P21: This idea of the Mind is united to the Mind in the same way as the Mind is united to the Body.

Dem.: We have shown that the Mind is united to the Body from the fact that the Body is the object of the Mind (see P12 and P13); and so by the same reasoning the idea of the Mind must be united with its own object, i.e., with the Mind itself, in the same way as the Mind is united with the Body, q.e.d.

Schol.: This proposition is understood far more clearly from what is said in P7 S; for there we have shown that the idea of the Body and the Body, i.e. (by P13), the Mind and the Body, are one and the same Individual, which is conceived now under the attribute of Thought, now under the attribute of Extension. So the idea of the Mind and the Mind itself are one and the same thing, which is conceived under one and the same attribute, viz. Thought. The idea of the Mind, I say, and the Mind itself follow in God/Nature from the same power of thinking and by the same necessity. For the idea of the Mind, i.e., the idea of the idea, is nothing but the form of the idea insofar as this is considered as a mode of thinking without relation to the object. For as soon as someone knows something, he thereby knows that he knows it, and at the same time knows that he knows that he knows, and so on, to infinity. But more on these matters later.