Weeeellll, well...
Driving into work yesterday, I was listening to a story...well, not sure if I was really listneing, but it was on and I was driving, though my mind may have been wandering.  At any rate, it was about Ernest Hemmingway and one of his wives, Martha Gellhorn.  Now, the only book of his I've read was "A Farewell to Arms", which I loved...but I haven't read anything else of his.  They said on the radio that he was one of the "great American writers", though I've heard a few opinions here and there stating that he wasn't so great...Guess I can't say as I only have one book to judge from, rather than a variety.  And I wouldn't think that you'd be able to call someone great based on one single thing they've done...

Or could you?  Seems you should judge a person's ability not on a singular instance (no matter how great), but on a pattern of consistency.  A star athelete, while maybe having a superb day on the field or on the court is usually judged by the overall performance over time.  They may be known for that one instance, but their consistency isn't there to be able to call them great.  One hit wonders in music may be able to slam out an astounding song, but have everything else fall flat.  One great song, one great effort, but one great artist?  Not so much.  Not to say they aren't artists...They are, just not great in the way you would say "one of the great American writers" would imply. 

Now to broaden this thought...Wouldn't it seem that basing things on patterns and consistency would apply to other aspects as well.  Not just the creativity or skills a person has, but on the kind of person they are as a whole.  If someone does one act of kindness (even though even just one thing is certainly good), yet on a regular basis is greedy and selfish, does that mean they are a kind person?  On the other end of the spectrum, if someone who is normally kind and giving does something selfish or unkind, does that make them a bad person?  That's where I get the whole consistency thing...Everyone has those odd days where they do something out of character.  Every once in a while a generally kind, giving person can have a moment - whatever the motivation (anger, self-preservation, lust, etc.) - that they do something greedily for themselves. 

One last note...While I would say you can't determine one's greatness from a singular event or thing that they've done, that's not to say THAT particular event/thing isn't the greatest of it's kind.  For example, a mediocre football star may, at one point, make one of the greatest catches ever witnessed in the game.  A musician may make one of the greatest songs ever written, even if he never does anything remotely comparable ever again, and a human may do one of the most kind, self-sacraficing acts ever known to man in that one moment, yet never have or never will do another again.  I guess the question then, would one single great act or event overwrite all others before or after?  Would that one thing, if great enough, wipe the slate of all other shortcomings?  I wouldn't think....but that's just me.