The Principles of Sufficient Reason

By | November 20, 2006

The great polymath Leibniz is famous for his advocacy of the “Principle of Sufficient Reason” (PSR). He states it in various ways throughout his writings, but it basically runs like this:

(PSR-1): Necessarily, for everything that exists, there is a sufficient reason why it exists and why it exists as it exists rather than otherwise.

This is a particularly strong form of PSR. Is there any reason (sufficient or not) for thinking that this is true or that it is false? Well, on behalf of its truth one might urge that PSR-1 is a methodological presupposition of rational inquiry. After all, if we didn’t presuppose quite generally that explanations are to be had for why things are and are as they are, then it is doubtful that inquiry on a great many topics of potentially significant importance would never happen. But even if that is so, it is not enough to justify PSR-1. The reason is that methodological presuppositions are often defeasible. In other words, they have an initial presumption in their favor, but it is a presumption that can be overcome. Suppose, for example, that some surprising event E occurs and, after investigating it for years and years with the utmost ingenuity, no promising leads come to light. At some point shouldn’t we start entertaining the possibility that E is simply a brute chance occurrence, one without a sufficient reason why it occurred when, where, and how it did? It would seem so. Arguably, there is now good reason for thinking that such events regularly happen at the quantum level. The exercise of free will may also be another counterexample. Why did I choose the way I did rather than some other way? Generally I have reasons for my choices, but must those reasons be sufficient in order for me to choose? If so, then I hope I never find myself in the position of Buridan’s ass.

A weaker version of PSR runs as follows:

(PSR-2): Necessarily, for everything that exists, there is a sufficient reason why it exists.

This version of PSR leaves open the possibility that how something exists may, in some respects, be a brute, unexplained fact, but nevertheless the fact that it exists still needs a sufficient reason. Thus, if I toss a deck of cards up into the air so that they scatter about, due to quantum indeterminacy, chaotic perturbations in the air, and the like, there may be no sufficient reason (at the moment I toss the deck) determining precisely which cards will land face up, which face down, and precisely where and when each card will land. But there is, presumably, a sufficient reason for the existence of the deck of cards. It didn’t just materialize out of thin air.

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