In his celebrated discussion of causation Hume identified four prima facie constituents in the relation of causation. As everyone knows, they are constant conjunction, contiguity in space and time, temporal priority, and necessary connection. As ordinarily understood, the causal relation is a binary relation relating causes to their effects, and so presumably are the four relations Hume discerns in it. But what do these four relations tell us about the nature of the entities they relate?

Constant conjunction is a relation between generic events, that is, kinds or types of events; constant conjunction makes no clear or nontrivial sense when directly applied to spatiotemporally bounded individual events. On the other hand, it is clear that the relation of temporal priority calls for individual, rather than generic, events as its relata; there appears to be no useful way of construing ‘earlier than’ as a relation between kinds of classes of events in a causal context.

What of the condition of contiguity? This condition has two parts, temporal and spatial. Temporal contiguity makes sense when applied to events; two events are contiguous in time if they temporally overlap. But spatial contiguity makes best sense when applied not to events but to objects, especially material bodies; intuitively at least, we surely understand what it is for two bodies to be in contact or to overlap. For events, however, the very notion of spatial location often becomes fuzzy and indeterminate.