Homer is composed out of only
twenty-four letters, so these few simple ideas constitute all the material
of knowledge. The mind can neither have more nor other simple ideas than
those which are furnished to it by these two sources of experience.

Locke differs from Descartes again in regard to extension and thought.
Extension does not constitute the essence of matter, nor thought the
essence of mind. Extension and body are not the same; the former is
presupposed by the latter as its necessary condition, but it is the former
alone which yields mathematical matter. The essence of physical matter
consists rather in solidity: where impenetrability is found there is body,
and the converse; the two are absolutely inseparable. With space the case
is different. I cannot conceive unextended matter, indeed, but I can easily
conceive immaterial extension, an unfilled space Further, if the essence
of the soul consisted in thought, it must be always thinking. As the
Cartesians maintained, it must have ideas as soon as it begins to be, which
is manifestly contrary to experience. Thinking is merely an activity of
the mind, as motion is an activity of the body, and not its essential
characteristic. The mind does not receive ideas until external objects
occasion perception in it through impressions, which it is not able to
avert. The understanding may be compared to a mirror, which, without
independent activity and without being consulted, takes up the images of
things. Some of the simple ideas which have been mentioned above represent
the properties of things as they really are, others not. The former class
includes all ideas of reflection (for we are ourselves the immediate object
of the inner sense); but among the ideas of sensation those only which come
from different senses, hence extension, motion and rest, number, figure,
and, further, solidity, are to be accounted _primary_ qualities, _i. e_.,
such as are actual copies of the properties of bodies. All other ideas, on
the contrary, have no resemblance to properties of bodies; they represent
merely the ways in which things act, and are not copies of things. The
ideas of _secondary_ or derivative qualities (hard and soft, warm and cold,
colors and sounds, tastes and odors) are in the last analysis caused--as
are the primary--by motion, but not perceived as such. Yellow and warm are
merely sensations in us, which we erroneously ascribe to objects; with
equal right we might ascribe to