In 1953 the Nobel Laureate Louis de Broglie, permanent secretary of the French Academie des Sciences, remarked that Pierre Duhem was "one of the most remarkable French physicists at the turn of the century."
...In his study of the growth, development, and scope of physical theories, he arrived at a positivistic philosophy of physics. He maintained that "truth" is not established by Newton's method of experiment plus induction; physical theory is an artificial construction. For him, physical theory was not an explanation, merely "a system of mathematical propositions whose aim is to represent as simply, as completely, and as exactly as possible, a whole group of experimental laws"...
Metaphysics, he insisted, must be completely separated from physics. He was not, however, an extreme positivist; he did not deny the existence of metaphysics. A convinced and devout Roman Catholic, he accepted his religious teaching; he believed in the existence of a reality external to man (he was by no means an idealist).
Noting the evolution of physical theory, he envisaged an "ideal" theory toward which it seemed to tend. To be sure, "man has a radical inability to reach the depths of reality" (cf. metaphysically, the religious basis of revelation).
Duhem's philosophy of science is well developed in an article (1905) denying a metaphysician's accusing him of having the "Physics of a Believer." He himself insisted upon the autonomy of the scientific method, its independence of metaphysical opinions. Physics draws a line of demarcation between the known and the unknown, but not between the knowable and the unknowable. A physics principle like the conservation of energy is not concerned with metaphysics or religious dogma (e.g., free will). One does have to admit that physical theory is premised on postulates that are independent of physics per se (e.g., the very search for a single, logical system). As in Plato's cave, however, although science deals with shadows (symbols), man may assume they are produced by solid objects. Moreover, although metaphysics cannot of itself favor or disfavor physical theory, the very harmony of that theory may reflect ontological order. Duhem goes further and allows for the possible use of analogy between the limit of the scientific world (beyond phenomena) and the real "natural world," (illustrated in a comparison of a modernized Aristotelian cosmology with general thermodynamics).