‘Let us consider the facts of reality. Theoretical physics is based on the assumption that there exist real events not depending upon our senses. This assumption must in all circumstances be maintained; and even physicists of positivist leanings make use of it. Even if this school maintains that the priority of the sense data is the sole foundation of physics, it is yet compelled, in order to escape an irrational solipsism, to assume that there are such things as individual deceptions and hallucinations; and these can be eliminated only on the assumption that physical observations can be reproduced at will. This, however, implies what is not evident a priori, namely, that the functional relations between sense data contain certain elements not depending upon the observer’s personality nor upon the time and place of observation. It is precisely these elements which we describe as the real part of the physical event and of which we attempt to discover the laws.
Whenever we observe an event taking place in nature we must assume that something is happening independently of the observer, and conversely we must endeavor to eliminate as far as possible the defects of our senses and of our methods of measurement in order to grasp the details of the event with greater perfection. There is a kind of opposition between these two abstractions: while the real external world is the object, the ideal spirit which contemplates it is the subject. Neither can be logically demonstrated and hence no reductio ad absurdum is possible if their existence is denied. The history of physics bears witness, however, that they have played a decisive part throughout its development. The choicest and most original minds, men like Kepler, Newton, Leibniz, and Faraday, were inspired by the belief in the reality of the external world and in the rule of a higher reason in and beyond it.’ (Max Planck, ‘Physics and World Philosophy’ in ‘The Philosophy of Physics’, chapter 1)