Ethics, Part II, Proposition 16
The idea of every mode, in which the human body is affected by external bodies, must involve the nature of the human body, and also the nature of the external body.
Proof
All the modes, in which any given body is affected, follow from the nature of the body affected, and also from the nature of the affecting body (by Ax. i., after the Coroll. of Lemma iii.), wherefore their idea also necessarily (by I. Ax. iv.) involves the nature of both bodies; therefore, the idea of every mode, in which the human body is affected by external bodies, involves the nature of the human body and of the external body. Q.E.D.
Corollary 1
Hence it follows, first, that the human mind perceives the nature of a variety of bodies, together with the nature of its own.
Corollary 2
It follows, secondly, that the ideas, which we have of external bodies, indicate rather the constitution of our own body than the nature of external bodies. I have amply illustrated this in the Appendix to Part I.
Benedict de Spinoza, The Ethics (Ethica Ordine Geometrico Demonstrata), translated from the Latin by R. H. M. Elwes.
First published in The Chief Works of Benedict de Spinoza, vol. II (London: George Bell and Sons, 1884).
This translation is out of copyright in the United States, where it was published in 1884, and wherever the copyright term is the author's life plus 100 years or less; its translator died in 1892.
Unit divisions, headings and the placement of notes are this site's.
Digital text: Project Gutenberg eBook #3800