Leibniz's Principle of Sufficient Reason (PSR) states that: for every fact F, there must be a sufficient reason why F is the case (https://plato.stanford.edu/entries/sufficient-reason/#WhatSuffReas). This applies to all contingent facts.

For Leibniz, a proposition is contingently true iff it is true in this world and false in another world. And a proposition is necessarily true iff it is true in every possible world. (https://plato.stanford.edu/entries/leibniz-modal/#NatMod)

However, if we define a fact to be necessary iff it cannot be otherwise, then doesn't the PSR lead us to conclude that all contingent facts are necessary? If for every contingent fact F, there is a sufficient reason F', which may itself be contingent, but then would be in turn grounded by another sufficient reason F'' ad infinitum, then wouldn't we be pressed to say that in our world (at least) the contingent fact is necessary in that it couldn't be otherwise?


  • 1
    No, the Principle of Sufficient Reason applies only to the actual world. Leibniz even implies that the reason this world was actualized is, ultimately, that it is the "best of possible worlds". As other possible worlds do not have that distinction, events in them are not subject to PSR.
    – Conifold
    Nov 15, 2022 at 2:59

2 Answers 2


Does the Principle of Sufficient Reason imply everything is necessary?

Yes, and for precisely the argument that you offered. Propositions that are contingent are true in this world. From that definition the rest follows easily: there is a Fact F which is the Sufficient Reason for the true proposition, which in turn is supported by its own series of Facts, and so forth.

Note that this argument works only in this world, where the contingent proposition is true. I cannot offer an answer for a different world, where the same proposition is false.

This question was interesting. This must have occurred to Leibniz, and I wonder what he did with it.

  • I wonder how many people are intuitively using the PSR to think and coming to absurd conclusions because of it
    – user63148
    Nov 15, 2022 at 2:46

It's called "modal fatalism, the view that there are no contingent truths": and yes, at least some philosophers say the PSR (on which the literature is vast) means there are no contingent facts. Peter van Inwagen argues that a variant on "Spinoza's PSR-based argument for modal fatalism" leads to an absurdity, specifically modal fatalism.

Peter van Inwagen (1983, pp. 202–204) has formulated an influential and elegant reductio ad absurdum of the PSR. Let p be the conjunction of all contingent truths. If p has an explanation, say, q, then q will itself be a contingent truth, and hence a conjunct of p. But then q will end up explaining itself, and that would be absurd

Besides which

There are few contemporary defenders of the PSR.

Not least, because then it seems nothing is contingent, everything is necessary and could not be otherwise.

  • I only answered cos I was googling the PSR before and you seem unwilling to :)
    – user63148
    Nov 15, 2022 at 2:37
  • why on earth was this downvoted??
    – user63148
    Nov 15, 2022 at 4:34
  • but why? it makes no sense
    – user63148
    Nov 16, 2022 at 2:36

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