Philosophers typically distinguish between a priori and a posteriori beliefs, knowledge, justification, and propositions. A belief is a priori if it is derived from reason, and it is a posteriori if it is derived from sense experience. Similarly, we would say that we know a priori that ‘a closed, n-sided figure has n interior angles’ because our knowledge is derived from reason in that we understand the concept of a closed, n-sided figure and thus know the statement is true. On the other hand, we know a posteriori that ‘Americans drive on the right’ because in justifying this belief, we appeal to sense experience; perhaps we have seen for ourselves that Americans drive on the right or we've read about it in a book or seen it in a movie.