@Twirlip,
To quote Aristotle, "A is A". Things are what they are.
To put it another way, Existence exists and only existence exists.
The metaphysicaly given are those things existing in nature. They are what they are. Could they have been other things? No. They simply are mountains, trees, stars, etc. This applies to living organisims as well. It is a waste of time to run circles around what if's. The fact remains that the mountain will still be a mountain weather you decide it's something else or not. This is metaphysicaly given. You can destroy the mountain, but that doesn't change the fact that it was a mountain. It doesnt change what it was or its current makeup now. It's still rock, insteand of a mountain it is now a pile of rubble. That is now the metaphysically given. But that is a man made change.
Mans interference with the world, the thigns man makes, did not have to be and are not metaphysicaly given. They are still however metaphysical. Before cars there was no such thing. They didn't exist. When the first car was made, when it was brought into existence, then and only then did they exist. You cannot undo their existence or make claim that if some infantismal difference had occured it would be somehow changed. The difference didn't occur. The car is a car. You can destory it, but it won't change the fact that it existed and that others like it exist. It can even be lost and forgotten, but that doesn't change the fact either.
Now you may argue that that there are yet "unknowable truths" and things yet to be discovered. Isn't it possible that maybe god exists or some other fantastic flight of fancy may indeed dwell within the universe?
My answer to this is that there are no "unknowable truths". Only undiscovered ones. You can say it is possible to travel at the speed of light for instance to which I would agree. It is
possible. We have not discovered how to do this yet, we may never discover how to do it safely. But I know it is possible because I know that light travels at the speed of light. Science tells me this truth. Reason interpreted facts to come to that conclusion and reason is what will guide us when we uncover other, greater truths.
The only things unknowable are those things which we have not yet discovered and the only things unclear are those things we see with irrationality.