Dr.EndoIndy said:If two explanations both fit, the useful question is which one predicts something the other does not.
There is a second half to this that has not been said yet. The version of this that has an answer is narrower than the version being asked. Narrow it and it becomes tractable; leave it broad and the thread will produce nine confident and incompatible replies.
If somebody has the primary source to hand I would rather cite it than paraphrase it.