Greg Mankiw links to a brief talk given by fellow Harvard professor Steve Marglin as part of an "Occupy Harvard" teach in. Marglin offers a course via the course called "Economics: A Critical Approach", and most of this talk is devoted to explaining why he thinks it's important to criticize "mainstream economics" and how Marglin was unable to get much of a hearing from his fellow members of the Economics Department at Harvard -- indeed he says he basically doesn't talk to them. He complains that the economics profession has become monolithic in accepting certain things, such as the efficacy of markets and the importance of efficiency, which he believes should not be so uncritically accepted, and as such that basic economics textbooks and courses (whether taught by "conservative" economists such as Mankiw or liberal ones such as Krugman) offer a univocal and uncritical examination of the issues at play.
There are actually two very different types of critique of mainstream economics which Marglin says that he deals with, though it's not clear from his talk whether he sees these types of critique as distinct. Firstly, there are what I would term economic critiques of "mainstream economics". One he mentions in particular is Keynes' critique of the idea that the normal state of an economy is full employment. I'm not clear that full employment is widely assumed to be a natural state for an economy these days -- certainly, economies with truly "full employment" are virtually never seen outside of very unusual circumstances. But this is definitely an economic critique. He also mentions more vaguely some "very good" Marxist critiques of economics. He doesn't go into detail as to what these are, but I would assume that these also deal with actual economics.
Most of the critiques he discusses, however, are what I would term moral critiques of economics. He talks about a critique from inequality -- as to whether a distribution of income in which some people have much and many others have little is right. He talks about an environmental critique -- as to whether means of growth are sustainable and environmentally just. He talks about Catholics Social Teaching (and though he doesn't state what this critique is, I assume he must then talk about the concept of the "universal destination of goods"), etc.
Now, although I imagine that Marglin and I see eye to eye on few political issues, I would very much agree that "what is economically efficient?" is very from from being the sum and total of thought one should give to many economic problems. Economics, to the extent that it is a study of how things work, doesn't answer questions of "what ought I to do". At best, it gives some idea of what happens in a system when you make some change. While it may tell you what the results of an action might be, it won't tell you whether the action (or the results) are good.
Personally, I'm fine with that. I don't see that economists have any especial ability to discern "the good", and so I'd be quite happy to have them stick to "what happens" and let people go off and discuss "what is 'the good'" via more established means of philosophy and theology. I like the idea of economics being a lesser science.
Marglin, however, seems to want to pull those questions of "ought" into economics itself and address them there. In this sense, his approach reminds me a bit of the Creationist or Intelligent Design objection that the physical and biological sciences don't give sufficient consideration to God and morality. For my part, while I can see the value of addressing economic critiques of mainstream economics in an economics course (though clearly, as Marglin concedes, one would actually need to learn the mainstream economics first before the critique) I don't think that addressing moral critiques of economics in an economics course is necessarily a good idea -- other than acknowledging that economics is not itself a full life guide to policy and action.
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