Like other physicists, Feigenbaum used an understated, tough-guy vocabulary to rate such problems. "Such a thing is obvious", he might say, meaning that a result could be understood by any skilled physicist after appropriate contemplation and calculation. "Not obvious" described work that commanded respect and Nobel prizes. For the hardest problems, the problems that would not give way without long looks into the universe's bowels, physicists reserved words like "deep."

Source: "Chaos", Prologue