The World, the Deceiver, and The Face in the Frost

In an appendix to The Foundations of Knowledge, Timothy McGrew provides the outline of a solution to the problem of the external world.1 McGrew argues that the probability of the existence of a deceiver who makes it appear that we live in a real external world must be lower than the probability of a real external world itself because the ontological commitments of the latter hypothesis will always necessarily be greater than those of the former. In the latter hypothesis, we posit a mental state of the deceiver as a cause of each of the apparently real things that seem to exist in the external world, but the deceiver himself also exists as an entity who is not merely the sum of all of these mental states. McGrew argues, further, that any time we conditionalize on some particular mental state of our own that we normally take to be caused by real objects in the external world, the gap in probability between realism and the deceiver hypothesis grows larger. He bases this argument on the probabilistic fact that if one theory is strictly simpler than some other theory, the confirmation a given piece of evidence affords to the simpler theory is always greater than the confirmation it affords to the more complex theory— the difference between the old probability of the simpler theory and its new probability is always greater than the comparable difference between the old and new probabilities of the more complex theory.2 McGrew’s argument thus shows, if we take it to be successful, that the prior probability of a deceiver scenario is lower than the prior probability of realism and also that, as we gradually conditionalize on more and more everyday evidence, the gap in probability between the two will continue to grow. This set of conclusions would seem to mean (since we have a great deal of sensory evidence that we normally