E4P35C1
Corollary — Part IV
Latin
Nihil singulare in rerum natura datur quod homini sit utilius quam homo qui ex ductu rationis vivit. Nam id homini utilissimum est quod cum sua natura maxime convenit (per corollarium propositionis 31 hujus) hoc est (ut per se notum) homo. At homo ex legibus suæ naturæ absolute agit quando ex ductu rationis vivit (per definitionem 2 partis III) et eatenus tantum cum natura alterius hominis necessario semper convenit (per propositionem præcedentem); ergo homini nihil inter res singulares utilius datur quam homo etc. Q.E.D.
English (Elwes 1883)
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. ii.), and to this extent only is always necessarily in harmony with the nature of another man (by the last Prop.); wherefore among individual things nothing is more useful to man, than a man who lives in obedience to reason. Q.E.D.
Modern English
There is no individual thing in nature more useful to a human being than a person who lives under the guidance of reason. For the most useful thing for a person is what agrees most closely with their nature (E4P31C), and that, self-evidently, is another human being. A human being acts absolutely according to the laws of their own nature when living under the guidance of reason (E3D2), and only to that extent necessarily agrees at all times with the nature of another human being (E4P35). Therefore among individual things nothing is more useful to a human being than a person who lives under the guidance of reason. Q.E.D.