 two types (or two parts) of theory, Duhem (1906, 52) took a cue from Rankine's (1855) distinction between "abstractive" and "hypothetical theories". According to Rankine (1855, 209), the difference between the two kinds of theory stems from the first operation we described in section "Abstraction Principles", i.e., the definition of properties. An abstractive theory will consider as its fundamental properties only those that can be "perceived by the senses": properties are introduced in the physical theory only by means of an abstraction. A hypothetical theory, on the contrary, will accept properties "not apparent to the senses" through conjectures about the underlying nature of the perceived objects or phenomena. Thus, explanatory hypotheses will be introduced as soon as the relevant properties have been chosen, bearing consequences on all of the edifice afterwards. However, unlike Duhem, Rankine did not devalue hypothetical theories. He took them as an indispensable "preliminary step" for the reduction of "the expression of the phenomena to simplicity and order, before it is possible to make any progress in framing an abstractive theory" (1855, 213). For him, the contrast between the two types of theory was a contrast between two different modes of unification, of the "tendency (...) to combine all branches of physics into one system". One way is to rely on the axioms of mechanics as "the first principles of the laws of all phenomena-an object for the attainment of which an earnest wish was expressed by Newton", the other being to rely on "propositions comprehending as particular cases the laws of the particular classes of phenomena comprehended under the more extensive classes". Hence, from Rankine's distinction it does not follow that an explanatory theory should be devalued. Is there any other reason that Duhem summons? For Duhem, explanatory hypotheses are actually "the germs that kill all mechanical theories", inasmuch as these hypotheses are not derived from any experimentation but from arbitrary restric