  Duhem’s early methodological views reflected late nineteenth-century positivism: Physical theory was no more than an aid to memory, summarizing and classifying facts by providing a symbolic representation of them; the facts of physical theory are different from common sense and metaphysics, especially from the mechanical theories fashionable at the time. Duhem’s position was immediately attacked by a Catholic engineer, Eugène Vicaire, on the grounds that separating physics from metaphysics implied that physics was the only real knowledge (another positivistic thesis), and thus conceded too much to skepticism. Vicaire raised an important point for turn-of-the-century Catholics because the Church was officially committed to neo-Thomism, with its generally rationalist apologetics (see Martin 1991, chap. 2). In his reply Duhem adopted a quasi-Thomist position: metaphysics is a real form of knowledge, more excellent than physics, but separated from it in that it has different objects and is governed by different methods. This immediate response fitted reasonably well into the framework of neo-Thomism, though it did not go as far as to reunite the disparate forms of knowledge into a hierarchical system of sciences including cosmology and metaphysics. Duhem’s mature position was somewhat different, containing three key ideas elaborated upon on various occasions: (1) the underdetermination of theory by fact, often referred to as the Duhem Thesis, (2) the rejection of metaphysics and models in physics, and (3) natural classification as the endpoint of physical theory.