From The Sciences, Sept/Oct 1994: Harvard economist Robert Dorfman on half a dozen principles that underlie economics. 1) Always think in alternatives; if there is no alternative, there is no economic problem. 2) Look for trade-offs. If one wants to protect the environment, what does one have to give up? 3) The question is never whether to do something or not, but always how much of it to do -- if anything at all. 4) Practically nothing is indespensable; there is a substitute for almost everything, even if not quite as satisfactory as the first choice. 5) Never forget that everry transaction is two sided: every purchase is a sale, and vice versa. 6) Analyze every situation as a game in which each participant responds to the actions of the others in the way most favorable to himself; the other participants take that into account and so on ad infinitum.