P72: A free man always acts honestly, not deceptively.

Dem.: If a free man, insofar as he is free, did anything by deception, he would do it from the dictate of reason (for so far only do we call him free). And so it would be a virtue to act deceptively (by P24), and hence (by the same Prop.), everyone would be better advised to act deceptively to preserve his being. I.e. (as is known through itself), men would be better advised to agree only in words, and be contrary to one another in fact. But this is absurd (by P31 C). Therefore, a free man etc., q.e.d.

Schol.: Suppose someone now asks: what if a man could save himself from the present danger of death by treachery? would not the principle of preserving his own being recommend, without qualification, that he be treacherous?

The reply to this is the same. If reason should recommend that, it would recommend it to all men.37 And so reason would recommend, without qualification, that men make agreements, join forces, and have common rights only by deception—i.e., that really they have no common rights. This is absurd.