"Never increase, beyond what is necessary, the number of entities required to explain anything" -- William of Ockham (1285-1349)
Also called Occam's Razor, this is the philosophical idea of "going with" the explanation which requires the least assumptions. It is sometimes paraphrased as "the simplest answer is best", but that is probably overly simplistic!
"Why didn't you tell us you were a Doctor before you were bandwagonned?"
"Simple; if I'd told you, the Mafia would have killed me last night."
This explanation seems simple, but by itself cannot be taken as proof. Maybe the player didn't roleclaim before, not because they were the Doctor, but because they were Mafia who wanted an 'out' in case they killed the Doctor at Night (see also Wine In Front Of Me).