The theory of knowledge, especially with regard to its methods, validity, and scope, and the distinction between justified belief and opinion. Some say that we can never know what is true. Some say there is no truth at all. I would say that the nature of epistemology makes it hard to say: do I even know anything about this? At a certain point, I have no knowledge about knowledge anymore, or any opinion about opinion! I might be a nihilist. However, such philosophical things are still REALLY interesting so I'm quite pleased!
Things are what they are. But the language used to describe them is man-made. After all, things still exist even without humans!
Fundamentally, I don't know.
I don't know anything. Whether any advice would work, what's there in the future, there is no way to know.
Experimenting - but still the nature of physics is that. Fundamentally I have no idea of anything in the future - hindsight bias of the successful will say they know, but they don't. They are merely reflecting on the past.
Philosophical view of there being no totally
verifiable knowledge.
I agree. We are subjective bodies, not seeing reality, but our
interpretation of reality, which is different. In the worst case, we may
just be in a simulation and reality may be non-existent after all!