P19: From the laws of his own nature, everyone necessarily wants, or is repelled by, what he judges to be good or evil.

Dem.: Knowledge of good and evil (by P8) is itself an affect of Joy or Sadness, insofar as we are conscious of it. And therefore (by IIIP28), everyone necessarily wants what he judges to be good, and conversely, is repelled by what he judges to be evil. But this appetite is nothing but the very essence, or nature, of man (by the Definition of Appetite; see IIIP9 S and Def. Aff. I). Therefore, everyone, from the laws of his own nature, necessarily, wants or is repelled by, etc., q.e.d.