Phelps has unusal physical features and for some biological reason he also tires the least as proved by recent scientific measures...this surely helps him comete better than his peers...he is at the right sport and the right time and has definitley combined it with disciplined approach to achieve wht he has done...
Of course his 12 golds and probably much more considering he is only 23 is no joke but then u cannot ignore his gifts which he has in plenty...greatness is deomonstrated with you achieve more with less and when u rise again after a fall...Phelps at this point has too much going for him we have to wait till the point when some of his gifts start to deteriorate he can then prove his greatness as well
