well enough for you now?

There is no Newtonian possible world, except in our imagination. It would make a fun science fiction story, but NOT serious philosophy. — fishfry

So modal logic is not serious philosophy according to you? I disagree

I already told you I'm talking about Leibniz's doctrine of possible worlds, not Lewis' strange claims (if he does in fact claim what you say he does, which I don't know). I don't agree with Lewis, I don't believe “possible worlds” (other than the actual world) exist. — Amalac

Modal realism is the view propounded by David Kellogg Lewis that all possible worlds are real in the same way as is the actual world: they are "of a kind with this world of ours."[1] It is based on the following tenets: possible worlds exist; possible worlds are not different in kind from the actual world; possible worlds are irreducible entities; the term actual in actual world is indexical, i.e. any subject can declare their world to be the actual one, much as they label the place they are "here" and the time they are "now".

https://en.wikipedia.org/wiki/Modal_realism

I'm afraid I don't know anything about Leibniz's views on the matter except that he said that we live in the BEST of all possible worlds. I'd hate to live in the other ones!

I did address it, I said if there is nothing logically contradictory about a world where Euclidean geometry holds or one where it doesn't, then by definition they are both possible worlds. If you are going to say that there is no possible world in which Euclidian geometry and newtonian physics are wholly true, then that must mean that their truth implies a logical contradiction, in which case you just have to tell me what that logical contradiction is. — Amalac

I don't need to do any such thing. I have already said several times that I agree that a Newtonian world is logically consistent, but still impossible.

The root problem here is that you insist on arguing with someone who rejects the doctrine of possible worlds in the first place. I don't know why your thread didn't attract any Wittgenstein or Leibniz scholars, since I am neither. I am totally the wrong person for you to be continually hammering with these irrelevant points.

Perhaps you could say that Euclidian geometry contradicts some basic axiom of mathematics or logic, in which case there are no possible worlds in which that's the case. — Amalac

I say again, for the fourth or fifth time: I accept that Euclidean geometry is logically consistent; and I reject the idea that there is a possible Euclidean world, except in our imaginations. But you wouldn't believe the stuff I have in my imagination. Everything is up there EXCEPT a Newtonian possible world. So again you are simply arguing with someone who already disagrees with your premise. Why don't you get that? I'm sorry your OP didn't attract any higher quality responses than mine, because I can't discuss this subject using your premises.

If you are asking: Assuming that the current laws of physics don't change in the future, is there any possible world in which the whole of newtonian physics is true? Then the answer is no, since as you pointed out newtonian physics were, at least partially, falsified by observation. — Amalac

Right.

But the thing is, logic is not limited by the laws of physics. — Amalac

Why are you explaining to me that which I have stipulated many times over already? It's like you're not reading what I'm writing.

When saying that there is some possible world in which newtonian physics is the case, what one means is that we can conceive of a world where newtonian physics wasn't ever falsified, and in which the behavior observed that falsified newtonian physics simply didn't happen. That's possible, since there is nothing logically selfcontradictory about such a state of affairs. But then you may still go back to determinism and deny this. — Amalac

I understand what you're saying. I disagree with your premise that whatever is logically possible is actually possible. Once you get that you'll stop hammering me with what I already understand and disagree with.

You say I'm just making you repeat yourself, yet you have asked me the same thing again about newtonian physics. — Amalac

Then we understand each other. I truly can't be of any assistance here. I don't know why there aren't any modal realists jumping in to have a better conversation with you than I'm capable of.

Well, I simply don't agree with you in the least about what you say here, so there's no point in discussing the matter of possible worlds further. — Amalac

Ahhhh, yes now we are in agreement. And I mean it when I say I am disappointed that no modal realists, Wittgenstein scholars, or Leibniz fans jumped into this thread. I'm the last person here who can be of any use to you because I find possible worlds incoherent. There are things that are logically possible yet still not possible. I'm not sure if there's a name for that position but if there is, that's what I am. I'm curious about that now. Maybe I'll google around.

There's no common ground. Perhaps the Stanford Enciclopedia of Philosophy can do a better job than I did: https://plato.stanford.edu/entries/possible-worlds/ — Amalac

A better job at what? At explaining something I understand and (without having studied the matter much) reject? Like I say, I actually share your frustration at my own limitations in this area. I'm totally unqualified to be having this conversation with you. Mostly I"m responding because you seem confused about the CMB, which is not transparent in any conceivable or even logically consistent world, and is by definition the limit of what we can see at any wavelength at all.

Ok then, if that's what you meant by “perfect window” then there are no problems. I accept that definition. — Amalac

I already noted in my first post that space is essentially perfectly transparent (except for some minor amount of interstellar dust) and that we can perfectly well see to the limit of the observable universe, namely the CMB.

It's not irrelevant, I was trying to clarify what I meant by “transparent”: When the average person speaks about X object being transparent, they probably have in mind something very different than what physicists have in mind. When saying «a window is transparent», they usually don't mean that it lets X,Y or Z amount of light To pasa through, they mean simply: you can see through a window. — Amalac

What average person? In some possible world? This is the most irrelevant point imaginable.

To quote Wittgenstein again: The meaning of a word is its use in a particular language game. The language game of describing things in daily life is not the same as the language game that results from doing physics. — Amalac

I'm not familiar enough with Wittgenstein to comment.

Human eyes. Have I defined it well enough for you now? — Amalac

Well then the CMB is indeed transparent or invisible to the human eye. Its color temperature is far far outside of our range of vision. It's visible only by powerful radio telescopes. You've just refuted your own premise.

So modal logic is not serious philosophy according to you? I disagree — Amalac

Model logic is not the same as possible worlds, but I haven't studied it. But what's wrong with disagreeing? We do disagree. I'm sorry there are no modal realists around here to engage with you.

Ok, but I'm not talking merely about the observable universe, I'm talking about both the observable universe and the parts of the universe we have not observed yet (that's why I said I meant “universe” in an all-encompassing sense). — Amalac

We can have no possible information about the part of the universe we can't observe. For all we know, in the unobservable universe, bowling balls fall up. There, I just contradicted my own position on this matter. I'm willing to accept that bowling balls might fall up in the universe at large, but not that there's a Newtonian possible world. My own ideas are clearly not well considered.

I use the term in the same sense as the one suggested by Bertrand Russell in this passage:

Space and time appear to be infinite in extent, and infinitely divisible. — Amalac

Two nonsensical premises, at odds with all of contemporary physics. Russell of course did not know about quantum physics or the Planck length.

If we travel along a straight line in either direction, it is difficult to believe that we shall finally reach a last point, beyond which there is nothing, not even empty space. — Amalac

False (or at least unproven and highly doubtful) premise hence unsound argument. But you already know this, right?

Again, I don't use the word “transparent” as it is used by physicists, but rather in the sense Wittgenstein uses it in my OP. You can substitute “see through” (meaning: such that you can see through it) every time I say “transparent” if you like. I guess it's a sense more akin to phenomenology than to physics. — Amalac

But the CMB is already transparent to visible light, to the human eye. Its color temperature can only be seen by powerful radio telescopes. You're refuting your own premises.

At any rate, what is wrong with this definition of transparent object?:

An object such that when a human sees it, through it some other object that is both not transparent and not translucent can be seen with clarity. — Amalac

Because by that definition, the CMB is indeed perfectly transparent. It can only be seen with a radio telescope.

Modal realism is the view propounded by David Kellogg Lewis that all possible worlds are real in the same way as is the actual world: they are "of a kind with this world of ours."[1] It is based on the following tenets: possible worlds exist; possible worlds are not different in kind from the actual world; possible worlds are irreducible entities; the term actual in actual world is indexical, i.e. any subject can declare their world to be the actual one, much as they label the place they are "here" and the time they are "now". — fishfry

I agree with you in this point, that sounds insane.

There are things that are logically possible yet still not possible. I'm not sure if there's a name for that position but if there is, that's what I am. I'm curious about that now. Maybe I'll google around. — fishfry

Good! If I got you curious about something, then our exchange about possible worlds wasn't entirely fruitless.

We can have no possible information about the part of the universe we can't observe. For all we know, in the unobservable universe, bowling balls fall up. There, I just contradicted my own position on this matter. — fishfry

I mean the part of the universe we can't observe at present, but may be able to observe in the future. We may never see those parts, but it's not logically impossible to see them. But then we're back to modal logic, and there we disagree.

If we could see them, then eventually (if space has some ultimate limit, even if we could never see it) we would have to reach something that is not transparent and not translucent by logical necessity.

I'll answer a few of your other points later, right now I've some work to do.

I agree with you in this point, that sounds insane. — Amalac

That's what I thought you were talking about. I don't know the diff between that and whatever Leibniz thought about the subject.

Good! If I got you curious about something, then our exchange about possible worlds wasn't entirely fruitless. — Amalac

I glanced at the SEP article on possible worlds and my eyes glazed. Never could get into this subject I'm afraid.

I mean the part of the universe we can't observe at present, but may be able to observe in the future. We may never see those parts, but it's not logically impossible to see them. But then we're back to modal logic, and there we disagree. — Amalac

Well we can see out another light year per year. Not very much in the scheme of things.

If we could see them, then eventually (if space has some ultimate limit, even if we could never see it) we would have to reach something that is not transparent and not translucent by logical necessity. — Amalac

Whatever we can see at any given moment is part of the ever-expanding observable universe. I don't see how we can reason past that.

I'll answer a few of your other points later, right now I've some work to do. — Amalac

You're far better off. I'm truly puzzled nobody else jumped in here about modal logic or possible worlds.

Let me leave you with this.

I wonder if physics allows for anything that's totally transparent to all energy. — fishfry

I have no idea but that "anything" would be impossible to detect as detection requires some form of interaction but transparency means none of that. Good day.

I have no idea but that "anything" would be impossible to detect as detection requires some form of interaction but transparency means none of that. Good day. — TheMadFool

Dark matter fits the bill. It doesn't interact electromagnetically at any frequency, it's only detectable as extra gravitational attraction.

https://en.wikipedia.org/wiki/Dark_matter