P35: Only insofar as men live according to the guidance of reason, must they always agree in nature.

Dem.: Insofar as men are torn by affects which are passions, they can be different in nature (by P33), and contrary to one another(by P34). But insofar as men live according to the guidance of reason, they are said only to act (by IIIP3). Hence, whatever follows from human nature, insofar as it is defined by reason, must be understood through human nature alone (by IIID2), as through its proximate cause. But because each one, from the laws of his own nature, wants what he judges to be good, and strives to avert what he judges to be evil (by P19), and moreover, because what we judge to be good or evil when we follow the dictate of reason must be good or evil (by IIP41), it follows that insofar as men live according to the guidance of reason, they must do only those things that are good for human nature, and hence, for each man, i.e. (by P31 C), those things that agree with the nature of each man. Hence, insofar as men live according to the guidance of reason, they must always agree among themselves, q.e.d.

Cor. 1: There is no singular thing in Nature that is more useful to man than a man who lives according to the guidance of reason.

For what is most useful to man is what most agrees with his nature (by P31 C), i.e. (as is known through itself), man. But a man acts entirely from the laws of his own nature when he lives according to the guidance of reason (by IIID2), and only to that extent must he always agree with the nature of the other man (by P35). Therefore, among singular things there is nothing more useful to man than a man, etc., q.e.d.

Cor. 2.: When each man most seeks his own advantage for himself, then men are most useful to one another.

For the more each one seeks his own advantage, and strives to preserve himself, the more he is endowed with virtue (by P20), or what is the same (by D8), the greater is his power of acting according to the laws of his own nature, i.e. (by IIIP3), of living from the guidance of reason. But men most agree in nature, when they live according to the guidance of reason (by P35). Therefore (by P35 C1), men will be most useful to one another, when each one most seeks his own advantage, q.e.d.

Schol.: What we have just shown is also confirmed by daily experience, which provides so much and such clear evidence that this saying is in almost everyone’s mouth: man is a God/Nature to man.

Still, it rarely happens that men live according to the guidance of reason. Instead, their lives are so constituted that they are usually envious and burdensome to one another. They can hardly, however, live a solitary life; hence, that definition which makes man a social animal has been quite pleasing to most. And surely we do derive, from the society of our fellow men, many more advantages than disadvantages.

So let the Satirists laugh as much as they like at human affairs, let the Theologians curse them, let Melancholies praise as much as they can a life that is uncultivated and wild, let them disdain men and admire the lower animals. Men still find from experience that by helping one another they can provide themselves much more easily with the things they require, and that only by joining forces can they avoid the dangers that threaten on all sides—not to mention that it is much preferable and more worthy of our knowledge to consider the deeds of men, rather than those of the lower animals. But I shall treat this topic more fully elsewhere.