 in the velocities are easy to notice (Duhem 1980, 76). Thus, Duhem preserves the basic principles of Lagrangian mechanics and redirects the difficulties and obstacles raised against the mechanistic program to its two presuppositions: the atomistic hypothesis of matter and its ultimate demand that physical concepts be reduced to a restricted number of primitive mechanical magnitudes such as mass, motion, shape and/or force (Duhem 1903, 270). The goal of unifying the whole of physics (the principle of logical unity) by the analytic approach to the mechanistic program, with its restricted mechanical conceptual basis (mass, motion, forces), is attained at the cost of a large indeterminacy of its parameters of masses and motions. And the consequence of the large indeterminacy and arbitrariness of this system of hidden masses in motion is the unfalsifiability of mechanical theories. In the analytic approach, mass and motion are mere parameters in mathematical equations, without the realistic or figurative connotations they have in the synthetic approach. The synthetic approach to mechanical explanation involves specifications and determinations of the masses, motions, sizes and forces. Why does Duhem see the unfalsifiability of the mechanistic method as a problem and why is it a source for his criticism? Is not this approach a legitimate heuristic approach as defended by Poincaré? Duhem does not think that the aim of theoretical physics is to promote the discovery of new laws. Such is the task of experimental physics. Duhem's Critical Analysis of Mechanicism and his Defense of a Formal Conception of Theoretical Physics José R. N. Chiappin – Cássio Costa Laranjeiras 48 Duhem pursues a rational explanation of the evolution of theoretical physics. From this viewpoint, he evaluates the methodological characteristics of the analytic approach to mechanistic theories. He does not think that hypotheses and methods that construct unfalsifiable physical theories make scientific growth rational and continuous. For him, rati