One anti-realist tradition has been around for a long time. At first sight it does not seem to worry about what the word ‘real’ means. It says simply: there are no electrons, nor any other theoretical entities. In a less dogmatic mood it says we have no good reason to suppose that any such things exist; nor have we any expectation of showing that they do exist. Nothing can be known to be real except what might be observed.

The tradition may include David Hume's A Treatise of Human Nature (1739). Its most recent distinguished example is Bas van Fraassen's The Scientific Image (1980). We find precursors of Hume even in ancient times, and we shall find the tradition continuing long into the future. I shall call it positivism. There is nothing in the name, except that it rings a few bells. The name had not even been invented in Hume's day. Hume is usually classed as an empiricist. Van Fraassen calls himself a constructive empiricist. Certainly each generation of philosophers with a positivist frame of mind gives a new form to the underlying ideas and often chooses a new label. I want only a handy way to refer to those ideas, and none serves me better than ‘positivism’.

Six positivist instincts

The key ideas are as follows: (1) An emphasis upon verification (or some variant such as falsification): Significant propositions are those whose truth or falsehood can be settled in some way.