A cosmological argument for the existence of God purports to infer the existence of a necessary being from the existence of one or more contingent existents. The justification for the inference is that one cannot consistently hold that x is contingent and that y does not exist, where y is a necessary being. The justification for this claim is generally thought to reside in some version of the Principle of Sufficient Reason (hereafter PSR). A typical expression of this principle is as follows: whatever exists must have an explanation of its existence either in the necessity of its own nature or in the causal efficacy of some other being.1 So, it is argued, since contingent existents do not have the explanation of their existence in themselves (else they would exist necessarily), they must have the explanation of their existence in the causal efficacy of some other being. Further, it is argued that this other being whose causal efficacy provides the explanation of the existence of a contingent existent cannot itself be contingent. For if the putative explanation of the existence of a contingent existent x is a contingent existent y, the state of affairs or fact that y-causes-x is itself contingent.2 So, y is not an explanation of the existence of x but at best a part of that which, according to the PSR, needs an explanation. By this reasoning it is thought that no contingent y can explain the existence of x. Thus if x requires an explanation (according to the PSR), the only y that can provide this is a necessary being. Clearly, some version of the PSR is required both to justify the claim that contingent beings need explanations for their existence and to support the claim that only one sort of explanation is possible.3 Yet the PSR is thought to be vulnerable to