are all in the habit of judging
as to the 'real' shapes of things, and we do this so unreflectingly that
we come to think we actually see the real shapes. But, in fact, as we
all have to learn if we try to draw, a given thing looks different
in shape from every different point of view. If our table is 'really'
rectangular, it will look, from almost all points of view, as if it had
two acute angles and two obtuse angles. If opposite sides are parallel,
they will look as if they converged to a point away from the spectator;
if they are of equal length, they will look as if the nearer side were
longer. All these things are not commonly noticed in looking at a table,
because experience has taught us to construct the 'real' shape from the
apparent shape, and the 'real' shape is what interests us as practical
men. But the 'real' shape is not what we see; it is something inferred
from what we see. And what we see is constantly changing in shape as we
move about the room; so that here again the senses seem not to give us
the truth about the table itself, but only about the appearance of the
table.

Similar difficulties arise when we consider the sense of touch. It is
true that the table always gives us a sensation of hardness, and we feel
that it resists pressure. But the sensation we obtain depends upon how
hard we press the table and also upon what part of the body we press
with; thus the various sensations due to various pressures or various
parts of the body cannot be supposed to reveal _directly_ any definite
property of the table, but at most to be _signs_ of some property which
perhaps _causes_ all the sensations, but is not actually apparent in any
of them. And the same applies still more obviously to the sounds which
can be elicited by rapping the table.

Thus it becomes evident that the real table, if there is one, is not the
same as what we immediately experience by sight or touch or hearing. The
real table, if there is one, is not _immediately_ known to us at all,
but must be an inference from what is immediately known. Hence, two very
difficult questions at once arise; namely, (1) Is there a real table at
all? (2) If so, what sort of object can it be?

It will help us in considering these questions to have a few simple
terms of which the meaning is definite and clear. Let us give the name
of 'sense-data' to the things that are immediately known in sensation:
such things as colours, sounds, smells, hardnesses, rou