But by the endeavour to persist in its being, in so far as it is attributable to mind and body in conjunction, we mean appetite and desire (III.ix.

note); therefore pleasure and pain are identical with desire or appetite, in so far as by external causes they are increased or diminished, helped or hindered, in other words, they are every man's nature; wherefore the pleasure and pain felt by one man differ from the pleasure and pain felt by another man, only in so far as the nature or essence of the one man differs from the essence of the other; consequently, any emotion of one individual only differs, &c.

Q.E.D.
Note .-- Hence it follows, that the emotions of the animals which are called irrational (for after learning the origin of mind we cannot doubt that brutes feel) only differ from man's emotions, to the extent that brute nature differs from human nature.

Horse and man are alike carried away by the desire of procreation; but the desire of the former is equine, the desire of the latter is human.

So also the lusts and appetites of insects, fishes, and birds must needs vary according to the several natures.