We move now to the second challenge that Kim raised against emergence. It is widely acknowledged that any theory that allows strong emergence has to respond to this problem, which concerns the alleged causal closure of the physical. Perhaps the causal-transformative account we have offered sounds plausible, as it stands; but, unless we can give a credible response to this issue, then it may yet be rejected.
The problem is well known, so we will give only a very short summary, cutting technical corners, including some that arise from our preferred account of causation. According to a standard way of thinking, everything at the base level is causally closed. Thus, a base-level state or event, B*, must be caused completely by another base-level state or states, B (whatever we take the relata of causal relations to be). Emergent phenomena seemingly threaten this view. Suppose E is emergently dependent on B. If E is supposed to be able, through downward action, to cause B*, then the base level cannot be causally closed. Just as bad, if E causes E*, another high-level phenomenon, but E * is supposedly emergently dependent on B *, then the causal closure of the base level is still threatened.
The issue seems especially urgent when it is mental states that are alleged to emerge from physical states. For here, it is said, the causal closure of the physical is under attack, if mental states are capable of downward causal action. And surely they must be. An agent’s decision to walk out of the room carries the molecules of her body out of the door. Accepting that a decision is an emergent, causally autonomous power seems to undermine the claim that, from complete knowledge of where all the particles of a body are, plus the laws of nature, you should be able to calculate where those particles will be at a later time.
Given this problem, it seems that the emergentist must either:
- (a) provide a good reason why causal closure is to be rejected, or
- (b) show that her account of emergence does not violate causal closure.
Option (a) is sometimes depicted as a rejection of physicalism, and thus as a non-starter. It is partly so as to avoid this that Wilson (2016) offers the subset view, which is a form of response (b). The powers of E are a subset of those of B; hence, there is a sense in which both E and B can be causes of B*. This allows that an effect is systematically overdetermined by E and B but without violating the principle of causal closure. She calls this non-reductive physicalism and acknowledges that it is short of strong emergence. However, we have given a positive account that we say deserves the name strong emergence, and we also explained why we prefer it to Wilson’s account. Do we, though, have a plausible response to the causal closure problem?