Tuesday, November 2, 2021

Leibniz on the PSR

According to the Principle of Sufficient Reason (PSR), every contingent fact has a sufficient reason. What does “sufficient” mean here? A natural thought is that it means that the reason is logically sufficient for the fact. My own work on the PSR rejects this natural thought. I say that a sufficient reason is one that suffices to explain the fact, not necessarily one that suffices for the fact to be true. I occasionally worry that this is too wimpy a take on the PSR, indeed a kind of bait-and-switch.

When I worry about this, it helps me to come back to Leibniz, whom nobody considers a wimp with respect to the PSR. How does Leibniz understand “sufficient”?

In the Principles of Nature and Grace, Leibniz talks of the

grand principe … qui porte que rien ne se fait sans raison suffisante; c’est-à-dire que rien n’arrive sans qu’il soit possible à celui qui connaîtrait assez les choses de rendre une raison qui suffise pour déterminer pourquoi il en est ainsi, et non pas autrement [great principle … which holds that nothing happens without sufficient reason; that is to say, that nothing happens without its being possible for someone who knows enough about how things are to give a reason that suffices to determine why it is so and not otherwise]. (my italics)

Leibniz does not say that the reason is sufficient to determine the fact. Rather, Leibniz carefully says that the reason is sufficient to determine why the fact occurred. You can read off the explanation, the answer to the why question, from the reason, but no claim is made that you can read the explained fact off from it.

Indeed, the only necessitation in the paragraph is hypothetical:

De plus, supposé que des choses doivent exister, il faut qu’on puisse rendre raison pourquoi elles doivent exister ainsi, et non autrement. [Further, supposing things must exist, it has to be possible to give a reason why they must exist so and not otherwise.] (my italics)

I wish Leibniz had this weaker picture of sufficient reason consistently. Sadly for me, he does not. In a 1716 letter to Bourguet he writes:

Mr. Clark … n’a pas bien compris la force de cette maxime, que rien n’arrive sans une raison suffisante pour le determiner. [Mr. Clark … has not understood well the force of the maxim that nothing happens without a reason sufficing to determine it.]

Oh well.

I comfort myself, however, that my philosophical hero does, after all, have two kinds of necessity, and hopefully the determination in the PSR involves the weaker one.

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