Economics, even capitalist economics, assumes that there are desiring beings, but it does not ask why these beings desire one thing rather than another. Economists say that they are interested not in the reasons for people’s preferences, but in revealed preferences, i.e. preferences that are enacted. Economics does not ask about the reasons for these preferences – why would a mass of people prefer to buy cocaine rather than books or cruise vacations...? – it takes into account and calculates the preferences expressed, i.e., the exchange that is acted out. Exchange is an act, not a belief, and it is a more or less risky exchange.
Dixxit 
