It’s about the relationship between our senses and our knowledge of the world. The essence of the question is whether we can know something is true if we don’t have direct evidence of it through our senses, and the usual example is this: “If a tree falls in a forest and no one is around to hear it, does it make a sound?” I used to teach some philosophy courses, and the students and I could debate this sort of thing in an earnest way for weeks on end. The answer, I think, is, “Of course it does, don’t be so ridiculous.”
