There are few more graphic illustrations of the role of thought in mediating experience that the fact that you cannot see, and have never seen, a cube.
But surely, you may think, I see cubes all the time! Every roll of the die, every sugar lump dropped in the tea, every square-sided cardboard box reveals a cube, and I see things like these every day. True enough. But you do not see a cube, you only see an object that you happen to know is a cube, and thus see it in your mind’s eye as cubic.
In a lecture in 1945, Maurice Merleau-Ponty laid out the matter clearly:
Even the objects right in front of me are not truly seen but merely thought. Thus I cannot see a cube, that is, a solid with six surfaces and twelve edges; all I ever see is a perspective figure of which the lateral surfaces are distorted and the back surface completely hidden. If I am able to speak of cubes, it is because my mind sets these appearances to rights and restores the hidden surface. I cannot see a cube as its geometrical definition presents it: I can only think it.
Once you think about this, you will realise that you could not possibly have ever seen a cube – perhaps the closest you could have come would be to see a square block aside a mirror at just the right angle so that you could see all its facings at once. But you would still then be seeing two distinct perspective figures that would only become one object in your mind.
We do not see the world as it is, we see stereoscopic images of the world and construct them, with the help of our mental models, into objects. But these objects exist in our mind, and not in our eyes, and so I say again that you cannot see, and never have seen, a cube.