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Physics/Duhem: the work of the physicist in 4 steps:
1. Definition and measurement of the physical quantities (also combined quantities) the symbols of the theory are given a physical meaning only to the extent that measurement methods for the quantities are given.
2. Formulation of hypotheses. Here, quantities are linked together.
3. The mathematical development of the theory e.g. the slowed down motion on the inclined plane is the responsibility of the logician and the mathematician alone.
4. Review by experiment. "Consistency with experience is the only criterion of truth for a physical theory."
Theoretical Physics/Duhem: cannot grasp the sensually perceptible phenomena. It cannot therefore decide whether these properties are qualitative or quantitative.
Theoretical physics confines itself to representing perceptible phenomena through signs, by symbols. It does not capture the reality of things.
Physics/Duhem: must respect the mathematical rules strictly. The mathematical symbols have only meaning under properly determined conditions. To define these symbols is to enumerate the conditions. E.g. because of the definition, an absolute temperature can only be positive, the mass of a body invariable.
Postulates: The theory has the principle of postulates, i.e. theorems which it can express as it likes, if there are no contradictions within the postulates, or between them. Only such rules weigh on the construction of the theory.
The physical theory can take any path that is logical. In particular, it is free to give no account of the experimental facts.
This is no longer the case when the theory is fully developed. Then it becomes necessary to compare the group of mathematical theorems with the group of experimental data. It may then turn out that it must be discarded, although it is logically correct because it is physically false because of the observation.
It is a mistake that all the conclusions made by the mathematician have a physical meaning. Similarly, (falsely) any calculation carried out during a measurement procedure would have to have a correspondence in a property of the examined body. That is not the case. Only the concluding formulas deal with the observed objects.
E.g. Dissociation of a perfect gas mixture into its elements, which were regarded as perfect gases.
Measuring: e.g. no thermometer can determine the highest temperature. The symbol for absolute temperature does not correspond to any of the measuring methods.
What is called a perfect gas in thermodynamics is only an approximate picture of a real gas.
Ziel und Struktur der physikalischen Theorien Hamburg 1998