I think that with the way physics turned out, what with general relativity and quantum mechanics, our folk insights and assumptions need to be cast aside. This, I think, means that metaphysics is impossible.
You'd be throwing the baby out with the bathwater.
Rather, like many things, it's a matter of keeping the right proportions. All we have to think up the world are our most basic intuitive assumptions, such as we should trust our perceptions and we should trust our sense of logic. You wouldn't want to throw that away because you'd have nothing left. And then, you still need to work out some rational view of the human experience, which, again, can only be grounded on our perceptions and sense of logic. Where does metaphysics start in this process? I believe it starts immediately when you decide to trust your perceptions and your sense of logic. Ultimately, it is the interplay between our own personal assumptions and that of others as conveyed through verbal interactions which provide the rational basis for our view of the world. Yet, we shouldn't forget all this is based on our initial assumptions. Even the idea that collaboration between human beings acting collectively justifies our trust is ultimately grounded in our initial assumption. What metaphysicians may sometimes be guilty of is pushing the game too far by piling up assumptions. Yet, Nobody is forced to act on their pronouncements, unlike what happens with a number of pronouncements by scientists or indeed religious institutions. Metaphysics should be seen in the same light as mathematics. It's there if you think you need it but you can live your entire life ignoring it altogether. Of course, unlike for mathematics, I couldn't give any precise example of an uncontroversially useful metaphysical idea but I also think that metaphysical considerations are more or less pervasive within the processes of rational thought, including rational thought underpinning all scientific work.
What Quantum Physics and Relativity have shown is that we need to be prepared to rethink entirely our model of how the world is, including rethinking, reinterpreting, the significance of our most basic assumptions.
We probably all intuitively tend to think of time as definitely something that exists independently of us and independently of space. Einstein's work forces us to reconsider this assumption: there's no such a thing as a universal time that would apply uniformly everywhere. Instead, all it seems we can do is measure locally the time dimension of continuous space-time intervals along causally connected paths. With the result that two twins could start from one location, split to take different routes through space-time but end up together in another space-time location having aged very differently, while having lived separate but perfectly normal lives in between. In effect, the time dimension seems to behave somewhat like a fourth space dimension.
So what is best discarded are any strong, religious-like commitment, to whatever may be our current interpretation of our intuitions rather than the intuitions themselves because trusting experimentation and collaboration with other people are also, ultimately, grounded in our intuitions that we should trust our perceptions and our sense of logic.
EB
