E4P27D
Demonstration — Part IV
Latin
Mens quatenus ratiocinatur nihil aliud appetit quam intelligere nec aliud sibi utile esse judicat nisi id quod ad intelligendum conducit (per propositionem præcedentem). At mens (per propositiones 41 et 43 partis II, cujus etiam scholium vide) rerum certitudinem non habet nisi quatenus ideas habet adæquatas sive (quod per scholia propositionis 40 partis II idem est) quatenus ratiocinatur; ergo nihil certo scimus bonum esse nisi id quod ad intelligendum revera conducit et contra id malum quod impedire potest quominus intelligamus. Q.E.D.
English (Elwes 1883)
The mind, in so far as it reasons, desires nothing beyond understanding, and judges nothing to be useful to itself, save such things as conduce to understanding (by the foregoing Prop.). But the mind (II. xli., xliii. and note) cannot possess certainty concerning anything, except in so far as it has adequate ideas, or (what by II. xl. note, is the same thing) in so far as it reasons. Therefore we know nothing to be good or evil save such things as really conduce, &c. Q.E.D.
Modern English
Insofar as the mind reasons, it desires nothing other than to understand, and judges nothing useful to itself except what conduces to understanding (E4P26). But the mind (E2P41) has certainty about things only insofar as it has adequate ideas — or, what is the same thing (E2P40S2), insofar as it reasons. We therefore know with certainty that only what truly conduces to understanding is good, and that only what can prevent us from understanding is bad. Q.E.D.