Feser's Philosophy of Mind, #1

In the first chapter of Feser's Philosophy of Mind, Feser attempts to justify belief in a physical external world using Occam's Razor. That is, given the two hypothesis that either the external physical world exists independently of us, or that it is a simply some form of illusion, application of Occam's Razor justifies belief in the first option.

There are several problems with this argument. First, Occam's Razor is a heuristic. It is simply a guideline, a good guess, when choosing between alternatives. But anyone familiar with search techniques in artificial intelligence knows that even good guesses can ultimately lead to less than optimal or even wrong conclusions. If you don't end up in a dead end, there might still be an untried path to a more favorable outcome.

Second, and more importantly, Occam's Razor only applies when all other considerations are equal. That is, Occam's Razor should be used only when both systems give the same independently verifiable answer to the same questions. Both the
Ptolemaic and Copernican systems predict the same positions of the planets in the night sky. But the Copernican system is simpler and so is justified by Occam's Razor. So the use of Occam's Razor is therefore not applicable in this case, because the answers to the same questions can be wildly different in the Realist and Solipsist systems.

Third, we can't really tell which system is simpler. Since we don't ultimately know what reality really is, any argument that the implementation of reality in one form is simpler than the implementation of reality in another is dubious, at best. No one has any idea what it takes to implement the reality that appears to be external to us, any more than we have any idea that we know what it takes to implement a mind that thinks there is an external reality.

Fourth, if a system with an entity which manipulates our minds is more complex than an external reality, then Feser has justified disbelief in Theism in general, and Christianity in particular, since God is one more complicating factor in an already complex system. Given that Feser is a theist, he may want to reconsider this use of the razor.

So Occam's Razor fails as a means to justify realism over solipsism. Granted, Feser conditions this justification with “If all this is right…" but, still, an inauspicious start to this book. The correct answer is that we choose one or the other simply because we choose one over the other. Post hoc rationalizations as to why we made a particular choice will vary depending on which system we chose.