The Many Inadequate Justifications of the Causal Closure Principle

The principle of the causal closure of the physical world (CCP) is a main rationale for physicalism. This article examines whether the justifications given for this principle are successful. I start by specifying the principle as claiming that every physical effect has a necessary and sufficient physical cause, thereby ruling out mental causes as well as causal overdetermination. I then proceed to the first justification, the argument from basic forces. It inductively extrapolates from our knowledge of known physical (conservative) forces to the conclusion that all forces must be of that nature, hence ruling out mental forces. However, the argument overlooks the fact that mental forces could be in principle conservative, and that even non-conservative forces are perfectly acceptable according to the Noether theorem. The second justification is the argument from the in-principle completeness of physics, whose success depends on insisting that physical causes be necessary and sufficient for their effects, which is just a restatement of CCP. Thirdly, the empirical argument has it that we have enough empirical knowledge of bodies to safely conclude that no nonphysical forces are at work in them. The problem with this is that the relevant empirical data (neuroscience of volitional actions) are not taken into account. Finally, the argument from scientific realism founders on the lack of a relevant metaphysical necessity, a necessity whose introduction would render CCP true by fiat. All in all, CCP lacks justification and comes out as a mere metaphysical doctrine begging the question against nonphysical interaction.