In this chapter, Ellis addresses whether top-down causation overdetermines a system. He asks “How can top-down causation be possible in the case of the implementation hierarchy, if the physics at the bottom is a causally closed system, determining all that happens through interactions of particles and fields mediated by forces and potentials? Isn’t the system already fully determined so there is no room for any kind of top-down causation?” In other words, this book is all about responding to reductionism.
This issue specifically arises when discussing the mind. “The claim can be made that physics does not just constrain what happens, it uniquely determines what happens in the brain. If basic physics determines all, the situation is causally closed and there is no room for higher level influences.” Ellis responds to this concern in this chapter, arguing that “…the underlying physics establishes the set of possibilities that can happen, but not the specific events that actually happen.”
Ellis offers five ways, which are not mutually exclusive, in which top-down effects can work:
1. Contextual constraints
2. Constraining structures
3. Changing the nature of lower level elements
4. Existence of lower level elements
5. Deleting lower level elements
This was a delightful chapter to read, both for being shorter and also for having more interesting examples. In essence, Ellis cites numerous examples in which high-level entities set a context in which the lower-level entities interact. In this way, the causality from above influences the outcomes of the lower-level causal forces. Reductionism cannot explain the high-level constraints. The simultaneous bottom-up and top-down causality is truly a beautiful interplay between the many different levels of existence.