If someone believes something, they think that it is true, but they may be mistaken. This is not the case with knowledge. For example, a man thinks that a particular bridge is safe, and he attempts to cross it; unfortunately, the bridge collapses under his weight. It could be said that the man believed that the bridge was safe, but that his belief was mistaken. It would not be accurate to say that he knew that the bridge was safe, because plainly it was not. For something to count as knowledge, it must actually be true.
The Aristotelian definition of truth states:
"To say of something which is that it is not, or to say of something which is not that it is, is false. However, to say of something which is that it is, or of something which is not that it is not, is true."