Who Is Doctor Who ?
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A question that still remains unanswered.  This much we do know, the Doctor, as he is simply known is an eccentric brilliant Time Lord of the planet Gallifrey who bored by the conventions and sedentary lifestyle of his people 'appropriated' a TARDIS and became a traveler in time and space. The Doctors' physiology included two hearts, a respiratory bypass system, a temperature of 60 Fahrenheit and has the ability to withstand a greater range of temperature and radiation than men, telepathic powers and, finally, the ability when his body becomes old or damaged, to regenerate his body at least twelve times.  On certain occasions, the Doctor could create a projection of his future selves to help them through his impending regenerations.  The Doctor is now over 750 years old and has so far regenerated seven times, on each occasion introduced a different personality and appearance.
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William Hartnell1st DOCTOR: The white haired old man known as the First Doctor, tended to be an observer, loath to interfere in history, a sometimes disagreeable and taciturn individual, and his often arrogant nature, occasionally led him into conflict with his human companions, who beneath the gruff surface was a charming and friendly explorer, who could offer great comfort to his close associates but remain an enigma to those he did not fully trust.
Patrick Troughton2nd DOCTOR:  Often hid his true personality beneath a clownish exterior, a veneer of absentminded foolery, a ploy which led his enemies to underestimate him.  Always ready to disarm his foes with a wry comment or via apparent bumbling mayhem and ineptitude, the second Doctor's mind was actually as sharp as ever.  Frequently he opted to play his recorder to help calm his mind and was not adverse to pass around confectionary to appease his foes.
Jon Pertwee3rd DOCTOR:  Exiled to Earth as a result of his first trial, the third Doctor was an expert in unarmed combat such as Venusian Aikido, and had a penchant for gadgetry such as the Sonic Screwdriver or his cars, Bessie and the Whomobile.  He was eccentric as he was suave and sophisticated, with a passion for flamboyant clothes.  He was capable of dealing equally with the high ranking ministers of the British government and the average man in the street.
Tom Baker4th DOCTOR:  Possessing many characteristics of his former selves, was often prone to stroll into potentially dangerous situations equipped with only his 20 foot long scarf, floppy hat broad grin, the occasional jelly baby, and his strong streak of morality.   His insatiable curiosity for other worlds, and his occasional aloofness from humans eventually caused him to gradually severe his connections with Earth, and resume his space traveling.

Peter Davidson5th DOCTOR:  Seemed younger in outward appearance than any of his prior bodies, but  retained the sharp, analytical mind of his predecessors. The youthful Doctor displayed limitless ingenuity in rectifying his own miscalculations.  Donning Edwardian cricketing garb with a stick of celery pinned to his lapel, the Doctor, although not ill tempered, nevertheless was easily exasperated, especially when trying to impart logic and sense.

Colin Baker6th DOCTOR:   Initial characteristics of the  Doctor emerging from a highly unstable regeneration proved most peculiar.  He displayed tendencies to burst into a situation, shouting and blustering and then examining the fallout of his actions.  This often led to tension and falling out with his companions.  Beneath the arrogance and bellowing was a shrewd quick-witted Doctor, preferring to present an image no one could take seriously, putting his foes of guard.
Sylvester McCoy7th DOCTOR:   A genuine mixture of all his predecessors, mixing the arrogance of the Sixth Doctor, with the whimsy of the Forth and taking a dash of the clown of the Second and the physicality of the Third, all tempered with the wisdom of the First and the analytical brain of the Fifth.  In many ways he was the arch manipulator, seeing each adventure akin to a chess match, moving a few pieces and seeing what retaliation the opposition made.
Paul McGann8th DOCTOR: Suffered the most traumatic and violent regeneration than any  had yet experienced.  Subjected to a heart probe, his body could not cope, and appeared to die on the operating table. Two hours later his body convulsed and regenerated in the morgue. This Doctor seemed to simply love life, endearing, unpredictable, suave, lunatic and caring.  This romantic character was more emotional, feeling no embarrassment about kissing his companion.