A convention is a social pattern we have chosen to prefer over whatever the raw world simply proffers. It is a sign of the operation of the mind, drawing the assent of a sufficient number of other minds so that the agreement will be widely operative. A convention is not a custom; a custom is a habit in which a sufficient number acquiesce. A custom can appear as a convention, but it is really a lesser act, the result of passive acceptance rather than of the imposition of design. It is the difference between learning to live by the annual flooding of the river or by a calendar.