Matter requires at least this: a part that cannot be broken apart any further. This means “matter” is whatever can pass from one side of a breaking machine to another. By “breaking machine” – which is certainly the wrong word – I mean a fire, an acid bath, a particle accelerator, an intense beam of light etc. Matter thus is what remains through a change, and is a proper and essential account both in metaphysics and natural science.
Matter is burdened by any number of dead-end accounts and assumptions about its nature. False imagination tends to make us take matter is “extended stuff” or “the tangible” – though this account would be of no value to the scientist (what work would it do to help him determine what matter is?) Again, for several centuries after Newton, matter was seen as the determined and mechanistic smallest parts of things (corresponding to point-values in quantitative descriptions). This account was purely theoretical and died off about a hundred years ago, which has led to confusion over whether “materialism” is has been disproved by contemporary science, and whether we should try to replace a doctrine like “materialism” with “naturalism” or “physicalism”. The answer is no. All that has fallen away is a theory about matter, but the essential account of matter as what remains through change hasn’t gone anywhere, and was assumed by Newton every bit as much as Aristotle or Nils Bohr. Nothing has changed about the validity of materialism as a doctrine. If it is false, it was false before Quantum Mechanics; if it is true, it is true in spite of Quantum Mechanics.
While the problem of mechanism has not disappeared, the question of materialism as mechanism has. The debate needs to be shifted back to a consideration of change, and specifically whether the being of things is nothing other than that which they arose from, and what they will resolve into.