To make our difficulties plain, 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.
Any one else who sees and feels and hears the table will agree with
this description, so that it might seem as if no difficulty would
arise; 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 colours, 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 to
the painter they are all-important: the painter has to unlearn the
habit of thinking that things seem to have the colour which common
sense says they 'really' have, and to learn the habit of seeing things
as they appear. Here we have already the beginning of one of the
distinctions that cause most trouble in philosophy--the distinction
between 'appearance' and 'reality', between what things seem to be and
what they are. The painter wants to know what things seem to be, the
practical man and the philosopher want to know what they are; but the
philosopher's wish to know this is stronger than the practical man's,
and is more troubled by knowledge as to the difficulties of answering
the question.