In part I of this post, I explained the basic idea behind Carl Sagan’s famous skeptical dictum, “Extraordinary claims require extraordinary evidence.” That idea was that to establish something extremely improbable, we need evidence that is incredibly strong. What could possibly be wrong with that?
I don’t think anything is wrong with the principle itself. Rather, I think where things go wrong is in its application; particularly, in the process of assessing what is improbable. It seems to me that we can shuttle easily and unconsciously between two very different kinds of improbability.
One kind involves reasonably knowable probabilities, probabilities we are in a position to assess. For instance, I referred in part I to the probability that Newt Gingrich would switch to the Democratic Party. Everyone, I think, would agree that that is highly unlikely. Would anyone think that is likely? These reasonably knowable probabilities tend to involve the likely behavior of familiar things. We have seen these familiar things behave so often, so many times, that we have a good idea of how likely they are to behave in a certain way.
The second kind, however, involves currently unknowable probabilities, probabilities we are not in a position to assess. We are not in a position to assess them because we really don’t have enough evidence yet. We don’t really know the likely behavior of certain things because we aren’t yet familiar enough with those things.
For instance, skeptics generally regard psychic phenomenon as extremely improbable. For example, on Skeptico website, we find this explanation of why psychic phenomena represent an extraordinary claim:
Psychic powers are extraordinary initially because of lack of scientific plausibility: that is, we have no known way for psychic signals to be sent and received. Lack of plausibility doesn’t mean something isn’t true, but it does make it extraordinary. The continued lack of good evidence for psychic powers, despite 125 years of looking, means that even more extraordinary evidence is now required to explain why the previous 125 years of looking were unsuccessful.
The author presents two reasons for why psychic powers represent an extraordinary claim:
1. We have no known way for psychic signals to be sent and received.
2. We have no good evidence for psychic powers after 125 years of looking.
These two reasons, I believe, reveal the fatal flaws in how the principle that “extraordinary claims require extraordinary evidence” is often applied. We will therefore look at them in detail in part III of this post.