P57: Each affect of each individual differs from the affect of another as much as the essence of the one from the essence of the other.

Dem.: This Proposition is evident from IIA1″ . But nevertheless we shall demonstrate it from the definitions of the three primitive affects.

All the affects are related to Desire, Joy, or Sadness, as the definitions we have given of them show. But Desire is the very nature, or essence, of each {individual} (see the definition of Desire in P9 S). Therefore the Desire of each individual differs from the Desire of another as much as the nature, or essence, of the one differs from the essence of the other.

Next, Joy and Sadness are passions by which each one’s power, or striving to persevere in his being, is increased or diminished, aided or restrained (by P11 and P11 S). But by the striving to persevere in one’s being, insofar as it is related to the Mind and Body together, we understand Appetite and Desire (see P9 S). So Joy and Sadness are the Desire, or Appetite, itself insofar as it is increased or diminished, aided or restrained, by external causes. I.e. (by the same scholium), it is the very nature of each {individual}. And so, the Joy or Sadness of each {individual} also differs from the Joy or Sadness of another as much as the nature, or essence, of the one differs from the essence of the other.

Consequently, each affect of each individual differs from the affect of another as much, etc., q.e.d.

Schol.: From this it follows that the affects of the animals which are called irrational (for after we know the origin of the Mind, we cannot in any way doubt that the lower animals feel things) differ from men’s affects as much as their nature differs from human nature. Both the horse and the man are driven by a Lust to procreate; but the one is driven by an equine Lust, the other by a human Lust. So also the Lusts and Appetites of Insects, fish, and birds must vary. Therefore, though each individual lives content with his own nature, by which he is constituted, and is glad of it, nevertheless that life with which each one is content, and that gladness, are nothing but the idea, or soul, of the individual. And so the gladness of the one differs in nature from the gladness of the other as much as the essence of the one differs from the essence of the other.

Finally, from P57 it follows that there is no small difference between the gladness by which a drunk is led and the gladness a Philosopher possesses. I wished to mention this in passing.

This will be enough concerning the affects that are related to man insofar as he is acted on.

It remains to add a few words about those that are related to him insofar as he acts.