Ethics, Part III, Proposition 54
The mind endeavours to conceive only such things as assert its power of activity.
Proof
The endeavour or power of the mind is the actual essence thereof (III. vii.); but the essence of the mind obviously only affirms that which the mind is and can do; not that which it neither is nor can do; therefore the mind endeavours to conceive only such things as assert or affirm its power of activity. 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