Douglas Adams : Forty-two.
David Hume : Out of custom and habit.
Carl Jung : The confluence of events in the cultural gestalt necessitated that individual chickens cross roads at this historical juncture, and therefore synchronicitously brought such occurrences into being.
B.F. Skinner : Because the external influences which had pervaded its sensorium from birth had caused it to develop in such a fashion that it would tend to cross roads, even while believing these actions to be of its own free will.
Aristotle : To actualize its potential.