The Duhem-Quine thesis states that theoretical assumptions cannot be tested in isolation, but rather whole systems of theoretical assumptions and auxiliary assumptions are subjected to empirical testing.
if our predictions are not in accordance with our theory, we can only conclude that one of our many theoretical assumptions must be wrong, but we cannot know which one, and we can always modify our system of assumptions, leading to various ways of immunity of theories (Stegmüller 1986).