Ethics, Part V, Proposition 21
The mind can only imagine anything, or remember what is past, while the body endures.
Proof
The mind does not express the actual existence of its body, nor does it imagine the modifications of the body as actual, except while the body endures (II. viii. Coroll.); and, consequently (II. xxvi.), it does not imagine any body as actually existing, except while its own body endures. Thus it cannot imagine anything (for definition of Imagination, see II. xvii. note), or remember things past, except while the body endures (see definition of Memory, II. xviii. note). 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