xli.); it follows that men, in so far as they live in obedience to reason, necessarily do only such things as are necessarily good for human nature, and consequently for each individual man (IV.

xxxi.
Coroll.); in other words, such things as are in harmony with each man's nature.

Therefore, men in so far as they live in obedience to reason, necessarily live always in harmony one with another.

Q.E.D.
Corollary I .-- There is no individual thing in nature, which is more useful to man, than a man who lives in obedience to reason.
For that thing is to man most useful, which is most in harmony with his nature (IV.xxxi.

Coroll.); that is, obviously, man.
But man acts absolutely according to the laws of his nature, when he lives in obedience to reason (III.Def.