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.