In the public imagination, physics is well on its way to giving a complete description of the fundamental nature of reality. From the scientific revolution onwards, the development of a rigorous experimental method has allowed continuous progress in understanding the nature of space, time and matter. Of course there is a long way to go; physicists have so far been unable to unify our best theory of the very big, i.e. general relativity, with our best theory of the very small, i.e. quantum mechanics. But at some point, it is supposed, these wrinkles will be ironed out and physicists will proudly present the public with the Grand Unified Theory of everything.
The trouble with this conventional wisdom is that physics, from Galileo onwards, has worked with a very austere vocabulary: its terms express only mathematical and causal concepts. Think about what physics tells us about an electron. Physics tells us that an electron has negative charge. What does physics have to tell us about negative charge? Rough and ready answer: things with negative charge repel other things with negative charge and attract other things with positive charge. Physics tells us that an electron has a certain amount of mass. What does physics have to tell us about mass? Rough and ready answer: things with mass attract other things with mass and resist acceleration. All the properties physics ascribes to fundamental particles are characterised in terms of behavioural dispositions. Physics tells us nothing about what an electron is beyond what it does. If the nature of an electron is exhausted by what physics tells us about it, then an electron is not so much a being as a doing.
More generally, physics provides us with mathematical models which capture the causal structure of the universe. This is very useful information; it enables us to manipulate nature in all sorts of extraordinary ways, allowing us to build lasers and microwaves, and to fly to the moon. But mathematical models abstract away from the concrete reality of their subject matter. A mathematical model in economics, for example, abstracts away from the concrete features of real world consumers, such as the nature of their labour and the specific things they buy and sell. Wherever there is mathematical-causal structure, there must be some underlying concrete reality realising that structure. But physics leaves us completely in the dark about the underlying concrete reality of the physical universe.
Contrary to popular opinion, the scientific revolution marks the point when fundamental natural science stopped trying to give a complete description of the universe, and instead focused on formulating useful mathematical models of its causal structure. This limited project has been extraordinarily successful, providing technology which has transformed our society beyond recognition. Somewhere along the line this impressive record has created in the public mind the conviction that physics is providing us with a complete account of the fundamental nature of the universe. But physics is successful precisely because it is aiming and succeeding at a much less ambitious task. The truth is that physics tell us nothing about the intrinsic nature of reality, never has done, and – so long as it continues to work with such an austere vocabulary – never will do.