In the absence of experimental evidence, basic beliefs of theoretical physicists may initially have almost a religious flavor, guided by faith and aesthetics. Fortunately unlike religion, these beliefs soon face the hard test of experiment.

The first principle of cosmology must be 'There is nothing outside the universe'. This is not to exclude religion or mysticism... But if it is knowledge that we desire... we need to seek answers to questions about the things we can see... only things that exist in the universe.

When the physicist poses the question, "What does it mean to do physics?" or "What is research in physics?" — his question is a preliminary question. Clearly, when you ask a question like that, and try to answer it, you are not "doing physics." Or rather you are no longer doing physics. But when you ask yourself, "What does it mean to do philosophy?" then you actually are "doing philosophy" — this is not at all a "preliminary" question but a truly philosophical one: you are right at the heart of the business.

The philosophy of physics is continuous with physics itself. Just as certain issues in the Foundations of Mathematics have been discussed by both mathematicians and by philosophers of mathematics, so certain issues in the philosophy of physics have been discussed by both physicists and by philosophers of physics. And just as there are issues of a more epistemological kind that tend to concern philosophers of mathematics more than they do working mathematicians, so there are issues that concern philosophers of physics more than they do working physicists.

Philosophy treats of physics where a more careful knowledge is required because the problems which come under this head are numerous... So the reader of Ctesibius or Archimedes and the other writers of treatises of the same class will not be able to appreciate them unless he has been trained in these subjects by the philosophers.

I want to emphasize the necessity for a sound mathematical basis for any fundamental physical theory. Any philosophical ideas that one may have play only a subordinate role. Unless such ideas have a mathematical basis they will be ineffective.

Not only must philosophy be in agreement with our empirical knowledge of Nature, but the origin and formation of the Philosophy of Nature presupposes and is conditioned by empirical physics. However, the course of a science's origin and the preliminaries of its construction are one thing, while the science itself is another. In the latter, the former can no longer appear as the foundation of the science; here, the foundation must be the necessity of the Concept.

I have tried to read philosophers of all ages and have found many illuminating ideas but no steady progress toward deeper knowledge and understanding. Science, however, gives me the feeling of steady progress: I am convinced that theoretical physics is actual philosophy. It has revolutionized fundamental concepts, e.g., about space and time (relativity), about causality (quantum theory), and about substance and matter (atomistics), and it has taught us new methods of thinking (complementarity) which are applicable far beyond physics.

The Philosophy of Nature takes up the material, prepared for it by physics out of experience, at the point to which physics has brought it, and again transforms it, without basing it ultimately on the authority of experience. Physics therefore must work into the hands of philosophy, so that the latter may translate into a true comprehension (Begriff) the abstract universal transmitted to it, showing how it issues from that comprehension as an intrinsically necessary whole.

After logic we must proceed to philosophy proper. Here too we have to learn from our predecessors, just as in mathematics and law. Thus it is wrong to forbid the study of ancient philosophy. Harm from it is accidental, like harm from taking medicine, drinking water, or studying law.

In classical physics, science started from the belief – or should one say, from the illusion? – that we could describe the world, or least parts of the world, without any reference to ourselves.

The combination of interest in physics and in philosophy was uncommon in the United States, and physicists who deviated from the narrow path were looked down upon by their colleagues. Philosophy of physics was definitely regarded as an aberrant, evasive sort of discipline, practiced by physicists who 'could not make it' in their straight profession and, in somewhat greater numbers, by philosophers whose knowledge of physics was inadequate.

Above all he believes that these mathematically based speculations solve thousand year old philosophical conundrums, without seriously engaging those philosophical issues. The belief that all of reality can be fully comprehended in terms of physics and the equations of physics is a fantasy.