  When you trace the causal history of any physical effect—that is, of anything physical that has a cause—you will  never need to appeal to anything non-physical. The physical  universe contains within itself the resources for a full causal explanation of  any of its (caused) elements, and in this sense is “complete”. The  point applies, then, to whatever might occur to or within our bodies. Any instance of bodily behavior has a sufficient physical  cause, which itself has a sufficient physical cause, and so on. In  tracing the causal history of what we do, we need never appeal to  anything non-physical.