A great many philosophers have claimed that, in addition to mundane a posteriori knowledge about the world, which depends on our having particular experiences, we have a priori knowledge, which is independent of experience insofar as we don’t have to have any particular experience to acquire it.
For example, “something that is red all over is not blue all over” is supposed to be a piece of a priori knowledge. Proponents of a priori knowledge would grant that you need to have experiences to acquire the concepts of redness and blueness, but you don’t need to observe any particular red thing or any particular blue thing to acquire those concepts. Further, once you acquire those concepts, you don’t need to do any more observations to grasp that nothing is ever completely red and completely blue. You can just put the concepts together in your mind and grasp the connection between them, according to the story, and that’s what makes it a priori.
Naturally, some have been skeptical about whether there is really any such knowledge. After all, how could we get knowledge of universally true propositions about the world without at least going out and performing observations? Part of Chisholm’s reply to skepticism about the a priori in his book Theory of Knowledge is as follows (p. 49):
The general reply to a skepticism which addresses itself to an entire area of knowledge can only be this: we do have the knowledge in question, and therefore, any philosophical theory implying that we do not is false.
While that’s unlikely to satisfy the skeptic, I think it’s one of the more plausible retorts in this area.