Although sometimes the words 'know' and 'believe' are used interchangeably, in a strict sense they are very different. Probably, no one would criticise you for saying, "I believe it's time for us to go" when you actually mean simply, "It's time for us to go".

However, belief frequently applies to something you are either unsure about or for which there is insufficient proof. For instance, I might say, "I believe that the single currency is a good thing", or "I believe that Wales will win next Saturday". These things may very well be false: the single currency may prove disastrous, and Wales - judging on recent form - may very well lose.

Knowledge, on the other hand, in its strict sense, only applies to things that are true. Therefore, it may be inappropriate to say, "I know that Wales will win" or that "I know which horse will win the 3:30 at Kempton", because there is an element of doubt involved (unless I have some proven psychic ability, such as Mystic Meg).