I say "an hour ago this table didn't exist", I probably mean that it was only made later on.
If I say "this mountain didn't exist then", I presumably mean that it was only formed later on - perhaps by a volcano.
If I say "this mountain didn't exist an hour ago", that is such a strange statement that it is not clear what I mean. Whether for example I mean something untrue but scientific. Perhaps you think that the statement that the mountain didn't exist then is quite clear, however one conceives the context. But suppose someone said "This mountain didn't exist a minute ago, but an exactly similar one did instead." Only the accustomed context allows what is meant to come through clearly.
 I might therefore interrogate someone who said that the earth did not exist before his birth, in order to find out which of my convictions he was at odds with. And then it might be that he was contradicting my fundamental attitudes, and if that were how it was, I should have to put up with it. 
Similarly if he said he had at some time been on the moon.
 I believe that every human being has two human parents; but Catholics believe that Jesus only had a human mother. And other people might believe that there are human beings with no parents, and give no credence to all the contrary evidence. Catholics believe as well that in certain circumstances a wafer completely changes its nature, and at the same time that all evidence proves the contrary. And so if Moore said "I know that this is wine and not blood", Catholics would contradict him.
 What is the belief that all human beings have parents based on? On experience. And how can I base this sure belief on my experience? Well, I base it not only on the fact that I have known the parents of certain people but on everything that I have learnt about the sexual life of human beings and their anatomy and physiology: also on what I have heard and seen of animals. But then is that really a proof?
 Isn't this an hypothesis, which, as I believe, is again and again completely confirmed?
 Mustn't we say at every turn: "I believe this with certainty"?
 One says "I know" when one is ready to give compelling grounds. "I know" relates to a possibility of demonstrating the truth. Whether someone knows something can come to light, assuming that he is convinced of it.
But if what he believes is