A common objection to the PSR is that indeterministic quantum effects lack sufficient reasons. However, the PSR that I am defending concerns explanation, which is the giving of reasons sufficient to explain the explanandum, not the giving of reasons logically sufficient for entailing the explanandum.
Quantum mechanical events do, however, have explanations. The experimental setup in which they happen has the property of giving rise to emissions with certain probabilities (John Haldane makes this suggestion in Smart & Haldane 2003, p. 126). Granted, on inde-terministic accounts of quantum mechanics, this explanation does not entail the outcome, and will only be a statistical explanation, perhaps involving small probabilities, but as the syphilis/paresis case in the previous section showed, that should not be a problem.
Still, one might be somewhat dissatisfied with quantum mechanical explanations. One might say that, yes, they are explanatory, but they lack some feature that better explanations have. But that is fine for the defense of the PSR. The PSR does not say that for every contingent proposition there is the best possible kind of explanation, but just that there is an explanation, "an 'explanation enough' " in Haldane's words (Smart & Haldane 2003, p. 126). And the kind of explanation that our PSR provides will be, as we shall see, enough to yield a cosmological argument - and that is the point here.
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