Ethics, Part IV, Proposition 27
We know nothing to be certainly good or evil, save such things as really conduce to understanding, or such as are able to hinder us from understanding.
Proof
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.
Benedict de Spinoza, The Ethics (Ethica Ordine Geometrico Demonstrata), translated from the Latin by R. H. M. Elwes.
First published in The Chief Works of Benedict de Spinoza, vol. II (London: George Bell and Sons, 1884).
This translation is out of copyright in the United States, where it was published in 1884, and wherever the copyright term is the author's life plus 100 years or less; its translator died in 1892.
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