The propositions cannot be separated from their context without an important loss of their meaning.
What is generally admitted for proposition is less conceded to discourses or scientific domains, even if they can be viewed as extended propositional positions. For instance, there is little difference between “The cat is on the roof” and the domain of religion built on the believe that God exists (or the justification of physics that there is a countable material reality) or a political discourse that ultimately promotes a definite option for changing or for preserving a certain social or political order.
If we admit some influential contexts to discourses and scientific domains, then we have also to follow their understanding by referring to those contexts. Some domains or discourses direct us to the knowledge of the persons that support or speak one of them. And, since the persons often represent general ways of intellectual or linguistic activity, there will not be surprising to speak about physics through a general approach of human rapport to the world.
Maybe this kind of approach does not seem to have anything related to the practice of scientific domain or the direct persuasiveness of a discourse. But nobody could say that the context of a proposition must be a proposition, too.