All emotions are related to desire, joy, or sorrow, as the definitions show which we have given of those emotions. But desire is the very nature or essence of a person and therefore the desire of one person differs from the desire of another as much as the nature or essence of the one differs from that of the other. Again, joy and sorrow are passions by which the power of a person or his effort to persevere in his own being is increased or diminished, helped, or limited. But by the effort to persevere in his own being, in so far as it is related at the same time to the mind and the body, we understand appetite and desire, and therefore joy and sorrow are desire or appetite in so far as the latter is increased, diminished, helped, or limited by external causes; that is to say they are the nature itself of each person. The joy or sorrow of one person therefore differs from the joy or sorrow of another as much as the nature or essence of one person differs from that of the other, and consequently the emotion of one person differs from the corresponding emotion of another. Hence it follows that the emotions of animals which are called irrational (for after we have learned the origin of the mind we can in no way doubt that brutes feel) differ from human emotions as much as the nature of a brute differs from that of a man. Both the man and the horse, for example, are swayed by the lust to propagate, but the horse is swayed by equine lust and the man by that which is human. The lusts and appetites of insects, fishes, and birds must vary in the same way; and so, although each individual lives contented with its own nature and delights in it, nevertheless the life with which it is contented and its joy are nothing but the idea or soul of that individual, and so the joy of one differs in character from the joy of the other as much as the essence of the one differs from the essence of the other. Finally, it follows from the preceding proposition that the joy by which the drunkard is enslaved is altogether different from the joy which is the portion of the philosopher,--a thing I wished just to hint in passing. _The Inconstancy of the Emotions_ The human body is affected by external bodies in a number of ways. Two men, therefore, may be affected in different ways at the same time, and therefore they can be affected by one and the same object in different ways. Again the human body may be affected now in this and now in that way, and consequently it may be affected by one and the same object in different ways at different times.