Philosophy defines what knowledge itself is, it doesn't define what different areas of knowledge are. That's because knowledge is a function of being human.

Philosophy is ultimately the study of being human, of understanding our existence and our thoughts. That is the bedrock underneath all different knowledges.

That said, there are a lot of questions philosophy cannot answer. But nobody else has the answer to those questions either.

What about metaphysics and religion? Most philosophers consider those speculation, so to philosophy, so those are things people "do" but they don't have any meaning independent from what happens in people's minds.

Hope that is a helpful answer.