What I gave is a simple reductio:

1. Suppose, "We are brains in vats" can be true even if it is not possible to justify such a proposition
2. "We are brains in vats" is (justifiably) false
3. Therefore, supposition (1) is false

(Your modus tollens was addressed in my first sentence.)

@Banno @Wayfarer

The missing foghorn between your ships passing in the night is that Banno thinks that idealism entails solipsism, unless I have misunderstood.

1. Suppose, "We are brains in vats" can be true even if it is not possible to justify such a proposition
2. "We are brains in vats" is (justifiably) false
3. Therefore, supposition (1) is false — Leontiskos

That's basically what I said.

The issue is that if realism is true then supposition (1) is true. Given that supposition (1) is false, realism is false. That's Putnam's reasoning.

The issue is that if realism is true then supposition (1) is true. — Michael

No, the issue is that if (2) is true then no one can presuppose (1), because the proposition in question is justifiable. The issue is that if (2) is true then "We are brains in vats" is not representative of global skepticism at all. (2) does not invalidate global skepticism, it invalidates the idea that "We are brains in vats" is representative of global skepticism. This is the third of three separate reasons why your argument fails.

No, the issue is that if (2) is true then no one can presuppose (1), because the proposition in question is justifiable. — Leontiskos

Anyone can suppose anything.

1. Suppose that I am a woman.
2. I am not a woman.
3. Therefore, supposition (1) is false.

If you don't believe that (2) is true then you might assert (1), perhaps because someone has tricked you into believing that I am a woman.

The issue is that if (2) is true then "We are brains in vats" is not representative of global skepticism at all. (2) does not invalidate global skepticism, it invalidates the idea that "We are brains in vats" is representative of global skepticism. — Leontiskos

I don't understand what you're saying here at all. All I can do to correct is you is to re-quote the IEP article on the brains in a vat argument:

This general characterization of metaphysical realism is enough to provide a target for the Brains in a Vat argument. For there is a good argument to the effect that if metaphysical realism is true, then global skepticism is also true, that is, it is possible that all of our referential beliefs about the world are false. As Thomas Nagel puts it, “realism makes skepticism intelligible,” (1986, 73) because once we open the gap between truth and epistemology, we must countenance the possibility that all of our beliefs, no matter how well justified, nevertheless fail to accurately depict the world as it really is. Donald Davidson also emphasizes this aspect of metaphysical realism: “metaphysical realism is skepticism in one of its traditional garbs. It asks: why couldn’t all my beliefs hang together and yet be comprehensively false about the actual world?” (1986, 309)

And again, if that is no more than that it can be made the consequent of a material implication, that is trivially right. So again, what is it to be "justified"? — Banno

Yes. And if B then A→B, for any A or B. So if we take justification as being the consequent of a material implication then that any truth is justified is trivial.

But is that what you mean? — Banno

I don't really understand what you're asking.

We have realists claiming that some truths are unknowable and antirealists claiming that all truths are knowable.

Assuming that knowledge is justified true belief, this reduces to the realist claiming that some truths are either unbelievable or unjustifiable and antirealists claiming that all truths are believable and justifiable.

What counts as justification is certainly an issue worth considering at some point, but I don't think it's particularly relevant to the current issue being discussed.

We can tentatively just say that it’s whatever distinguishes knowledge from a mere true belief.

(do I need to add that if it is justified, then it is by that very fact justifiable?) — Banno

Sure, justified entails justifiable. But justifiable does not entail justified. Antirealists are only claiming that if a proposition is true then it is knowable (justifiable), not that if a proposition is true then it is known (justified).

Returning back to a previous comment of yours:

Just to be clear, my target here is the idea that a proposition can only be true if justified. — Banno

Note that these mean different things:

1. If a proposition is true then it can be justified
2. A proposition can only be true if justified

In propositional logic:

3. p → ◊Jp
4. ◊p → Jp

At least, (4) is my best attempt at formulating (2). The position of the "can" is a little confusing.

I don't really understand what you're asking. — Michael

Yeah, you do.

If, for antirealists, as you say, all truths are believable and justifiable, then for any given truth there is some justification. On your account, a proposition can only be true if it has a justification.

Which is not so.

If, for antirealists, as you say, all truths are believable and justifiable, then for any given truth there is some justification. On your account, a proposition can only be true if it has a justification. — Banno

You’re making the same mistake. I explained it clearly above:

Note that these mean different things:

1. If a proposition is true then it can be justified
2. A proposition can only be true if justified

In propositional logic:

3. p → ◊Jp
4. ◊p → Jp

At least, (4) is my best attempt at formulating (2). The position of the "can" is a little confusing. — Michael

Do you understand what (1) and (3) mean?

To better explain it, I am saying:

1. If something is mortal then it can die

You are misinterpreting/misrepresenting this as:

2. Something can only be mortal if it’s dead

Not I.

The set of true propositions is on your account a proper subset of the set of propositions with a justification.

Hence a proposition can only be true if it has a justification.

But not all true propositions have a justification.

Unless you are saying that to be justified is to be the consequent of some implication, in which case, trivially, any true proposition is justified.

Hence my question - what is it to be justified?

2. Something can only be mortal if it’s dead — Michael

No. I'm saying that somethign can be mortal only if it has a death.

The set of true propositions is on your account a proper subset of the set of propositions with a justification. — Banno

No it's not. That would be:

1. p → Jp

That's not what is argued. What is argued is:

2. p → ◊Jp

Do you understand the difference?

No. I'm saying that somethign can be mortal only if it has a death — Banno

What does that mean? It doesn't even appear to be grammatically correct.

If, for antirealists, as you say, all truths are believable and justifiable, you can drop the modality. p→Jp

If a truth is justifiable, then for that truth there is some justification.

Otherwise you are saying that there might be truths with no justification. But that would contradict you "all truths are believable and justifiable"

It says that if something is mortal, then there is an something which is the death of that thing. Pretty plain.

The missing foghorn between your ships passing in the night is that Banno thinks that idealism entails solipsism, unless I have misunderstood. — bert1

:ok:

If, for antirealists, as you say, all truths are believable and justifiable, you can drop the modality. p→Jp — Banno

No we can't. Dropping modality changes meaning.

These mean different things:

1. All truths are believed and justified
2. All truths are believable and justifiable

↪Michael It says that if something is mortal, then there is an something which is the death of that thing. Pretty plain. — Banno

This still doesn't explain what that means.

In ordinary English we say that if something is mortal then it can die; we don't say that if something is mortal then something it its death. I understand the former, not the latter.

If a truth is justifiable, then for that truth there is some justification. — Banno

Were @Michael to disagree with this, he would have to show us a justifiable truth with no justification.

(What is happening here is that there is a shifting back and forth between the View from Nowhere (justifiability) and the View from Somewhere (particular acts of justification).)

Another example:

1. If the vase is fragile then it can break
2. The vase can be fragile only if it has a break

These do not mean the same thing. (1) is true and (2) is false.

Were Michael to disagree with this, he would have to show us a justifiable truth with no justifcation. — Leontiskos

No I don't. Just as the realist doesn't have to show us an unknowable truth.

1. If the vase is fragile then it can break
2. The vase can be fragile only if it has a break — Michael

One can show that a vase is fragile without breaking it, but can one show that a truth is justifiable without justifying it?

No I don't. — Michael

Sure you do. If you want to deny A→B then you must give an example of A^~B.

The point of that was to show that there is a meaningful difference between these two propositions:

1. If A is B then it can C
2. A can be B only if it has C

Banno is repeatedly misinterpreting/misrepresenting (1) as (2).

Sure you do. If you want to deny A→B then you must give an example of A^~B. — Leontiskos

Firstly, I don't. One approach is to show that A→B is a contradiction or is in some other sense incoherent. Antirealists often do this by addressing the meaning of the word "true" and explain that this meaning is inconsistent with unknowability.

Secondly, it is the realist who denies p → ◇Kp, and so if you follow your own reasoning you must provide an example of an unknowable truth.

No we can't. Dropping modality changes meaning. — Michael

Sure. Do you really want to say that if a proposition is true than in some possible world there is a justification? Fine, then for you every truth has a justification.

1. All truths are believed and justified
If every truth is justified, then every truth has a justification.

2. All truths are believable and justifiable
If every truth is justifiable, then for every truth there is some justification.

They have the same implication, that every truth has a justification. Straight forward stuff. But not every truth has a justification. Hence not all truths are believed and justified, and not all truths are beleivable and justifiable.



Your understanding of logic has been repeatedly shown to be lacking. There's no reason to take you seriously on such issues.

The point of that was to show that there is a meaningful difference between these two propositions:

1. If A is B then it can C
2. A can be B only if it has C

Banno is repeatedly misinterpreting/misrepresenting (1) as (2). — Michael

The question is whether that distinction is relevant when it comes to justification. (1) is not demonstrative. Mortality, for example, is an inductive inference. It's not at all clear that justifiability is inductive, such that one can claim that a proposition is justifiable without actually justifying it. Yet that is what you require.

Secondly, it is the realist who denies p → ◇Kp, and so if you follow your own reasoning you must provide an example of an unknowable truth. — Michael

Except that you haven't produced a single example of a real person who holds to your definition of realism. So there's that.

Do you really want to say that if a proposition is true than in some possible world there is a justification? Fine, then for you every truth has a justification. — Banno

That simply does not follow.

If some entity is a person then in some possible world it has a spouse. Therefore, every person has a spouse?

No, obviously not.

If every truth is justifiable, then for every truth there is some justification. — Banno

Just no.
"can be justified" does not entail "is justified".
"can be killed" does not entail "is killed".
"can be broken" does not entail "is broken".

It's honestly crazy that I have to explain this to you.

Your understanding of logic has been repeatedly shown to be lacking. There's no reason to take you seriously on such issues. — Banno

No one on this forum takes you seriously. I'm just continuing an exchange with Michael.

That simply does not follow. — Michael

Only if you do not wish to allow for justifications in other possible worlds. Hey, you are the one who wants to introduce modality... I think quite unnecessarily.

"can be justified" does not entail "is justified — Michael

Sure. But "can be justified" entails "has a justification". The alternative would be to supose that some truth can be justified yet has no justification, which is absurd.

Your other analogs do not work.

Sure. But "can be justified" entails "has a justification". — Banno

What is the difference between "is justified" and "has a justification"?

Only if you do not wish to allow for justifications in other possible worlds. — Banno

What? It doesn't follow because it doesn't follow, just as the spouse example doesn't follow.

Your other analogs do not work. — Banno

They work perfectly. They show that your re-phrasing of the claim has changed the meaning of the claim.

What is the difference between "is justified" and "has a justification"? — Michael

This is your game. you get to decide, I supose. I have asked you to tell me what you take a justification to be. Presumably "is justified" means we have the justification to hand, but perhaps not so for "has a justification".

This is your game. you get to decide, I supose. — Banno

You're the one who has introduced new grammar, so you need to explain it.

1. p can be justified
2. p is justified
3. p has a justification

I can't help but think that you're equivocating. You say that (1) entails (3), say that (2) is false, somehow use that to conclude that (3) is false, and so use that to conclude that (1) is false.

(1) entails (3). (2) entails (3). I don't know the truth value of p. Presumably it is either ⊤ or ⊥.