Explanatory rationalism is the view that there is a satisfactory answer to every why-question. Equivalently, it is the view that there are no brute facts, where a brute fact is a fact that neither has, nor can have, an explanation. Are there some truths which simply must be accepted without explanation? Consider the conjunction of all truths. Could this conjunctive truth have an explanation? Jonathan Bennett thinks not:
Let P be the great proposition stating the whole contingent truth about the actual world, down to its finest detail, in respect of all times. Then the question 'Why is it the case that P?' cannot be answered in a satisfying way. Any purported answer must have the form 'P is the case because Q is the case'; but if Q is only contingently the case then it is a conjunct in P, and the offered explanation doesn't explain; and if Q is necessarily the case then the explanation, if it is cogent, implies that P is necessary also. But if P is necessary then the universe had to be exactly as it is, down to the tiniest detail — i.e., this is the only possible world. (Jonathan Bennett, A Study of Spinoza's Ethics, Hackett 1984, p. 115)
Bennett's point is that explanatory rationalism entails the collapse of modal distinctions.
The world-proposition P is a conjunction of truths some of which are contingent. So P is contingent. Now if explanatory rationalism is true, then P has an explanation in terms of a Q distinct from P. Q is either necessary or contingent. If Q is necessary, and a proposition is explained by citing a distinct proposition that entails it, and Q explains P, then P is necessary, contrary to what we have already established. On the other hand, if Q is contingent, then Q is a conjunct of P, and again no successful explanation has been arrived at. Therefore, either explanatory rationalism is false, or it is true only on pain of a collapse of modal distinctions.
That is a cute little argument, one that impresses van Inwagen as well who gives his own version of it, but I must report that I do not find it compelling. Why is P true? We can say that P is true because each conjunct of P is true. We are not forced to say that P is true because of a proposition Q which is a conjunct of P.
I am not saying that P is true because P is true; I am saying that P is true because each conjunct of P is true, and that this adequately and noncircularly explains why P is true. Some wholes are adequately and noncircularly explained when their parts are explained. Suppose three bums are hanging around the corner of Fifth and Vermouth. Why is this theesome there? The explanations of why each is there add up (automatically) to an explanation of why the three of them are there. Someone who understands why A is there, why B is there, and why C is there, does not need to understand some further fact in order to understand why the three of them are there. Similarly, it suffices to explain the truth of a conjunction to adduce the truth of its conjuncts. The conjunction is true because each conjunct is true. There is no need for an explanation of why a conjunctive proposition is true which is above and beyond the explanations of why its conjuncts are true.
Suppose the three bums engage in a ménage à trois. To explain the ménage à trois it is not sufficient to explain why each person is present; one must also explain their 'congress': not every trio is a ménage à trois. A conjunction, however, exists automatically if its conjuncts exist.
Bennett falsely assumes that "Any purported answer must have the form 'P is the case because Q is the case'. . ." This ignores my suggestion that P is the case because each of its conjuncts is the case. So P does have an explanation; it is just that the explanation is not in terms of a proposition Q which is a conjunct of P.
I conclude that Professor Bennett has given us an insufficient reason to reject the Principle of Sufficient Reason.
I apply a similar critique to Peter van Inwagen's version of the argument in my "On An Insufficient Argument Against Sufficient Reason," Ratio, vol. 10, no. 1 (April 1997), pp. 76-81.