Or why we can never believe in miracles.
If you tell me that there is a statue in a church of the Virgin Mary it is rational of me to accept your assertion because my life experience tells me that this is quite possible - statues often hang out in churches. If you have no history of lying to me randomly about everyday events and there is no other reason to suspect that you could be mistaken there is no logical reason to doubt you.
This is the Antecedent Probability Principle. I accept what you tell me because it lies within what I know to be probable.
If, however, you also say "and it's hovering 6 feet off the floor" I then have cause to doubt. I know from experience that statues don't hover and that there is a greater likely hood of your assertion being false - for whatever reason. If I am to behave rationally I must assume you are mistaken.
In order for me to believe you I then need far more evidence than normal. What is happening here is that I proportion my belief in what you have told me in relation to the net evidence for it. The more rationally unlikely the event, the more unlikely it is to be true and the more evidence is therefore needed for it. This is the philosopher’s version of Carl Sagan’s argument that extraordinary claims need extraordinary evidence.
If someone chooses to accept weak evidence for extraordinary events above their known experience of the physical world they are therefore thinking irrationally.
However, it may be that the hovering statue is still hovering, in which case you can go and test it so that the standard of evidence matches the scale of its improbability. If after doing some simple tests yourself it would then be rational to believe that some miraculous intervention is occurring.
But note, while it may well be rational to think that way, it may still be an error as it may be that your understanding of the phenomena is just incomplete.
To give an old example, a solar eclipse would seem miraculous if you hadn't lived in a culture that has the science to understand it.
Also, the fact that you have done some tests that convinces YOU, it does not mean that when you tell someone else that it is rational for THEM to believe you. They may take note of what you say, take your tests into consideration but still reject it because they rightly need more objective analysis if they cannot witness the event themselves.
And so on.
The logical conclusion of this argument is that miracles cannot exist. This is because the more extraordinary the event, the less credible it must be, and as a miracle defies a natural law - which is impossible - they therefore cannot exist.
I'm sure this begs the question. Maybe someone who has properly studied this can take the argument further?
Life, don't talk to me about life - Marvin the Paranoid Android
™ Version 4.2