⁂ Concepts · Logic · No. 67
Sufficient reason

Radioactive decay: a precise half-life, and no answer about this atom.

Sufficient reason

That everything has an explanation - and the question of whether anything is entitled to assume so.

Theme
Logic, language and interpretation
Tradition
Western
Term fixed
1714

In plain terms

Nothing is just so. Whatever is the case, there is a reason it is that way rather than another - that is the principle, and almost every enquiry proceeds as though it were true.

It has to be, or investigation would have no stopping condition and no starting one. An accident investigator who accepted that the aircraft simply came down would not be doing the job. A doctor who accepted that the patient is simply unwell has stopped before the point of the visit.

But as a claim about the universe rather than a working rule it is much bolder. It says there are no brute facts anywhere - nothing that is the case for no reason at all. And there is now a serious candidate for exactly that, which is why the principle is not the safe background assumption it was for three hundred years.

As a philosopher would put it

Leibniz makes it one of two principles governing all reasoning: nothing is without a reason why it should be so rather than otherwise. Applied cosmologically it produces the argument from contingency, since the totality of contingent things would itself require an explanation.

Spinoza's use is stronger and its consequences are severe. If everything has a reason, and reasons necessitate, then nothing could have been otherwise: contingency disappears and the world follows from the divine nature with the necessity of a theorem. Van Inwagen's modern version of that argument is why many philosophers now treat unrestricted PSR as unacceptable rather than merely unproven.

Hume attacks the principle directly - we can conceive of something beginning to exist without a cause, so no contradiction is involved - and Kant relocates it, making it a regulative principle of the understanding that governs how we must investigate rather than a truth about things in themselves. Della Rocca's contemporary defence runs the other way, arguing that any restriction of the principle is arbitrary and that its critics rely on it selectively.

Where you meet it

A given atom of carbon-14 will decay. The half-life is 5,730 years, which is known to considerable precision, and the statistical behaviour of a large sample is entirely predictable.

Why did that particular atom decay at that particular moment rather than a thousand years later? On the standard interpretation of quantum mechanics there is no answer, and not because the answer is unknown. There is no property of the atom, no prior state and no hidden variable that determines it. Bell's theorem and the experiments following it constrain what a local hidden-variable account could look like, and the constraints are severe.

That is a candidate brute fact in the most ordinary object there is. Interpretations differ - Bohmian mechanics restores determinism at the cost of nonlocality, and many-worlds relocates the question rather than answering it - so the case is not closed. But a principle that Leibniz treated as one of the two foundations of reasoning now has to argue for itself against a laboratory result.

What is actually disputed

Whether the principle can be held in a restricted form.

Unrestricted PSR appears to entail Spinoza's necessitarianism, which almost nobody accepts. So the natural move is to restrict it - everything has a reason except the fundamental facts, or except the free choices, or except the quantum events. The objection, pressed hardest by Della Rocca, is that every such restriction is arbitrary: if brute facts are permissible anywhere, the demand for explanation loses its authority everywhere, and the principle was doing its work precisely by being unrestricted.

Defenders reply that the same is true of any principle with exceptions and that this proves too much. What is not disputed is the methodological version: treating the world as though everything has an explanation is how enquiry proceeds, and the fact that it has repeatedly turned up explanations where none was expected is the best evidence the principle has.

Commonly confused with

  • Causation c. 330 BC The principle demands a reason, which may be a cause, a ground, or a purpose. Not every explanation is causal.
  • Determinism 1677 Determinism says the past fixes the future given the laws. Sufficient reason is broader and includes the laws themselves.
  • the demand for evidence Asking for a reason to believe something is epistemic. This is a claim about the world - that there is a reason things are as they are, whether anyone knows it.
  • refusing to accept coincidence Coincidences have reasons; they simply have separate ones. The principle is not the claim that events are connected.

Defined against

  • Chance If some events are genuinely undetermined, they are the counterexample the principle cannot absorb

Philosophers

  • Gottfried Wilhelm Leibniz Names the principle and makes it foundational
  • Baruch Spinoza Uses it to conclude that nothing could have been otherwise
  • David Hume Denies that a beginning without a cause involves any contradiction
  • Immanuel Kant Relocates it from a truth about things to a rule for enquiry
  • Avicenna The contingency argument that depends on it

Works

Schools

Related concepts

Causation c. 330 BC · Explanation 1948 · Contingency c. 1020 · Necessity c. 330 BC · Chance c. 330 BC

Leibniz names it and treats it as one of the two great principles of reasoning alongside non-contradiction; Spinoza had already used it without the name to argue that everything follows necessarily.