Comment by Timwi
5 days ago
A math professor at my uni said that a statement in mathematics is “obvious” if and only if a proof springs directly to mind.
If that is indeed the standard, then it's easy to see how something that is vaguely plausible to an outsider can be obvious to someone fully immersed in the field.
Not quite 'obviously', but mathematical folklore has it that 'clearly' is used to mark the difficult conceptual step in a proof.