Or so says Brad DeLong in an insightful contribution to the ongoing debate on economic heterodoxy. His point, I think, is not just that neoclassical economics provides the right (sorry!) way to think, but also that it gives you plenty of ammunition for tearing to shreds market fundamentalists. But then this raises an obvious question: why are the heterodox critics of "mainstream" economics mostly on the left of the political spectrum?
Every first-year graduate student learns the First Fundamental Theorem of Welfare Economics, which says essentially that provided a long list of conditions are satisfied, a market equilibrium is efficient in a particular way--that is, you cannot make someone better off without making someone else worse off. Now you can read the theorem in two, radically different ways. One is to say: "There you have it! We knew Adam Smith was right all along, but here it is stated in mathematically precise way and proved to everyone's satisfaction. Now let the government get out of the way and have the markets work their magic." The other is to say: "Wow, hold on! You mean we need so many conditions for markets to produce efficient outcomes? No externalities, no returns to scale, no market power, markets for everything and for every point in time... I better get my theorems of the second-best straight!"
If you are the first kind of person... Well, I have never understood how you could be the first kind of person if you have also understood the First Fundamental Theorem of Welfare Economics. Which is what I think Brad is also saying (although you would not know it from some of his posts...)