Next above plants in the scale of being come animals. Since the higher always contains the lower, but exhibits a further realization of form peculiar to itself, animals share with plants the functions of nutrition and propagation. What is peculiar to them, the point in which they rise above plants, is the possession of sensation. Sense-perception is therefore the special function of animals, and they possess, therefore, the nutritive and the sensitive souls. With sensation come pleasure and pain, for pleasure is a 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.