Let us concentrate attention on the table. To the eye it is oblong, brown and shiny, to the touch it is smooth and cool and hard; when I tap it, it gives out a wooden sound. But as soon as we try to be more precise our troubles begin. Although I believe that the table is 'really' of the same colour all over, the parts that reflect the light look much brighter than the other parts, and some parts look white because of reflected light. I know that, if I move, the parts that reflect the light will be different, so that the apparent distribution of colours on the table will change. It follows that if several people are looking at the table at the same moment, no two of them will see exactly the same distribution of colors, because no two can see it from exactly the same point of view, and any change in the point of view makes some change in the way the light is reflected. For most practical purposes these differences are unimportant, but here we have already the beginning of one of the distinctions that cause most trouble in philosophy.

What is the underlying theme of the passage?