Alfred Marshall, who on this, as on so many other things, laid down the rules to which economists have since adhered, noted that “the economist studies mental states rather through their manifestations than in themselves; and if he finds they afford evenly balanced incentives to action, he treats them prima facie as for his purposes equal.”5 He almost immediately added that this simplification, which indeed has rendered profound scientific service to economics, was only a “starting-point.”
