Fundamental metaphysics...
Any starting point is fairly arbitrary, but if we go back to Socrates, Plato and Aristotle, we see the beginnings of an argument that went on for about 2000 years.
In simple terms, what goes on in our heads is all we know (I call that experience - it's as good a term as any). The basic question is, where does this experience come from? There are two basic answers - the arguments are all around which takes primacy and how reliable each is. Everyone acknowledged the fallibility of human perception (except for the more recent strict rationalists like Ayn Rand, but they weren't very rational...).
The rationalist school argues that knowledge originates in the mind. Aristotle talked about perfect forms. This got teased and twisted into the idea that all knowledge originates from preconceived ideas that we then impose on the stream of perceptual experience. The primacy was given to the ideas, into which our experience was squished. So, we are born with the concept of "chair" and we learn to associate a bunch of different things we perceive with the idea "chair". In this sense, reality exists within our mind and the world is an imperfect - and imperfectly perceived - example of our internal reality.
The empiricist school gave primacy to experience. The external world was "real" and we imperfectly perceive it. Based on this imperfect perception, we build ideas about the world in our minds. Thus the concept of chair has been developed over time as are exposed to chairs in various shapes and sizes. Of course, our perception of reality is all messed up (Socrates' Cave is the best known example of this - our mind apprehend nothing more than hints and shadows of reality).
This all stopped when someone taught the Scots how to read. Hume came along and managed to dismember both schools of thought, demonstrating that neither school could be considered rational (Hume was, technically, a rationalist, but has never made sense to me). Thus knowledge based on either rationalist or empiricist metaphysics was irrational and fatally flawed: reality and reason were unreconcilable. He dealt with the ideas of the big names of his time - French (Descartes), English (Locke) Irish (Berkeley), and a few others (Leibniz, Spinoza). None were ablet o withstand his arguments. It too a German to put and end to the nonsense.
Kant argued that our minds require a few basic concepts (synthetic a priori knowledge). These concepts are then applied to interpret our experience to produce knowledge. The concepts were the big abstracts - things like space, time causality and the like. The trick in deriving these principles was to take a conclusion about the necessary structure of knowledge. For Kant, it became apparent that the mind's structuring makes experience possible. Kant basically accepted the Empiricist position - that knowledge grows from experience of reality. But he also identified its limits: the statements that must be true for us to build knowledge from experience.
One of Kant's key concepts was the idea that, "there are mind-independent objects that persist over time". While he never actually proved this to be true, he demonstrated that any presumptions of knowledge required that this be true. The lack of proof was always problematic - people like Kierkegaard and Neitzche picked up on this and started wondering what reality would be like if this weren't true. The results were some of the more interesting philosophy that we've ever seen. Sadly, the one most likely to give us a meaningful explanation wasn't able to complete his project.
What we got instead was a Austrian transplanted to Cambridge who linked knowledge to language. Wittgenstein talked about metaphysical problems resulting from trying to express them in language. Language is how we express knowledge and it's inadequate to express knowledge about metaphysics (what is reality...). To put this another way, in his first big work, he concludes with "Whereof one cannot speak, thereof one must be silent". In his second big work, he tells us we cannot (meaningfully) speak of metaphysics due to the limits of language. Thus the true nature of "reality" is an intractable problem (Wittgenstein is the most obscure writer I've ever read - so if anyone disagrees with that interpretation, that's hardly surprising).
That's pretty much where I sit. All the ideas I've encountered about "what is reality" are flawed. I find Kant's transcendental idealism curiously pragmatic: I like it because it creates a practical basis to work from, not because it proved that basis was true. I find the strict modern empiricism of people like Popper and Ayer fails to address the flaws that were pointed out a couple of hundred years ago. Pure rationalism is similarly weak - and was almost entirely supplanted by Kant's idealism.
I know enough to distrust my own knowledge. I don't know what reality is, but I'm damn sure no-one else does either. I automatically distrust anyone who says they know better.
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