Ethics, Part V, Proposition 1
Even as thoughts and the ideas of things are arranged and associated in the mind, so are the modifications of body or the images of things precisely in the same way arranged and associated in the body.
Proof
The order and connection of ideas is the same (II. vii.) as the order and connection of things, and vice versa the order and connection of things is the same (II. vi. Coroll. and vii.) as the order and connection of ideas. Wherefore, even as the order and connection of ideas in the mind takes place according to the order and association of modifications of the body (II. xviii.), so vice versa (III. ii.) the order and connection of modifications of the body takes place in accordance with the manner, in which thoughts and the ideas of things are arranged and associated in the mind. Q.E.D.
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.
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Digital text: Project Gutenberg eBook #3800