plains perfection and it is perfection that explains why unification should be aimed at, or at least why it is reasonable to strive for it. The fact that natural classification will always remain an unjustifiable claim on the positivist method does not make it unjustifiable: Physical theory confers on us a certain knowledge of the external world which is irreducible to merely empirical knowledge; this knowledge comes neither from experiment nor from the mathematical procedures employed by the theory, so that the merely logical dissection of theory cannot discover the fissure through which this knowledge is introduced into the structure of physics; through an avenue whose reality the physicist cannot deny, any more that he can describe its course, this knowledge derives from a truth other than the truth apt to be possessed by our instruments; the order in which theory arranges the results of observation does not find its adequate and complete justification in its practical or aesthetic characteristics; we surmise, in addition, that it is or tends to be a natural classification; through an analogy whose nature escapes the confines of physics but whose existence is imposed as certain on the mind of the physicist, we surmise that it corresponds to a certain supremely eminent order (1906, 334-335). Marie Gueguen and Stathis Psillos Anti-Scepticism and Epistemic Humility in Pierre Duhem's Philosophy of Science 69 Are Relations the Epistemic Limit? A natural question at this point is this: why does Duhem insist that knowledge can extent only up to relations among "hidden realities whose essence cannot be grasped" (1906, 297). The answer to this lies, by and large, with Duhem's account of representation and the role of hypotheses in science. At one point Duhem notes that "an intelligence that see essences" would classify the laws according to the "natural order" (1893b, 68). But we humans do not see "essences". And that's the problem for Duhem. If we (have to) rely only on representations of "essences", then we can never