A note on the completeness of 'physics'

Abstract

1. One argument for physicalism states that all physical effects are due to physical causes, and hence that anything having physical effects must itself be physical. Let us call this now familiar style of argument the ‘causal argument’ for physicalism, and its crucial premiss, that all physical effects are due to physical causes, the ‘completeness of physics’ (cf. Crane 1995). But how are we to understand ‘physics’ in this context? If we tie the meaning of ‘physics’ to any specific details of present physical theory then it seems unlikely that physics is complete. A short glance at the history of science reveals that proposed lists of fundamental forces and basic entities usually turn out to be wrong. So advocates of the causal argument need to abstract away from current physical theory. But then it seems that they need to say something more about the shape of ideal or future physics, lest their thesis of the completeness of physics lose all substantial content. This note aims to show that this demand for clairvoyance is misplaced. Users of the causal argument do not need any detailed assumptions about ideal or future physics. All they need is some way of understanding ‘physics’ which makes it plausible that physics so understood is complete. We shall make the point by showing that there is more than one way of so understanding ‘physics’. In particular, we shall identify two plausible completeness theses. Each such thesis can be plugged into the causal argument, and each then generates its own version of ‘physicalism’. Which completeness thesis you ought to be interested in thus depends on the purpose to which you want to put the causal argument. You should be interested in our first completeness thesis if you want to investigate the relationship between the mental and the non-mental. The relevant completeness thesis is then the claim that the non-mental is complete. If you plug this thesis into the causal argument, it then generates the conclusion that the mental must be identical with the non-mental. Alternatively, you may be interested in the relationship between manifest qualitative phenomena, like colours and sounds and smells, and the underlying quantitative features of the material world, like size and shape and motion. If so, the relevant completeness thesis is that the quantitative is complete. With this premiss, the causal argument then generates the conclusion that the non-quantitative must be identical with the quantitative. In effect our aim here is to shift attention away from debates about the meaning of the word ‘physics’ (cf. Crane, 1991, Papineau, 1991). Tangles