Chauncey Wright

All observers not laboring under hallucinations of the senses are agreed, or can be made to agree, about facts of sensible experience, through evidence toward which the intellect is merely passive, and over which the individual will and character have no control.

Natural Selection never made it come to pass, as a habit of nature, that an unsupported stone should move downwards rather than upwards. It applies to no part of inorganic nature, and is very limited even in the phenomena of organic life.

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.