We, the atom and I, have been on friendly terms, until recently. I saw in it the key to the deepest secrets of Nature, and it revealed to me the grea… - Max Born

To come now to the last point: can we call something with which the concepts of position and motion cannot be associated in the usual way, a thing, or a particle? And if not, what is the reality which our theory has been invented to describe?

The answer to this is no longer physics, but philosophy, and to deal with it thoroughly would mean going far beyond the bounds of this lecture. I have given my views on it elsewhere. Here I will only say that I am emphatically in favour of the retention of the particle idea. Naturally, it is necessary to redefine what is meant. For this, well-developed concepts are available which appear in mathematics under the name of invariants in transformations. Every object that we perceive appears in innumerable aspects. The concept of the object is the invariant of all these aspects. From this point of view, the present universally used system of concepts in which particles and waves appear simultaneously, can be completely justified. The latest research on nuclei and elementary particles has led us, however, to limits beyond which this system of concepts itself does not appear to suffice.

The lesson to be learned from what I have told of the origin of quantum mechanics is that probable refinements of mathematical methods will not suffice to produce a satisfactory theory, but that somewhere in our doctrine is hidden a concept, unjustified by experience, which we must eliminate to open up the road.

I must give some attention to the delicate question of religion, on which I have touched already. In my father's generation this question still was discussed with passion. Since then a sort of truce has existed in the countries of the West, while in the communist states of the East atheism has been made into the State religion. It is not advisable to blow on the embers of such controversies. But as I am talking about the limitations of our physical world picture I cannot but say that I do not believe in transgressions of the laws of nature. As these laws are of a statistical nature, and therefore allow deviations from the norm, I must define more clearly what I mean. The statistical deviations themselves obey certain laws. The miraculous events of religious tradition, however, are of a different kind, they lie on a different plane altogether; they are meant to prove something lying entirely beyond scientific consideration, such as the power of prayer, the intervention of divine power for or against certain men or nations.

I am now convinced that theoretical physics is actually philosophy. It has revolutionized fundamental concepts, e.g., about space and time (relativity), about causality (quantum theory), and about substance and matter (atomistics). It has taught us new methods of thinking (complementarity), which are applicable far beyond physics.