In purporting to explain the occurrence of some event or process we cite the causal factors which, we assert, brought it about or keeps it in being. The explanation is a true one if those factors did indeed bring it about or keep it in being. In discussing explanation I shall henceforward (unless I state otherwise) concern myself only with true explanations. I believe that there are two distinct kinds of way in which causal factors operate in the world, two distinct kinds of causality, and so two distinct kinds of explanation. For historical reasons, I shall call these kinds of causality and explanations ‘scientific’ and ‘personal’; but I do not imply that there is anything unscientific in a wide sense in invoking personal explanation.

The deductive-nomological (D-N) model of scientific explanation2 teaches that the causal factors involved in scientific explanation are of two kinds—states of affairs (or events, as I shall sometimes call them) occurring at instants of time and describable independently of their consequences, and laws which state that all states of affairs of one sort are followed by states of affairs of another sort. An event (E) is explained if a description of its occurrence is entailed conjointly by a description of the occurrence of a prior state (the initial conditions, or cause) (B) and a statement of a law of nature (L). The operation of a law (L∧) is explained if it is deducible from a statement of the operation of a higher level law, (L2) perhaps together with some description of boundary conditions (i.e. a state of affairs) (B).