Second, this means that the connection must be made at the level of sentences. I'll suppose that Davidson's position would allow that the connection could be made at the level of imperatives as well as declaratives, however. Perhaps the simplest answer is because it is sentences that express thoughts. Gretchen wants Elwood to close the door. This is the sentence we use to describe Harold's belief, and it is the sentence we would expect him to use to express it.
X does A .
Kilimanjaro is big. Kilimanjaro is cloudy. Kilimanjaro is a long ways away. Kilimanjaro is big. From this, our theorist derives the following.