What is the principle of sufficient reason?

Baruch Spinoza, answered from the texts and cited to the page.

Baruch Spinoza answers

The principle runs through everything I demonstrate in the Ethics, though I never give it a single canonical name the way later writers would. Its content is this: of everything whatsoever, a cause or reason must be assigned, either for its existence or for its non-existence.¹ There is no brute fact, no bare "it simply is so" that stops inquiry.

Every existing thing has a reason why it exists; every non-existing thing has a reason why it does not. The illustration I use is deliberate. A triangle either necessarily exists or it is impossible that it should exist — and which of these holds follows not from the triangle's own nature but from the order of universal nature in extension.²

The triangle's essence does not settle the question of its existence; something external to it must account for why it is or is not actual. The square circle, by contrast, carries its own reason for non-existence inside its nature, since the concept involves a contradiction.³ And substance carries its own reason for existence inside its nature, since its nature involves existence — which is what Proposition VII establishes and what the demonstration of God's necessary existence in Proposition XI turns on.⁴

This asymmetry is the engine of the whole of Part I. Where a thing's essence does not settle its existence, we must look outside — to the order of causes in which it stands. Where a thing's essence does involve existence, no external cause could annul it, for any such cause would have to be drawn either from the thing's own nature, which would involve a contradiction, or from a substance of another nature, which by Proposition II could have nothing in common with it and therefore no power over it.⁵

The divine existence is thus not a special exemption from the principle but its most rigorous application: precisely because no reason for God's non-existence can be found anywhere, his existence is necessary.⁶ What this means for finite things is equally clear. The essence of things produced by God does not involve existence — their nature, considered in itself, settles neither that they exist nor how long they persist.⁷

God is therefore not only the cause of their coming into existence but of their continuing in existence; remove the sustaining cause and there is nothing in the finite essence itself that would maintain the thing in being.⁸ The principle of sufficient reason is thus what binds the whole modal world to substance: every finite mode requires a reason, that reason is always traceable through the causal order back to God, and God alone requires no reason outside himself because his essence just is the sufficient reason for his own existence.

Sources

1. Of everything whatsoever a cause or reason must be assigned, either for its existence, or for its non-existence.

   Ethics

2. But the reason for the existence of a triangle or a circle does not follow from the nature of those figures, but from the order of universal nature in extension. From the latter it must follow, either that a triangle necessarily exists, or that it is impossible that it should exist.

   Ethics

3. the reason for the non-existence of a square circle is indicated in its nature, namely, because it would involve a contradiction.

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4. the existence of substance follows also solely from its nature, inasmuch as its nature involves existence.

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5. a reason or cause which would annul the divine existence cannot be drawn from anything external to the divine nature, such cause must perforce, if God does not exist, be drawn from God's own nature, which would involve a contradiction.

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6. neither in the nature of God, nor externally to his nature, can a cause or reason be assigned which would annul his existence. Therefore, God necessarily exists. Q.E.D.

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7. This proposition is evident from Def. i. For that of which the nature (considered in itself) involves existence is self-caused, and exists by the sole necessity of its own nature.

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8. God is not only the cause of things coming into existence, but also of their continuing in existence...whenever we contemplate their essence, we see that it involves neither existence nor duration; consequently, it cannot be the cause of either the one or the other.

   Ethics