Books on Amazon
Hypotheses/Criterion/Duhem: Certain fundamental hypotheses cannot be refuted by any experiment because they form definitions in reality, and certain expressions used by the physicist only receive meaning through them.
E.g. Free fall cannot be refuted (like Putnam's robot cats) When a heavy body falls free, its acceleration is constant. This cannot be disproved in the experiment because it forms the definition of what one understands under free fall!
Le Roy: "Laws are not verifiable if they are taken strictly, because they themselves constitute the criterion."
Duhem: the words "free fall of a heavy body" now have a twofold meaning: for those who do not know the theories, they have a real meaning, for the physicist they have a symbolic meaning.
Duhem: the theory would not have fulfilled its task if the second meaning was not the expression of the first. Only that the symbolic sense does not have the accuracy of the real and concrete case we observe.
Ziel und Struktur der physikalischen Theorien Hamburg 1998