Ethics, Part III, Proposition 5
Things are naturally contrary, that is, cannot exist in the same object, in so far as one is capable of destroying the other.
Proof
If they could agree together or co--exist in the same object, there would then be in the said object something which could destroy it; but this, by the foregoing proposition, is absurd, therefore things, &c. 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