P18: A Desire that arises from Joy is stronger, other things equal, than one that arises from Sadness.

Dem.: Desire is the very essence of man (by Def. Aff. I), i.e. (by IIIP7), a striving by which a man strives to persevere in his being. So a Desire that arises from Joy is aided or increased by the affect of Joy itself (by the Def. of Joy in IIIP11 S), whereas one that arises from Sadness is diminished or restrained by the affect of Sadness (by the same Schol.). And so the force of a Desire that arises from Joy must be defined both by human power and the power of the external cause, whereas the force of a Desire that arises from Sadness must be defined by human power alone. The former, therefore, is stronger than the latter, q.e.d.

Schol.: With these few words I have explained the causes of man’s lack of power and inconstancy, and why men do not observe the precepts of reason. Now it remains for me to show what reason prescribes to us, which affects agree with the rules of human reason, and which, on the other hand, are contrary to those rules. But before I begin to demonstrate these things in our cumbersome Geometric order, I should like first to show briefly here the dictates of reason themselves, so that everyone may more easily perceive what I think.

Since reason demands nothing contrary to nature, it demands that everyone love himself, seek his own advantage, what is really useful to him, want what will really lead man to a greater perfection, and absolutely, that everyone should strive to preserve his own being as far as he can. This, indeed, is as necessarily true as that the whole is greater than its part (see IIIP4).

Further, since virtue (by D8) is nothing but acting from the laws of one’s own nature, and no one strives to preserve his being (by IIIP7) except from the laws of his own nature, it follows:

(i) that the foundation of virtue is this very striving to preserve one’s own being, and that happiness consists in man’s being able to preserve his being; (ii) that we ought to want virtue for its own sake, and that there is not anything preferable to it, or more useful to us, for the sake of which we ought to want it; and finally (iii) that those who kill themselves are weak-minded and completely conquered by external causes contrary to their nature.

Again, from IIPost. it follows that we can never bring it about that we require nothing outside ourselves to preserve our being, nor that we live without having dealings with things outside us. Moreover, if we consider our Mind, our intellect would of course be more imperfect if the Mind were alone and did not understand anything except itself. There are, therefore, many things outside us which are useful to us, and on that account to be sought.

Of these, we can think of none more excellent than those that agree entirely with our nature. For if, for example, two individuals of entirely the same nature are joined to one another, they compose an individual twice as powerful as each one. To man, then, there is nothing more useful than man. Man, I say, can wish for nothing more helpful to the preservation of his being than that all should so agree in all things that the Minds and Bodies of all would compose, as it were, one Mind and one Body; that all should strive together, as far as they can, to preserve their being; and that all, together, should seek for themselves the common advantage of all.

From this it follows that men who are governed by reason—i.e., men who, from the guidance of reason, seek their own advantage—want nothing for themselves that they do not desire for other men. Hence, they are just, honest, and honorable.

These are those dictates of reason which I promised to present briefly here before I began to demonstrate them in a more cumbersome order. I have done this to win, if possible, the attention of those who believe that this principle—that everyone is bound to seek his own advantage—is the foundation, not of virtue and morality, but of immorality. After I have shown briefly that the contrary is true, I shall proceed to demonstrate this in the same way I have followed up to this point.