To wrap up the previous two posts, I have a few thoughts on the common invocation of the principle that “extraordinary claims require extraordinary evidence.” We have already seen that this principle doesn’t do what its common proponents would like it to do, for the reason that extraordinary claims constitute extraordinary evidence. However, there is a further problem. Normally, when people invoke this principle, they mean by “extraordinary claims” the types of claims that are associated with religion, e.g. that immaterial beings such as God or angels exist, or that certain miraculous events have occurred, or some such thing (which is English for res).
However, in virtue of what does the claim that, say, immaterial beings exist count as “extraordinary”? Why isn’t the claim that material beings exist also extraordinary? The fact that we experience material beings cannot be invoked to answer this question, since this fact only indicates that there is a (very) high posterior probability that material things exist. But the objector wants to say that it is a priori improbable that immaterial beings exist. It seems to me that no possible reason can be given for thinking that the prior probability of material beings is any higher than the prior probability of immaterial beings.
What about the fact that we don’t seem to experience immaterial beings? In fact, this does not, in general, render their existence any less probable, since the absence of sensation of immaterial things is just what we would expect–on either hypothesis. In other words, the fact that I don’t see an angel in front of me does not render the probability of its existence one bit less, since my not seeing it fits equally well with its existence and nonexistence.
Thus, someone who wants to maintain that it is highly improbable (and thus an “extraordinary claim”) that immaterial beings exist must also maintain that it is highly improbable (with respect to prior probability) that material beings exist, and that it is only with respect to posterior probability that their existence is probable. But this commits one to the view that it is highly improbable a priori that anything at all exists. This seems rather problematic. At the very least, there seems to be no reason that can be given for believing it.
Someone could also try to avoid this problem by saying that there is, in fact, evidence that immaterial beings, at least rational ones, do not exist; namely, that we would expect them to reveal themselves in some way if they did. But this is exactly what most religions claim has happened, and what witnesses of alleged miracles claim has happened. Since the people who deny the existence of immaterial beings are generally the same ones who reject claims of miraculous happenings as a priori improbable, they are trapped in a vicious circle. For if it is not improbable a priori that immaterial beings exist, then that they should interact with the visible world, and hence that miracles should happen, is also not improbable a priori, and the evidence needed to confirm miracles is not extraordinary, since they are not especially unexpected. Thus, the probability of immaterial beings cannot be rendered low by their alleged lack of interaction with the world. It is either low a priori, or it is not low at all. And if it is low a priori, then so is the probability of anything existing at all, which seems absurd.
What it comes down to, then, is that the skeptic wants to say that it is a priori improbable that anything exists which is not subject to empirical observation. But no possible reason can be given why this might be true. I might as well say that it is a priori improbable that anything exists outside of my field of vision. Remember, we are speaking about prior probabilities, probabilities before consideration of evidence. Thus, the fact that there is plenty of evidence that things exist outside my field of vision does not render the comparison invalid; what reason can be given for denying it before considering this evidence? Furthermore, there is plenty of posterior evidence that increases the posterior probability of unobservable things existing. For instance, there is the fact that even many observable things have escaped human observation for most of the world’s history. Many other reasons could be given.
In short, the claim of the skeptic that it is somehow “extraordinary” to posit the existence of immaterial beings functionally amounts to nothing more than a kind of solipsism, in which one selects a mostly arbitrary set of beings–say, what I can see right now–and asserts that it is highly improbable that any other beings exist. Hardly convincing.