Knowing what knowing is

When people say "I do not think it, I know it," they are saying that they are sure of it. Knowledge is important because we want bodies of knowledge that can be relied upon, and also because we need to distinguish between what we know and what we only think is the case, in all sorts of situations. When people say "I do not think it, I know it," they seem to be assuring us that what they think is indeed the case. Knowledge claims are a bit like promises. And promises can be a bit of a gamble:

Consider a boy sitting an exam, a boy who is not sure of an answer, but puts it down anyway. Suppose that his answer turns out to be correct. Would you say that he did not know the answer?

I wonder if the following has some intuitive plausibility:

Proposition p is known by subject S when S's justification for believing p guarantees that p is true.

How much of a guarantee S would need to provide would presumably depend upon the kind of uses that such knowledge might be put to, so it is conceivable that philosophers could disagree about what knowledge is without any of them being wrong!