Brad Delong is complaining about Marx, and defending Keynes from charges of being a central planning advocate again. I wrote two comments, and these seem to have merged into one thing. So far, Delong has not run them, and I don't know that he will--he only runs a few of my comments. So here they are.
"A somewhat comprehensive socialisation of investment" (as Keynes advocated, in his own words) is pretty damn socialist, really. It strikes right to the heart of wealth and power, in arguing for democratic governance of the financial system. But Keynes was not an advocate of minutely-detailed planning, no. Is this unfair to the rich? I suppose so. But Keynes goes on to explore the alternatives and finds them wanting.
A Marxist might say that Marx's "dictatorship of the proletariat" would use Keynsianism. Indeed, what else could it use? But for Marxists a Keynsian system would be only a step on the way to a fully Communist system, which did not use money at all.
Delong: "[Marx] believes there's something wrong about credit."
He believed there was something wrong about money, at least if he held to the viewpoint expressed "The Power of Money" (1844.) In TPOM Marx objected that money falsifies human relations, a point still worth attending to, but one difficult to erect an economics on. To some extent it seems to me that, in that essay, Marx was objecting to the constraints of physicality and embodiment.
I think Delong was objecting that Marx had not read, or was not, Keynes. But Keynes read Marx, not the other way around, and I do not see how it could have been otherwise. Is it not possible to regard Marx as an important and useful historical theorist or philosopher, without insisting that he was entirely right or entirely wrong? A forerunner of a still-emerging social science, in the way that, perhaps, Copernicus was a forerunner of what has since become physics? The Copernican system still used perfect circles and epicycles; it was not particularly useful for computation. But by putting the Sun at the center, Copernicus made it possible for Kepler and then Newton to do their work. Marx put system at the center of economics, rather than the wealthy or powerful, and paved the way for thinking about economics without defining economics in terms of central figures or groups. Which might put Keynes in the position of Kepler or Newton. And, as with physics, economics advances, one tombstone at a time. It is only 65 years since Keynes death; only a few more years since his major works were published. That is not so very long for a revolution in thought.