P16: The idea of any mode in which the human Body is affected by external bodies must involve the nature of the human Body and at the same time the nature of the external body.

Dem.: For all the modes in which a body is affected follow from the nature of the affected body, and at the same time from the nature of the affecting body (by A1″ ).

So the idea of them (by IA4) will necessarily involve the nature of each body. And so the idea of each mode in which the human Body is affected by an external body involves the nature of the human Body and of the external body, q.e.d.

Cor. 1: From this it follows, first, that the human Mind perceives the nature of a great many bodies together with the nature of its own body.

Cor. 2: It follows, second, that the ideas which we have of external bodies indicate the condition of our own body more than the nature of the external bodies. I have explained this by many examples in the Part 1 Appendix - Modernized.