Ethics, Part II, Proposition 23

The idea or knowledge of the mind (II. xx.) follows in God in the same manner, and is referred to God in the same manner, as the idea or knowledge of the body. But since (II. xix.) the human mind does not know the human body itself, that is (II. xi. Coroll.), since the knowledge of the human body is not referred to God, in so far as he constitutes the nature of the human mind; therefore, neither is the knowledge of the mind referred to God, in so far as he constitutes the essence of the human mind; therefore (by the same Coroll. II. xi.), the human mind thus far has no knowledge of itself. Further the ideas of the modifications, whereby the body is affected, involve the nature of the human body itself (II. xvi.), that is (II. xiii.), they agree with the nature of the mind; wherefore the knowledge of these ideas necessarily involves knowledge of the mind; but (by the last Prop.) the knowledge of these ideas is in the human mind itself; wherefore the human mind thus far only has knowledge of itself. Q.E.D.