Originally posted by twhitehead
I disagree.
First some basic definitions.
Correct me if I am wrong, so that we are on the same page:
My understanding is that a prior probability exists as an assumed probability of events based on partial knowledge. We then run some experiments that essentially test the probability. We combine our initial estimate with the results of the experiment an ...[text shortened]... ble. It may even be more probable than the chance of picking 42 at random from the real numbers.
My understanding is that a prior probability exists as an assumed probability of events based on partial knowledge.
The 'loose' meaning of prior probability, which is unfortunately the one it is conventionally given, is that it is the probability before you have looked at
some evidence, not necessarily before you have looked at ALL the previous evidence in the past before looking at that new evidence. Unfortunately that often causes confusing because that means that probability is a prior probability in respect to that new evidence but posterior probability in respect to any old evidence before observing that new evidence.
To avoid that confusion, I had been giving 'prior probability' a less conventional 'strict' meaning of prior probability where it means the probability before you have looked at ANY evidence relevant to the theory; because only that strict 'true prior probability' tell you nothing about what is or is not causally possible.
Perhaps we can both agree to, for now on, call that 'strict' prior probability a '
true prior', to avoid possible confusion.
We then run some experiments that essentially test the probability.
Was it tested before in the past? Do we take into account prior knowledge we have of the actual external world that was gained by past observations?
We combine our initial estimate with the results of the experiment and come up with a new estimate we call the posterior probability.
Was the said 'prior probability' a posterior probability in respect to knowledge gained earlier in the past?
This mechanism can never ever result in certainty that something is impossible.
Not ABSOLUTE certainty. But if we simply say we cannot ever have certainty, period; we would then be in the uncomfortable position that we must reject ALL scientific facts (because we cannot know for a 'fact' that it is impossible for a given theory to be possible to be true ) except those that are just tautologies such as mathematical facts.
Now, suppose I say I have a coin that has 'heads' on one side and 'tails' on the other. It has no snakes. If I throw the coin, 'snakes' will be an impossible result.
LOGICALLY impossible, therefore causally impossible, yes.
Your description above seems to suggest that I know that snakes is an impossible result ...
LOGICALLY impossible, therefore causally impossible, yes.
through my knowledge of natural laws ...
NO, through it being LOGICALLY impossible! so natural laws have nothing to do with it here!
It is LOGICALLY impossible to select out of 'heads' or 'tails' 'snake'.
I say no. I say that the impossibility of 'snakes' has nothing whatsoever to do with natural law
EXACTLY!
THEREFORE it isn't impossible because it violates natural law but rather because it is LOGICALLY impossible!
I say that throwing 'snakes' is a logical impossibility in my situation.
You understand!