I just finished reading the entry about property physicalism in The Oxford Handbook of Philosophy of Mind, which was written by Ansgar Beckermann. Over the course of the article, he presents the idea of “emergent properties,” makes reference to “reduction” several times, and considers the view that some emergent properties may be irreducible.

Here are his examples of uncontroversial emergent properties:

  • Solidity and fluidity – “Atoms are neither solid nor fluid; only collections of molecules have these properties.”
  • Breathing, feeding, and reproduction – “Molecules again don’t breath or nourish, nor do they reproduce; only living beings have these capacities.”

Beckermann then introduces the unity of the world thesis and gives examples.

The idea of the unity of the world just consists in the claim that all properties of higher level entities have a reductive explanation in terms of the properties and the arrangement of their lower level parts. That salt is water-soluble has an explanation in terms of the properties of ions that salt molecules consist of. That soap has the capacity to loosen dirt can be explained by its molecular structure. That animals have the capacity to digest has a chemico-physiological explanation, etc. It is, of course, an empirical question whether all higher-order phenomena can be explained in this way. The unity-of-the-world thesis is not a priori true. But it seems to be an aim of science to show that it is an empirical truth.

The other point of interest in this article is his description of the contrast to the unity of the world thesis.

Why is salt water-soluble? To answer this question,it will not do to just point to the fact that water has a certain molecular structure. Nothing short of a reductive explanation will suffice. One has to show that having this molecular structure explains water-solubility. If such an explanation can be given, everything is all right. The thesis of the unity of nature once more is proven to be true. If not, there would be, at this point at least, a breach in nature, as it were. The water-solubility of salt would be revealed to be an emergent property – a real new property with effects that cannot be accounted for by the microstructure of salt.

This seems like a useful way of framing the debate over whether there are emergent properties or whether everything can be reduced to physics and chemistry. Yet I wonder why Beckermann assumes that an emergent property would have to be “a breach in nature,” i.e., supernatural. It seems like there should be some third option between naturalistic reductionism and supernaturalistic emergentism.