If God exists, then the PSR for contingent propositions is true. Why? Because God's activity ultimately explains everything. This is going to be clearest on views on which God's activity alone explains everything, and that is going to be most plausible on Calvinisttype views but also seems correct on any theological account that has a strong view of divine concurrence with creaturely activity. Moreover, the inference from God's being the creator and sustainer of everything to the claim that divine activity provides the explanation of everything contingent, or at least of everything contingent that is otherwise unexplained (this variant might be needed to handle creaturely free will), is a highly plausible one. Thus, someone who has good reason to accept theism has good reason to accept the PSR.
Now one might think that this is a useless justification for the PSR if we are going to use the PSR to run a cosmological argument, since then the cosmological argument will be viciously circular: the conclusion will justify the PSR, whereas the PSR is a premise in the argument.
However, recently, Daniel Johnson (forthcoming) has come up with a very clever account showing that a cosmological argument based on the PSR could still be epistemically useful, even if the PSR is accepted because of the existence of God (he also applies the view to the possibility premise in the ontological argument). Suppose that, as Calvin and Plantinga think, there is a sensus divinitatis (SD), which, noninferentially, induces in people the knowledge that God exists - at least absent defeaters - and tells them something about God's power and nature.
Suppose that Smith knows by means of the SD that God exists. From this, Smith concludes that the PSR is true - this conclusion may not involve explicit reasoning, and it is one well within the abilities of the average believer. Smith then knows that the PSR is true. Next, Smith sinfully and without epistemic justification suppresses the SD in himself and suppresses the belief that God exists. If Calvin's reading of Romans 1 is correct, this kind of thing does indeed happen, and it is why nontheists are responsible for their lack of theism. However, the story continues, the suppression is not complete. For instance, Smith's worshipful attitude toward God turns into an idolatrous attitude toward some part of creation. It may very well happen, likewise, that Smith does not in fact suppress his belief in the PSR, although he forgets that he had accepted the PSR in the first place because he believed in God. Indeed, this situation may be common for all we know.
Johnson then claims that Smith remains justified in believing the PSR, just as we remain justified in believing the Pythagorean theorem even after we have forgotten from whom we have learned it and how it is proved. Thus, Smith continues to know the PSR. The cos-mological argument then lets Smith argue to the existence of God from the PSR, and so Smith then can justifiably conclude that God exists. Of course, unless Smith has some additional source of justification for believing the PSR, Smith has no more justification for believing that God exists than he did when he learned about God from his SD. So the argument has not provided additional evidence, but it has restored the knowledge that he had lost.
We have a circularity, then, but not one that vitiates the epistemic usefulness of the argument. Irrational suppression of a part of one's network of belief can be incomplete, leaving in place sufficient beliefs allowing the reconstruction of the suppressed belief. A similar thing happens not uncommonly with memory. Suppose I am trying to commit to memory my hotel room number of 314. I note to myself that my hotel room number is the first three digits of n. Later I will forget the hotel room number, but remember that it is identical to the first three digits of n, from which I will be able to conclude that the number is 314. My reason for believing the number to be identical to the first three digits of n was that the number is 314, but then, after I lose, through a nonrational process of forgetting, the knowledge that the number was 314, I will be able to recover the knowledge by using a logical consequence of that very piece of knowledge. In doing so, I do not end up with any more justification for my belief about the room number than I had started out with, but still if I started out with knowledge, I end up with knowledge again.
This means that an argument where a premise was justified in terms of the conclusion can be useful in counteracting the effects of nonrational or irrational loss of knowledge. This means that the cosmological argument could be useful even if none of the arguments for the PSR given earlier worked, and even if the PSR were not self-evident, for some people may know that the PSR is true because they once knew that God exists. They lost the knowledge that God exists but retained its shadow, the entailed belief that the PSR is true.
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