If c causes e and c is a supervenience base of c, then c causes e.
These principles are perhaps more clearly expressed by means of a visual aid: see Figures 1 and 2. (We represent causation with a single arrow and supervenience with a double arrow pointing from the supervenience base to the supervening event.) SC I and SC II respectively tell us that when the top single arrow in each of Figure 1 and 2 occurs, so does the bottom single arrow.
Figure 1: Supervenience-Causation I
e Figure 2: Supervenience-Causation II c
One further remark about what SC I and SC II mean is in order. The principles speak of supervenience bases of events, though the usual notion of supervenience is defined for properties. What we mean by “e is a supervenience base of e”, when e and e are events, is that e and e are simultaneous events involving the same object(s) and e is an instance of some property which is a supervenience base of a property that is instantiated by e. More precisely:
The event in which x instantiates P at t is a supervenience base of the event in which y instantiates Q at t’ if and only if x = y, t = t’, and P is a supervenience base of Q. We believe that Kim introduces SC I at pp. 39-41, where he expresses the principle by saying that an instance of a property can only cause an instance of a supervenient property “by causing its supervenience base” (p. 40).9 The grounds for this claim have to do with the alleged fact that there are “two seemingly exclusionary answers” to the question “What is responsible for, and explains, the fact that [e] occurs on this occasion?” These two answers are:
(a) Because c caused e.
(b) Because e, a supervenience base of e, is instantiated on this occasion.
Kim says that a “tension [is] created by (a) and (b)” and that a “simple and natural way of dissipating” this tension is to conclude that c is a cause of e. It is not clear to us why these answers are “seemingly exclusionary”, or even that they are seemingly exclusionary (they don’t strike us as seemingly exclusionary),10 but we will leave it to others to reconstruct and evaluate the reasoning that leads Kim to accept SC I, because an adequate reconstruction of it would require a separate paper11 and we are not, at any rate, singling this assumption out for criticism.
The reasoning supporting SC II is more transparent. SC II is introduced, obliquely, at p. 41, where Kim states that “There are strong reasons for thinking that [in the kind of situation represented in Figure 1] [c'] is a cause of [e']”. These reasons are:
[c'] is (at least) nomologically sufficient for [c], and the occurrence of [c] on this occasion depends on, and is determined by, the presence of [c'] on this occasion. Since ex hypothesi [c] is a cause of [e], [c'] would appear to amply qualify as a cause of [e] as well.
The idea appears to be this: c supervenes on c'; therefore c' is at least nomologically sufficient for c (this depends on reading the innermost modal operator in the definition of supervenience as “at least” nomological necessity). By assumption, c is a cause of e. Any event that is nomologically sufficient for the occurrence of a cause of an event is also a cause of it; therefore c' is a cause of e'.
The final “general metaphysical constraint” required by the argument, according to our reconstruction, is: