I'm going to use the science fiction movie The Matrix as an example to elaborate on why I now reject metaphysics as a necessary discipline and why I adopt a pragmatic epistemology as a result. In the movie, human beings are basically resting, unconscious, in closed pods. Various wires are connected to their nervous systems, and a big computer simulates a fake world that they live in. Even though everyone thinks the world they live in is the real world, in reality they are resting in a pod, completely unaware of this fact.
Consider the life of someone in the Matrix. His natural predisposition to the correspondence theory of truth causes him to assume that everything he sees simply is what it is. He sees a dog and he concludes that what is happening is that he is seeing an actual dog, rather than a computer generated image.
The simulated world of the Matrix is his phenomena. It is the world of his experience, and in this example it has been cleanly divorced from the world as it is. The world as it actually is (the noumena) consists of pods with people in them and a bunch of computers running the show.
Now consider; is it possible for this man in the Matrix to ever know he is in the Matrix? Particularly if he grew up in it from childhood and had no reason to ever suspect that he lived in a simulation? And if we dig even further, can the computers (which, in this movie, were conscious creatures like human beings) know that they aren't in a similar simulation? No matter how deep you dig, there could always, theoretically, be another layer underneath your current experience that you are unable to make contact with.
It may be reasonable to infer, from the fact that we have sense experience, that there are noumena. But the only thing we can infer about noumena is that they generate our experiences. Whether they do this by simply being the things we see, through a complicated computer simulation, or any number of other means, we can never know. Even if we dig a layer deeper, we can't ever know that we're at the "final layer" of reality.
In our day to day lives, we are no different than someone in the Matrix. I'm not asserting that we all live in an advanced computer simulation; I am asserting that it is impossible to prove or disprove this notion. I am also asserting that it is impossible to prove or disprove the notion that we aren't living in a computer simulation.
Fortunately, as a pragmatist, I don't really have to care. William James, an early pragmatist thinker, has a brilliant quotation that I think sums up my view on the issue quite nicely:
"There can be no difference anywhere that doesn't make a difference elsewhere - no difference in abstract truth that doesn't express itself in a difference in concrete fact and in conduct consequent upon that fact, imposed on somebody, somehow, somewhere and somewhen."If there are no practical implications to believing I'm in the Matrix when compared to believing that I'm in the real world, then the theories seem pragmatically equal. The Matrix theory actually ends up losing out purely for pragmatic reasons; it is more awkward and difficult to integrate into day to day life. Since it offers no predictive power with regards to my experience, it is not useful.
But suppose someone did prove a theory of metaphysics and connect it to sensory experience somehow. Does the whole pragmatic house of cards fall down? Not at all.
Even if everything we see, hear, touch, etc. is simply as it seems to us, the fact remains that we cannot remember it all, and we have a finite capacity for understanding it. Our own limitations as finite beings prevent us from ever understanding the world with perfect metaphysical clarity.
By necessity of being finite, we have to make fuzzy generalizations, imperfect predictions, and decisions that might not work out like we expect. To put it bluntly, we are not reliable enough instruments of understanding to ever assume that what we hold in our minds is metaphysical truth, even if said truth was somehow able to be experienced directly.
This forces us into the pragmatic position. We have to use the theories, values, etc. that work the best because we simply cannot hold some perfect, Platonic form of reality in our mind. We have to go into everything assuming that what we think is true might be false if we are to remain adaptable and thrive in the finite, imperfect environment of our own minds.