P8: The knowledge of good and evil is nothing but an affect of Joy or Sadness, insofar as we are conscious of it.

Dem.: We call good, or evil, what is useful to, or harmful to, preserving our being (by D1 and D2), i.e. (by IIIP7), what increases or diminishes, aids or restrains, our power of acting. Therefore (by the Definitions of Joy and Sadness in IIIP11 S), insofar as we perceive that a thing affects us with Joy or Sadness, we call it good or evil. And so knowledge of good and evil is nothing but an idea of Joy or Sadness which follows necessarily from the affect of Joy or Sadness itself (by IIP22).

But this idea is united to the affect in the same way as the Mind is united to the Body (by IIP21), i.e. (as I have shown in IIP21 S), this idea is not really distinguished from the affect itself, or (by the general Definition of the Affects) from the idea of the Body’s affection; it is only conceptually distinguished from it. Therefore, this knowledge of good and evil is nothing but the affect itself, insofar as we are conscious of it, q.e.d.