An "idealist" theory of meaning, as I am using the term, might go like this (in its
simplest form): the meaning of such a sentence as 'electrons exist' is a function of
certain predictions that can be derived from it (in a pure idealist theory, these would have to be predictions about sensations); these predictions are clearly a function of the theory in which the sentence occurs; thus 'electrons exist' has no meaning apart from this, that or the other theory, and it has a different meaning in different theories.