Einstein would be one of the greatest theoretical physicists of all time even if he had not written a single line on relativity.

Statements like ‘a quantity x has a completely definite value’ (expressed by a real number and represented by a point in the mathematical continuum) seem to me to have no physical meaning.

But I believe that there is no philosophical highroad in science, with epistemological signposts. No, we are in a jungle and find our way by trial and error, building our road behind us as we proceed. We do not find signposts at crossroads, but our own scouts erect them, to help the rest.

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America has grown by expansion in a practical vacuum; the pioneers of the West had to overcome terrific natural obstacles, but negligible human resistance. The Russia of today had to conquer not only natural but human difficulties: she had to break up the rotten system of the Czars and to assimilate backward Asiatic tribes; now she has set herself the task of bringing her brand of modernization to the ancient civilizations of the Far East. For this purpose it is indispensable to have a well-defined doctrine full of slogans, which appeals to the needs and instincts of the poverty-stricken masses. Thus one understands the power which Marx's philosophy has gained in the East.
What can we scientists do in this conflict? We can join the spiritual, religious, philosophical forces, which reject war on ethical grounds. We can even attack the ideological foundations of the conflict itself. For science is not only the basis of technology but also the material for a sound philosophy.

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My advice to those who wish to learn the art of scientific prophecy is not to rely on abstract reason, but to decipher the secret language of Nature from Nature's documents, the facts of experience.

Physics as we know it will be over in six months.

In combination with other infernal contraptions, like rockets to deliver bombs at large distances, chemical, biological and radioactive poisons, such a war must mean a degree of human suffering and degradation which is beyond the power of imagination. No country would be immune, but those with highly developed industry would suffer most. It is very doubtful whether our technological civilization would survive such a catastrophe. One may be inclined to regard this as no great loss, but as a just punishment for its shortcomings and sins: the lack of productive genius in art and literature, the neglect of the moral teachings of religion and philosophy, the slowness to abandon outdated political conceptions, like national sovereignty. Yet we are all involved in this tragedy, and the instinct of self-preservation, the love of our children, makes us think about a way of salvation.

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.

There are metaphysical problems, which cannot be disposed of by declaring them meaningless. For, as I have repeatedly said, they are "beyond physics" indeed and demand an act of faith. We have to accept this fact to be honest. There are two objectionable types of believers: those who believe the incredible and those who believe that "belief" must be discarded and replaced by "the scientific method."

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.