A lot of the Dhamma is passed down by interactions that go beyond the suttas, like life lessons, some practices, and explanations of specific topics or terms. If we see that most schools believed in something, then we can expect the belief to come from an earlier common source, probably the Buddha himself.
I was wondering if this principle that is so often applied in comparative studies of the EBT hasn’t been already applied to ideas that are outside the suttas. For example, we could apply it to things like the in-between state and try to see if it’s early or not. From what I’ve read, it seems most early schools indeed believed in it.
In practical terms, we could create a table with lines containing all early schools and columns containing hot topics, like those discussed in the kathavatthu. There would be great chance that the most frequent views in each column would be the earliest. This could be a handy way to learn about early Buddhism beyond the early suttas themselves.
Has this already been done? Is anyone aware of an extensive study comparing all early schools?