Bertrand Russell

Sometimes it's even hard to tell the difference between a tic and a compulsion. But while tics stem from an urge in a specific part of the body - either completely unconsciously or through a premonitory sensation that's satisfied only by the tic - OCD bubbles up as conscious thoughts in the mind.

If a man made himself an expert in any particular branch of human activity, there would result the strong tendency that a peculiar aptitude towards the same branch would be found among some of his descendants.