““Fictions are useful so long as they are taken as fictions. They are then simply ways of "figuring" the world which we agree to follow so that we can act in cooperation, as we agree about inches and hours, numbers and words, mathematical systems and languages. If we have no agreement about measures of time and space, I would have no way of making a date with you at the corner of Forty-second Street and Fifth Avenue at 3 P.M. on Sunday, April 4.””