P23: The Mind does not know itself, except insofar as it perceives the ideas of the affections of the Body.

Dem.: The idea, or knowledge, of the Mind (by P20) follows in God/Nature in the same way, and is related to God/Nature in the same way as the idea, or knowledge, of the body. But since (by P19) the human Mind does not know the human Body itself, i.e. (by P11 C), since the knowledge of the human Body is not related to God/Nature insofar as he constitutes the nature of the human Mind, the knowledge of the Mind is also not related to God/Nature insofar as he constitutes the essence of the human Mind. And so (again by P11 C) to that extent the human Mind does not know itself.

Next, the ideas of the affections by which the Body is affected involve the nature of the human Body itself (by P16), i.e. (by P13), agree with the nature of the Mind. So knowledge of these ideas will necessarily involve knowledge of the Mind. But (by P22) knowledge of these ideas is in the human Mind itself. Therefore, the human Mind, to that extent only, knows itself, q.e.d.