Plato thought that the philosopher was someone who can realize what everyone else is seeing is merely shadows and can walk out of the metaphorical cave and into reality. By understanding reality, the philosopher acquires knowledge, that is (in my definition here), beliefs about the world that are accurate to how the world is. So, for example, if A-theory of time is correct, and we believe in A-theory of time, this counts as knowledge. Thus, the philosopher becomes wise and enlightened. Sounds nice, but is this even possible?

If our brains are the product of evolution, then they have been tuned to react to stimuli local to our frame of reference. We cannot assume that we can understand the secrets of the cosmos. In fact, we cannot assume that anything we consider "knowledge" today, is even an accurate representation of reality as we know it.

Furthermore, if there is a possibility that something could be disproved, then it does not count as knowledge. Someone could read as much as they want regarding any topic in, let's say philosophy, and still not have acquired any knowledge whatsoever except for maybe a history of ideas. Without complete, absolute certainty (an impossibility - is absolute certainty incoherent?), there cannot be any knowledge. It's not as if someone can read about A-Theory and B-Theory about Time, and then look in the back of the book for answer key. It's unknown. It is impossible to know if we have ever come to the correct conclusion.

But since there cannot be any certainty about anything, does this mean that any opinion is equally valid? Does this destroy the entire enterprise of rational inquiry?

The radical subjectivist dilemma is something I have been struggling with for a long time now. It's very contradictory and self-refuting, but at the same time appeals to my skepticism.