Good point. One can "believe" in the existence of the tooth fairy, but one does not -- in the same sense of the word -- "believe" in the existence of his mother. Belief in the first proposition (tooth fairy) requires faith, the belief in something for which there is no evidence or logical proof. The second proposition (mother) is that kind of knowledge which follows from sensory evidence. There is also that kind of knowledge (like the Pythagorean theorem) which follows from a logical proof. In between mother and the Pythagorean theorem are those propositions we provisionally accept (or in common usage "believe"), like relativity and evolution, because they are scientific theories -- logical and falsifiable explanations of the available data (which data is knowledge obtained via sensory evidence).