The following is the account of an argument that I had with my friend the other day: At 7.45 PM my friend asked me what time the mail room would be open till. I said that I did not know but that it was unlikely that the mail room would be open this late. He then said that he was not busy at the moment so he might as well just go and check it out; he then came back five minutes later with his mail and said that it was in fact open. I said that although it happened to be open, the probability of the mail room being open at 7.45 PM was quite low so taking that chance wasn't necessarily the best decision. But my friend argues that the probability of the mail room being open at 7.45 PM was 100% because there is no instance in which he could go down to the mail room at 7.45 PM and find it closed! Compare this to a multiple choice quiz with four options for every question. Intuitively it seems that putting down A, B, C or D would give you a 25% chance of being correct. But if the answer is B and you mark it D, then you have a 0% chance of being correct. How are the two cases different, if they are different? Basically I want to get to the question of whether or not probability depends on the amount of information one has.