Some friends have been debating what Keynes thought of Marx and wanted to get hold of a letter he wrote to Bernard Shaw about this. So I hunted down volume 28 (Social, Political and Literary Writings) of The Collected Writings of John Maynard Keynes, with the assistance of my sister and the helpful librarians in the Christ Church library.
The letter (to George Bernard Shaw, 2 December 1934) is not yet out of copyright, so I give just the portion of it that bears on this.
My feelings about Das Kapital are the same as my feelings about the Koran. I know that it is historically important and I know that many people, not all of whom are idiots, find it a sort of Rock of Ages and containing inspiration. Yet when I look into it, it is to me inexplicable that it can have this effect. Its dreary, out-of-date, academic controversialising seems so extraordinarily unsuitable as material for the purpose. But then, as I have said, I feel just the same about the Koran. How could either of these books carry fire and sword round half the world? It beats me. Clearly there is some defect in my understanding. Do you believe both Das Kapital and the Koran? Or only Das Kapital? But whatever the sociological value of the latter, I am sure that its contemporary economic value (apart from occasional but inconstructive and discontinuous flashes of insight) is nil. Will you promise to read it again, if I do?
Could modern critics of Keynes level the same criticisms against The General Theory (perhaps excepting the bit about carrying fire and sword around half the world)?
Personally I think most of what passes for macroeconomics makes scholastic physics look good — that the field is clearly pre-scientific — so I'm not convinced it's that easy to dismiss Marx (or Keynes for that matter) as having been entirely superseded by newer theory.