First, we must sort out the different uses of the word: I know my uncle, and I know that Abraham Lincoln was born in 1809. These are two very different mental states or processes. In philosophy, we are usually concerned with examples of the latter type - what we might call propositional knowledge: knowing that a proposition (a statement or sentence) is true.
One philosophical tradition suggests that knowledge is justified, true belief: it is justified, meaning that we have some reason to embrace it; it is true, because we would not say that one can know a falsehood ("I know that 2 + 2 = 5"); and it is a belief, because one can only know something if one also believes it.
A slightly different spin on knowledge is given by British philosopher A.J. Ayer, who writes
that the necessary and sufficient conditions for knowing that something is the case are first that what one is said to know be true, secondly that one be sure of it, and thirdly that one should have the right to be sure. This right may be earned in various ways; but even if one could give a complete description of them it would be a mistake to try to build it into the definition of knowledge.
Ayer is indicating that there would be different types of justifications for different types of facts: “I know that my neighbor’s car is blue” and “I know that my knee hurts” and “I know that 7 + 5 = 12” represent various types of knowledge. My answer to the question “how do you know?” will not only by different for each of the three, but will be of a different type.
