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The first question we should face is: What is the aim of a physical theory? To this question diverse answers have been made, but all of them may be reduced to two main principles:
"A physical theory," certain logicians have replied, "has for its object the explanation of a group of laws experimentally established."
"A physical theory," other thinkers have said, "is an abstract system whose aim is to summarize and classify logically a group of experimental laws without claiming to explain these laws...
Now these two questions — Does there exist a material reality distinct from sensible appearances? and What is the nature of reality? — do not have their source in experimental method, which is acquainted only with sensible appearances and can discover nothing beyond them. The resolution of these questions transcends the methods used by physics; it is the object of metaphysics.
Therefore, if the aim of physical theories is to explain experimental laws, theoretical physics is not an autonomous science; it is subordinate to metaphysics...
Now, to make physical theories depend on metaphysics is surely not the way to let them enjoy the privilege of universal consent.
--
Pierre Maurice Marie Duhem, translated by Philip P. Wiener (1991). The aim and structure of physical theory. Princeton University Press. p. 10. ISBN 069102524X. 

 

Pierre Duhem

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I just want to explain what I mean when I say that we should try to hold on to physical reality.
We are ... all aware of the situation regarding what will turn out to be the basic foundational concepts in physics: the point-mass or the particle is surely not among them; the field, in the Faraday-Maxwell sense, might be, but not with certainty. But that which we conceive as existing ("real") should somehow be localized in time and space. That is, the real in one part of space, A, should (in theory) somehow "exist" independently of that which is thought of as real in another part of space, B. If a physical system stretches over A and B, then what is present in B should somehow have an existence independent of what is present in A. What is actually present in B should thus not depend the type of measurement carried out in the part of space A; it should also be independent of whether or not a measurement is made in A.
If one adheres to this program, then one can hardly view the quantum-theoretical description as a complete representation of the physically real. If one attempts, nevertheless, so to view it, then one must assume that the physically real in B undergoes a sudden change because of a measurement in A. My physical instincts bristle at that suggestion.
However, if one renounces the assumption that what is present in different parts of space has an independent, real existence, then I don't see at all what physics is supposed to be describing. For what is thought to be a "system" is after all, just conventional, and I do not see how one is supposed to divide up the world objectively so that one can make statements about parts.

 

Albert Einstein

 

Mental illness, of course, is not literally a "thing" — or physical object — and hence it can "exist" only in the same sort of way in which other theoretical concepts exist. Yet, familiar theories are in the habit of posing, sooner or later — at least to those who come to believe in them — as "objective truths" (or "facts"). During certain historical periods, explanatory conceptions such as deities, witches, and microorganisms appeared not only as theories but as self-evident causes of a vast number of events. I submit that today mental illness is widely regarded in a somewhat similar fashion, that is, as the cause of innumerable diverse happenings. As an antidote to the complacent use of the notion of mental illness — whether as a self-evident phenomenon, theory, or cause — let us ask this question: What is meant when it is asserted that someone is mentally ill?
In what follows I shall describe briefly the main uses to which the concept of mental illness has been put. I shall argue that this notion has outlived whatever usefulness it might have had and that it now functions merely as a convenient myth.

 

Thomas Szasz

 

What now is the answer to the question as to the bridge between the perception of the senses and the concepts, which is now reduced to the question as to the bridge between the outer perceptions and those inner image-like representations. It seems to me one has to postulate a cosmic order of nature — outside of our arbitrariness— to which the outer material objects are subjected as are the inner images... The organizing and regulating has to be posited beyond the differentiation of physical and psychical... I am all for it to call this "organizing and regulating" "archetypes." It would then be inadmissible to define these as psychic contents. Rather, the above-mentioned inner pictures (dominants of the collective unconscious, see Jung) are the psychic manifestations of the archetypes, but which would have to produce and condition all nature laws belonging to the world of matter. The nature laws of matter would then be the physical manifestation of the archetypes.

 

Wolfgang Pauli

 

There is an ancient Chinese saying "He who labours with his mind rules over he who labours with his hand". This kind of backward idea is very harmful to youngsters from developing countries. Partly because of this type of concept, many students from these countries are inclined towards theoretical studies and avoid experimental work.

In reality, a theory in natural science cannot be without experimental foundations; physics, in particular, comes from experimental work.

 

Samuel C. C. Ting

 

Three principles — the conformability of nature to herself, the applicability of the criterion of simplicity, and the "unreasonable effectiveness" of certain parts of mathematics in describing physical reality — are thus consequences of the underlying law of the elementary particles and their interactions. Those three principles need not be assumed as separate metaphysical postulates. Instead, they are emergent properties of the fundamental laws of physics.

 

Murray Gell-Mann