‘I have held it my duty as a scientist as well as my duty as a Christian never to cease being the apostle of common sense, the sole foundation of all scientific, philosophical, and religious certainty.’ So Duhem told a friend since youth in a letter of which four more phrases are known owing to the perspicacity of E. Picard, perpetual secretary of the Academie des Sciences, who made them the culminating point of his great eulogy of Duhem.^l In the same letter Duhem met head-on the classic objection to the central role of common sense. Were not its claims, he asked, ‘tantamount to some philosophical and religious beliefs, all resting on worthless reasonings which invariably imply undefinable notions, so many empty words void of meaning?’ As he tried to come up with an answer Duhem noticed that

the same could be said in connection with all the sciences, including those which are considered the most rigorous among them — physics, mechanics, and even geometry. The foundations of any of these constructs are formed by notions, which one pretends to understand although one cannot define them, or are formed by principles which one feels assured about, although one has no proof of them whatever. These notions, these principles are formed by common sense. Without this basis provided by common sense, a basis not at all scientific, no science can maintain itself; all of its solidity comes from there.