Alright, assuming that you are correct, it only shows that my example is not a good one, but this falls outside of my main point. My main point is simply to make you understand what I mean when I say "we cannot be mistaken about our perceptions" which is not what you think it means. Let's try again.

When we observe the stars from the earth, we observe that they twinkle. In reality, stars don't twinkle and it is an atmospheric effect. Let's say I am a teacher asking a student what he sees when looking at the stars. He should say "I observe that they twinkle". He will be correct because that is indeed what he should see when looking at the stars. Not drawing any conclusions about the real things observed, he is "not mistaken about his perceptions". That is all that I mean by this. We might disagree on the label, but you just need to understand what I mean behind the label I use.

So we still disagree because for you, the statements "mistaken about the real thing perceived" and "mistaken about the perception" are the same thing, and for me, they are not. Let me try to explain why in another way:

Let's say I feel sick. I go to the doctor and, upon testing everything (let's say we are able to test everything), it reveals that I am not really sick. Thus the feeling was not reflective of reality, but at the time that I felt sick, I felt sick. I was not dishonest with the doctor, because the feeling itself was real, even though the sickness was not. This is an example of being mistaken about the real thing perceived (the presence of real sickness) while not being mistaken about the perception (the feeling of sickness).

The physical is that which relates to physics, which is the natural science that involves the study of matter and its motion and behaviour through space and time, along with related concepts such as energy and force. — Sapientia

If I understand correctly, we at least agree that space is part of the physical. And we previously agreed that my syllogism works for things as colours and shapes. But how can we have colours and shapes without space? For us to perceive or even imagine colours and shapes, these must occupy an area (2D space). IE, shrink the area down to zero, and we can no longer perceive these colours and shapes. Thus space is an essential attribute of colours and shapes. Let's recap:
-Colours and shapes exist in the real world (as proven by the syllogism).
-Space is essential for the colours and shapes to exist, therefore space exists.
-Space is part of the physical world (if I understand your definition correctly), therefore the physical world exists, at least the portion of the physical that includes space.

Regarding Idealism: If we agreed that shapes and colours are things in themselves (as per the syllogism), then they exist independently of the activity of the mind. This does not refute the claim that some objects of knowledge are dependent on the mind, but it refutes the claim that all objects are.

No. As you said, you first perceived it as a table. You were not "mistaken in your perception" because a table is what you perceived. Then you later perceived a desk. Again, you were not "mistaken in your perception" because a desk is what you perceived. Let's say the real thing was in fact a rock (that looked like a table from afar). Then both your perceptions were wrong in identifying the real thing, but you were not "mistaken in your perception", because even though we are not certain about the real thing that we perceive, we are certain about the perception itself. I think you are using the term "perception" incorrectly.

Another example: Let's say I perceive a purple unicorn. I am not certain if it is an illusion or reality (though likely an illusion). One thing is certain though: I am perceiving a purple unicorn, and not a green dog.

Am I right? — TheMadFool

That is right.

The point being what if mind precedes the physical? — TheMadFool

You are correct too. Indeed, it appears the argument is refuted if there exists a being which can create (or imagine) something out of nothing, and then pass it on to us. Such a power is typically attributed to God. I think this is why Buddhism survives this argument, if I understand Buddhism correctly, as such: The physical world does not exist, but a God (pantheistic) exists which imagines it all.

Note: I know your quote was from the Christian Bible, not from a Buddhist book. But since Christianity (at least most branches of it) believe that the physical world is real, then the argument was never a threat to it.

You can be mistaken about the real thing you are perceiving, but how can you be mistaken about a perception? To use the specific example, how can you mistakenly call a desk a table if you are perceiving a desk? The only way I see how is because the person doesn't speak english very well and thus thought that a desk was called "table" in english. But this would be an error in language, not in perception.

or it will be rejected on the basis that "space", " time" and "energy" are subjective, not objective, and are just words or concepts. — Chief Owl Sapientia

While I agree with the points you say previously, I disagree with this one. For if space, time and energy are subjective, then they do not exist in themselves in the real world. But my argument refutes that, insofar as we perceive them. Just plug the terms in the syllogism.

unless your definition is accepted, which it probably will not be by most people who deny the physical. — Chief Owl Sapientia

That was my attempt at defining "physical". What is your (or the idealist's) definition then? Maybe we can resume the argument from there.

Are you conceding that you cannot refute idealism — Chief Owl Sapientia

Let's make sure we agree on the definition first. Idealism: system of thought in which the objects of knowledge are held to be in some way dependent on the activity of mind. Is that adequate?

How can you say that it's undoubtable that what you perceive is a table when you could be perceiving something else, and incorrectly calling it a table — Metaphysician Undercover

If I perceive a table, then I perceive a table. If I perceive a desk, that I perceive a desk. Calling a desk a table does not change the perception (if I understand you correctly when you say "calling"). If I call a desk a table, then I am dishonest, but that does not change my perception of it.

Interesting. I never thought of the term "physical" being so general. If your definition of "physical" contains all of these concepts too, then what do you consider non-physical things, if any?

So we agree that such things as colours, shapes, smells and sounds exist in the real world. That was the point of my original argument. It refutes the belief that no such things exists, that these are all illusions.

I then categorized these things as physical things, because my understanding of a physical thing is whatever has a spatial, time, or energy component to it; or can be perceived by the senses (sight, smell, touch, hear, or taste). It looks like we disagree on the definition of the term "physical", but that's okay, because at this point, we are merely arguing about labels.

Interesting. I never heard of that theory before. I think I can still refute it, as so: If I perceive a physical thing and my brain is in a vat, then it is possible that I have never encountered that thing in my life. However, that illusion of the physical thing still came from somewhere. In this case, it was created by the designer of the brain-in-a-vat experiment, right? Thus the question is brought back: how did the creator of the experiment come about the idea of the physical thing which I perceive?

I can generalize the problem this way: every effect requires an adequate cause. The effect of me perceiving a physical thing requires a cause. It could be caused by the designer of the brain-in-a-vat experiment. But then, that requires an adequate cause too. Eventually, I think it must come from the existence of the real physical thing (or something similar to it).

It sounds like we will not agree on the definition of the term "physical". What if I simplify my original argument and use the term "colour"? It changes the conclusion a bit, but not really the intent of the argument. Here it is:
1. We cannot imagine things we have not experienced in the past.
2. We perceive colours.
3. Therefore colours exist. Not necessarily the ones I directly perceive right now, (because I could be dreaming right now), but these colours must have existed at least in the past in order to enable me to imagine them right now.
Any objections? If not, then the same argument can be used for shapes, smells, sounds and so on.

We can now see that 1 is true but 2 is dubious as there's a possibility that our perceptions could be mentally generated, having no real physical correlate. — TheMadFool

There seems to be a logical fallacy in that statement: How can our perception of a thing be always mentally generated, since you agreed in premise 1 that it is not possible to imagine things that have not been perceived. Let's say I perceive a table. It could be that this table is mentally generated, because I perceived a similar table in the past. How can I explain the perception of the past? It could be that that too was mentally generated from a perception of another table before that, and so on ... But How did I get the very first perception? Logically, the first perception of a table must come from a real table, since we agree in premise 1 that it is not possible to imagine things that have not been perceived. Also, I would rectify the syllogism as so:
1. It is not possible to imagine things that have not yet been perceived.
2. We perceive physical things.
3. Therefore the physical things we perceive exist somewhere. Not necessarily the direct things we perceive, but similar things that would enable us to imagine these direct things we perceive.

But I think what would be desired of anything that was truly physical, would be that it is real independently of any perception of it; its reality would not be dependent on it being perceived. — Wayfarer

As I understand it, our disagreement lies around the definition of the term "physical". Let's take a step back. Instead of using the term "physical", let's use the term "colour" in the argument, as so:
1. We cannot imagine things we have not perceived in the past.
2. We perceive colours.
3. Therefore colours exist. Not necessarily the ones I directly perceive right now, (because I could be dreaming right now), but these colours must have existed at least in the past in order to enable me to imagine them right now.
Do you agree with the new simplified argument?

I still think there is a misunderstanding. While it is debatable that the table that I perceive is real or an illusion, the undeniable fact is that I perceive a table. As long as the thing that I perceive has a shape or colour, then it is physical. Even if the table is an illusion, then it is an illusion of a physical thing. In contrast, non-physical things would be concepts such as logic, morality and justice, because they don't have physical attributes such as shape, colour, or energy. If these are illusions, then they would be illusions of non-physical things.

So the only obstacle I see to a mutual understanding is to agree on the definition of a thing being "physical". Once clarified, then the original argument is easily understood.

Investigate what it means for something to be physical, does it leave anything out? — jkop

Yes. Concepts such as logic, morality, and justice are not perceived through the senses, and yet are undeniably real (at least logic for most people). They are therefore non-physical things. I would also add spiritual things like souls, angels and God, but these concepts can be controversial and so we can leave them out. If you disagree, what would you consider physical things versus non-physical?

Science can't really tell you if the world is real or not. — Wayfarer

I agree with this, but I think there is still a misunderstanding on the term "physical". Let me try to define it another way: A thing is physical if it has spatial attributes such as length, height, volume, etc. It is also physical if it has attributes related to energy, such as speed, force, noise emission etc. Finally, it is physical if it can be detected (by instruments) and measured. Under that new (less than perfect) definition, a table is definitely physical, even if only an illusion. My whole argument is not prove that the table that I perceive is real, but that if the perception of physical things exist, then there must exist a physical world somewhere.

An idealist would dispute that the table is made of matter. — dukkha

But science claims that tables are made of matter. Surely I can appeal to the authority of science on this. At the very least, the scientific claims become the default position, and so the idealist disputing the claim agreed upon by science would have the onus of proof, not me.

Begs the question. — dukkha

I defined "physical" as matter and energy. I perceive a table. The table is made of matter, thus is physical. Therefore I perceive a physical world.

From the fact that you perceive something it does not follow that the world is physical — jkop

I guess not always. But how about perceiving things directly through the senses (seeing, hearing, smelling, touching, ...)? Aren't things that are perceived through the senses necessarily physical?

I perceive a table in front of me. This table has not been demonstrated by you to be a physical object, whether that object is only imaginary or otherwise. — Sapientia

As previously stated, I defined "physical" as: matter and energy. I would now add "anything that is perceived by the senses" for further clarity. Now a table is made of matter (atoms). Thus the table I perceive is physical.

things like "blue" are perceptual, and thus lead to an ideal (or "spiritual") world. — Sapientia

But the definition of perceptual is "involving perception especially in relation to sensory experience", and "sensory" is related to physical things. Thus according to that definition, this would make the perception of "blue" lead to acknowledging a physical world, not idealism, would it not?

By what method are you assessing whether or not a candidate is adequate? — Sapientia

To say that "the existence of X is an adequate cause for my perception of X" is really the common sense hypothesis and is therefore the default position. If you were to object to this, you would have the onus of proof to refute the claim, not me.

You can't jump straight into how it is that we perceive a physical world without first establishing that we perceive a physical world. — Sapientia

Well, I perceive a table in front of me. This table is a physical object, even if that object is only imaginary.

things like "blue" are perceptual, and thus lead to an ideal (or "spiritual") world. — Sapientia

I am not sure I understand this. Can you present the argument?

You've said that there must be a cause, yet there is no necessary connection between the one and the other. Or, if there is, can you demonstrate this? — Sapientia

Sure. Indeed, the existence of a physical world is not a necessary cause to the effect of my perception of physical things; but it is an adequate cause, thus a candidate. To refute this candidate, we would need to find other adequate causes to take its place. I can think of only one, which is God. Since God (should He exist) can create things from nothing, then He could create the idea of physical things in our minds without physical things existing in the real world.

Why do you not in return though, need to explain where the physical world comes from? — dukkha

I think your concern misses the mark. If only a spiritual world exists, even if it is uncaused, then we could never perceive a physical world, due the limitation of our imagination, as explained previously. But we do perceive a physical world. If a physical world exists, even if it is uncaused, then it explains the phenomenon of our perception. Now you demand a cause for the existence of the physical world. That is indeed left unanswered, but it stands outside of the original topic of discussion, which was to explain how it is that we perceive a physical world.

Are you assuming that the world would be made of your perceptions or imaginations? Why else would you claim that the question whether the world is physical would depend on our ability to perceive physical things? — jkop

My argument is not from cause to effect, but tracing the effect back to the cause. The effect of our perception of the physical world requires an adequate cause. That cause must be a real physical world. Could it be something else?

Well, a tree. Your reasoning rests on the premise that when we imagine a tree we're imagining a physical thing. But a spiritualist might reject this premise and say that when we imagine a tree we imagine a spiritual thing (or some other non-physical thing). — Michael

Does this mean that, to a spiritualist, a tree (even imaginary) is a spiritual thing, not a physical one? If so, then how does he differentiate between physical and spiritual things?

So they'll say that these are spiritual concepts, not physical concepts. — Michael

How can that be? Spiritual things such as angels, souls, logic and morality do not possess such things as colours and shapes and sounds and textures and other sensory qualities. Can you find a spiritual thing that does?

You seem to be assuming that "physical" is a simple impression, like "blue", but that is doubtful. I have certainly experienced "blue", but how do I know whether I have experienced "physical"? — Sapientia

We do not experience "physical", but "blue", which is a physical concept. As long as you have experienced "blue", you need to explain where this experience comes from, if not from a physical world.

The extreme spiritualist could argue that when we imagine things like chairs and tables we're imagining complex arrangements of the latter things, not the former. — Michael

I agree with this. But these concepts (colours and shapes and sounds and textures and other sensory qualities) are physical concepts, not spiritual ones. How can the extreme spiritualist explain where these come from, if not from a physical world?

Well, as a possible reductio ad absurdum of your reasoning (unless you accept the conclusion), given that we can imagine such things as angels and souls, it must then follow that such things exist? — Michael

I do believe in these things, but I see your general point. The concept of "angels" can be deconstructed into "living beings" and "spiritual", both of which we perceive elsewhere. As for the soul, I think we do not have a clear perception of it. As such, I would not use my argument to prove the existence of these two concepts.

As long as the blind share our background capacities and language there is little that prevents them from understanding descriptions of colours and imagining what colours are like. — jkop

I would disagree. The blind can understand the theory behind the colour blue, that is, a certain frequency range of light wave, but he could not "see" the colour blue in his mind.

But whence the assumption that the question whether the world is physical or non-physical would somehow depend on our ability to imagine non-physical things? It doesn't. — jkop

Actually it depends on our ability to imagine physical things. Because if we cannot imagine physical things prior to experiencing them, then how can we explain our perception of physical things if these things don't exist anywhere? If we cannot provide an adequate alternative explanation for that phenomenon, then we must conclude that physical things exist.

I don't know how you define the spiritual (or the physical) so it's kinda hard to comment. — Michael

No problem:
Physical: matter and energy.
Spiritual: all that exists which is not physical. EG: living beings such as angels and souls, but also non-living beings such as logic and morality.

It took me a couple of tries but I think I finally understand what you wrote haha. Indeed, it sounds similar to my argument. To put it simply, an effect requires an adequate cause; and the existence of spiritual things is not an adequate cause for our perception of physical things.

I may have used the wrong term. Nevertheless, I argue against the concept that I have defined, even if the term is the wrong one.

Perception on its own is not a proof for reality. I perceive things in dreams but these perceived things are not real. At least, not in the dream itself.