By the expression ‘closed causal chain’ I mean a process (a causally connected sequence of events) that loops back in such a way that each event is indirectly a cause of itself. In this chapter I will explore whether such processes might actually occur and whether certain scientific theories that suggest them might be correct. Since these are questions about what might exist, they are highly ambiguous. For there are various notions of possibility – including, for example, logical consistency, compatibility with the laws of physics, feasibility given current technology, and consonance with known facts – and the status of closed causal chains may be examined with respect to any of these notions. However, since our interest is not confined to just one kind of possibility, there is no need to choose between them, and so we can put our problem as follows. In what senses (if any) are closed causal chains possible, and in what senses (if any) are they impossible?

Another way in which the issue before us is somewhat unclear is with respect to what is to qualify as a closed causal chain – whether the often cited, potential examples really involve causation, strictly speaking. For one might well wonder if any relation that an event can bear to itself could really count as the causal relation, the relation of making something happen. I shall try to mitigate (or, rather, avoid) this problem by raising the questions of possibility, not primarily with respect to the abstract notion, ‘closed causal chain’, but rather with respect to three specific theories, and by construing the expression, ‘closed causal chain’, by reference to the processes they postulate.