In so far as men live in obedience to reason they necessarily do only such things as are good for human nature, and consequently for each individual man
As every man seeks most that which is useful to him, so are men most useful one to another. For the more a man seeks what is useful to him and endeavours to preserve himself, the more is he endowed with power to act... [and] to live in obedience to reason. Men are most in natural harmony, when they live in obedience to reason;; therefore men will be most useful one to another, when each seeks most that which is useful to him..
What we have just shown is attested by experience... Yet it rarely happens that men live in obedience to reason, for things are so ordered among them, that they are generally envious and troublesome one to another. Nevertheless they are scarcely able to lead a solitary life, so that the definition of man as a social animal has met with general assent; in fact, men do derive from social life much more convenience than injury. Let satirists then laugh their fill at human affairs, let theologians rail, and let misanthropes praise to their utmost the life of untutored rusticity, let them heap contempt on men and praises on beasts; when all is said, they will find that men can provide for their wants much more easily by mutual help, and that only by uniting their forces can they escape from the dangers that on every side beset them: not to say how much more excellent and worthy of our knowledge it is, to study the actions of men than the actions of beasts. But I will treat of this more at length elsewhere.
Spinoza advises that living by reason and doing good for others is in our own best interest. Unfortunately most men do not live according to reason and discord is rampant. Still the mutual help that social life brings is obviously beneficial. He will address this point in Part 5.