There is a ton of shit you can reason successfully about. Hell, math provides literally an infinite number of things you can reason about, including, conveniently enough, actual infinities. Still, some bros never have enough, and the stubborn fucks insist on trying to reason about experience. Of course, they all run into the same problem: it can't. fucking. be done.
Sure, you can understand experience, but that's completely different. Understanding just puts labels on everything so your brain can file it neatly away. That color you're seeing? Yellow. The shape? Curved. The curved yellow thing? Banana. Fucking CHECK. Everything you know about bananas, you know because you had an experience, and you understood; what the hell could reason possibly tell you beyond that? If you're trying to get to the 'essence' of a banana, its 'ultimate reality', then enjoy your aneurysm.
Aristotle went on and on about 'essences' but no one has ever seen an essence because you can't see essences. There's no way to access them. We're stuck with phenomena, our subjective experiences, and those don't tell you shit about objective, ultimate realities, the noumena. Sorry, that's just the way it is. When bros try to reason their way across the divide, they make assholes of themselves.
You look skeptical. Alright hotshot, Mr. Thinks-he's-Chuck-Norris, I defy you to come up with one thing you can know about a banana that doesn't start with experience. Take as long as you like. I'll wait.
Done? Really? Nothing? You couldn't figure out what the 'banananess' of a banana is? Cool story - so why do you keep trying to do that with all of reality? If you can't even conquer the objective nature of a piece of fruit, good luck reasoning out the objective nature of the entire Universe.
It's not your fault. You're certainly not the first to fail and try; some bros have made way more audacious claims than 'I've figured out bananas'. Descartes thought he figured out the mind, just because he 'thinks' or some shit. But all he did was identify a manifestation, an appearance of the mind in a thought. How? Fucking experience, that's how. Thinking is an experience too! And then he reasoned in circles for a while, said the soul was immortal without a fucking shred of proof, and published it. How could we know the soul is immortal? What an asshole.
The ancient Greeks? Those motherfuckers could reason their way to anything. "The world had a beginning - an infinite regress is impossible!" Okay, fair enough. "The world has no beginning - otherwise, there would have been nothing to start it!" Yeah, that makes sen- waaaaiiit a second. What? Both of those seem reasonable, since we can't experience either. That's exactly the sort of bullshit they loved to pull, those dicks. If you were busy thinking, "reason shouldn't be able to prove contradictions like that!" you get a gold star - that's exactly the problem. Time and space don't belong to reason, they belong to your experiences, except no one could have experienced the beginning of time. When you start applying reason to made-up experiences no one has actually had, you can prove anything you want, and things just stop making sense.
The same shenanigans happen with free will and determinism. We've all watched two bros go red in the face arguing over free will - the determinist just keeps going, "Well how does it work? How does it work? Everything is causality!" and the defender of free will just keeps going, "I don't know, but how do you keep choosing?!" It's like watching a Twilight fan try to solve a Rubik's cube. Except that the laws of nature belong to experience, and freedom is the ability to act outside those laws. So yeah, maybe it's true that every single experience we have is determined, because that's how we understand and organize experiences. But essences can be free, and we'd have no way of knowing - if you could explain freedom, it would just be more complicated determinism, which is exactly why that argument goes in circles. Apples and oranges, bro, apples and fucking oranges.
Look, there's a lot of shit about our experiences that we don't know; it's a great big fucking world out there. But there are no problems of reason that we can't solve, because we have complete access to reason. Which means if you find a problem you can't solve, it's not a reasonable problem. You've overstepped your boundaries, and your confusion is the cue to toe the fucking line. Metaphysics can't tell us anything anything about ultimate reality, so stop wasting your energy.
"How is Metaphysics in General Possible?" is part three of Immanuel Kant's Prolegomena To Any Future Metaphysics, which you can find online in its entirety, free of charge, on Wikisource.