Now that we have completed our survey of the conceptual problems of quantum physics, what are we to make of it all? One thing that should be clear is that there is considerable scope for us all to have opinions and there is a disappointing lack of practicable experimental tests to confirm or falsify our ideas. Because of this, I intend to drop the use of the conventional scientific ‘we’ in this chapter and to use the first person singular pronoun wherever I am stating an opinion rather than describing an objective fact or a widely accepted scientific idea. This is not to say that everything in the earlier chapters has been free of personal bias, but I have tried to maintain a greater basis of objectivity there than will be appropriate from now on.

I first want to refer briefly to a way of thinking about philosophical problems known as ‘positivism’. Encapsulated in Wittgenstein's phrase ‘of what we cannot speak thereof should we be silent’, positivism asserts that questions that are incapable of verification are ‘non-questions’, which it is meaningless to try to answer. Thus the famous, if apocryphal, debate between mediaeval scholars about how many angels can dance on the point of a pin has no content because angels can never be observed or measured so no direct test of any conclusion we might reach about them is ever possible. Opinions about such unobservable phenomena are therefore a matter of choice rather than logical necessity.