A truism of rhetoric

Repeating myself, there is a truism of rhetoric of the form:

Being in the same category is "excited" when both X and Y are mentioned and even more excited when three things from the same category are mentioned. The category remains dormant until at least two things from it are mentioned. 
Again: Truism 4 says "things remain the same" and this is applied to a comparison of past versus present; so the "thing" has assumed constancy in time. With the rhetorical truism, the impulse is the same as T4 but, in its case, "thing" is the next member of the category. So the assumed constancy is between members of the category. With eg the bird category and the migrates (yes/no) attribute, the attribute is not entirely independent of the category. If someone mentions terns migrating and swallows migrating then the next mention of a bird ("robin") carries with it the assumption that the subject matter is birds that migrate. Changing the scope from migrating birds to birds in general calls for the use of "but".
I am still trying to figure this out. "Things remain the same" applied to rhetoric, says that the scope of the topic remains the same. (First shivers in the direction of "context".)