pleasant
sensation, and pain the opposite. Hence arises the impulse to seek the
pleasant and avoid the painful. This can only be achieved by the power
of movement. Most animals, accordingly, have the power of locomotion,
which is not possessed by plants, because they do not require it,
since they are not sensitive to pleasure and pain. In his books upon
animals Aristotle attempts to carry out the principle of development
in detail, showing what are the higher, and what the lower, animal
organisms. This he connects with the {298} methods of propagation
employed by different animals. Sex-generation is the mark of a higher
organism than parthenogenesis.

The scale of being proceeds from animals to man. The human organism,
of course, contains the principles of all lower organisms. Man
nourishes himself, grows, propagates his kind, moves about, and is
endowed with sense-perception. But he must have in addition his own
special function, which constitutes his advance beyond the animals.
This is reason. Reason is the essential, the proper end and activity
of man. His soul is nutritive, sensitive, and rational. In man,
therefore, the world-reason which could only appear in inorganic
matter as gravitation and levitation, in plants as nutrition, in
animals as sensation, appears at last in its own proper form, as what
it essentially is, reason. The world-reason, so long struggling
towards the light, has reached it, has become actual, has become
existent, in man. The world-process has attained its proximate end.

Within human consciousness there are lower and higher grades, and
Aristotle has taken great pains to trace these from the bottom to the
top. These stages of consciousness are what are ordinarily called
"faculties." But Aristotle notes that it is nonsense to talk, as Plato
did, of the "parts" of the soul. The soul, being a single indivisible
being, has no parts. They are different aspects of the activity of one
and the same being; different stages of its development. They can no
more be separated than the convex and concave aspects of a curve. The
lowest faculty, if we must use that word, is sense-perception. Now
what we perceive in a thing is its qualities. Perception tells us that
a piece of gold is {299} heavy, yellow, etc. The underlying substratum
which supports the qualities cannot be perceived. This means that the
matter is unknowable, the form knowable, for the qualities are part of
the form. Sense-perception, the