John Dos Passos

Every time you log in to Facebook, every time you click on your News Feed, every time you Like a photo, every time you send anything via Messenger, you add another data point to the galaxy they already have regarding you and your behavior.

Whenever you have dynamic interactions between 300 million people and the American economy acting in really complex ways, that introduces a degree of almost chaos theory to the system, in a literal sense.

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.