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.