system
Given the following post, respond as if you're commenting on Reddit:
human
What does Duhem mean when he says that no theory or hypothesis in physics is ever falsifiable?
gpt
Here's what Duhem wrote in *The Aim and Structure of Physical Theory*:

> The prediction of the phenomenon, whose nonproduction is to cut off debate, does not derive from the proposition challenged if taken by itself, but from the proposition at issue joined to that whole group of theories; if the predicted phenomenon is not produced, not only is the proposition questioned at fault, but so is the whole theoretical scaffolding…

This is an account of scientific discovery from observations, which invoke a modus tollens. Suppose the truth of a theory, P, implies the observation of some phenomenon, Q. However, if Q is not observed, then P can't be true: 

P → Q 
¬Q 
∴ ¬P

This means we have to reject P. However, Duhem's idea is that there is always one or more more auxiliary theories, a, in conjunction with P. This changes our result to:

(P & A) → Q
¬Q
∴ ¬(P v A)

Where, A=(a1+a2+…+an).

Hence, there is no way of deciding from this logical schema whether P or A or both is the cause of ¬Q. ¬Q only implies that either P or A or both is at fault. According to Duhem, there is no way of isolating any P of auxiliaries and therefore it is in principle impossible to ever falsify any P beyond doubt. 

As for Quine, i *think* this is pretty close to what he means by 

> …our statements about the external world face the tribunal of sense experience not individually but only as a corporate body

How should we then decide between two competing theories given that neither is strictly falsifiable? According to Duhem, the best we can hope for is that:

> The day arrives when good sense comes out so clearly in favour of one of two sides [of conflicting theories] that the other side gives up the struggle even though pure logic would not forbid its continuation.