The Second Doctor was very different to his predecessor. A more playful, whimsical air disguised dark undercurrents and a sharp mind. And as the Doctor changed, so did the adventures. Trips into earth's history gave way to besieged Moonbases, Martian Ice Warriors and Space Pirates. Not to mention the heyday of the emotionless Cybermen, whom the Doctor froze into their ancient tombs once more. It wasn't until he was put on trial by his own people - The Time Lords - for interference, that the Second Doctor was forced to regenerate.
Logic, my dear Zoe, merely enables one to be wrong with authority.
Our lives are different to anybody else's. That's the exciting thing, that nobody in the universe can do what we're doing.
I am not a student of human nature. I am a professor of a far wider academy of which human nature is merely a part.
There are some corners of the universe which have bred the most terrible things. Things which act against everything we believe in. They must be fought.