" [ ] Quine holds that the truth of some sentence , which he calls observation sentences, is tied directly to experience (more precisely, to patterns of excited nerve endings), further sentences derive their empirical content from their connections with observation sentences and their logical relations to one another. The truth of the resulting theory depends only on how well it serves to explain or predict true observation sentences. Quine plausibly maintains that there could be two theories equally capable of accounting for all true observation sentences, and yet such that neither theory can be reduced to the other (each theory contains at least one predicate that cannot be defined using the resources of the other theory). Quine has at different times embraced different ways of thinking of this situation. According to one way, both theories are true. I see no reason to object to the view that empirically equivalent theories ( however one characterizes empirical content) are true or false together. According to Quine's other view, a speaker or thinker at a given time operates with one theory and, for him at that time, the theory he is using is true and the other theory false. The position may illustrate what Quine means when he says that truth is 'immanent' [see Ontological relativity and Other Essays, N.Y.: Columbia, 1969 ]. This conception of the immanence or relativity of truth should not be confused with the pedestrian sense in which the truth of sentences is relative to the language in which they occur. Quine's two theories can belong to, and be stated in, the same language; indeed, they must be if we are to understand the claim that the theories conflict. It is not easy to see how the same sentence(without indexical elements), with interpretation unchanged, can be true for one person and not for another, or for a given person at one time  and not at another. The difficulty seems due to the attempt to import epistemological considerations into the concept of truth, "

Donald Davidson, 
The Structure and Content of Truth, "Journal of Philosophy", June 1990, p. 306.