In 'My Early Beliefs' he lectures a younger generation of (Marx-influenced) Cambridge students on his own debt to the founder of English analytical philosophy G.E. Moore. Moore, unread today, was part of a reaction against the Hegelian social philosophy of T.H. Green, Edward Caird and MacTaggart that identified the forward march of the Empire with social progress.
Moore, with Russell, and later, Wittgenstein, were trying to develop a philosophy that might best be described as "anti-essentialist", i.e. it refused the distinction between 'appearance' and 'essence', and ruled impermissable all appeals to 'underlying forces' and so on. Moore called such beliefs the 'naturalistic fallacy'. Much of what they argued (though they were never credited with the insight, because the English analytic philosophy was in turn rubbished by the post-structuralists) has, in a confused way, become the orthodoxy in the humanities and social sciences.
The appeal for Keynes is understandable. He was trying to save economic theory from the dogma that markets were 'self-equilibriating'. To him that seemed like a Newtonian conception of underlying forces that simply were not present, and could provide no confidence in the prospect that the market would correct itself. Moore's anti-essentialism gives Keynes' economic theory a philosophical justification.
It is not surprising, then, that Keynes was quite so indifferent to Marx's charms. Marx, despite attempts to read him otherwise, is in his method, very much an Hegelian. His account of the laws of capitalist development rests four-square on the distinction between immediate appearance, and underlying essence. His entire approach to theoretically reconstructing capital, starting from its simplest abstract form, the commodity, would have struck Keynes as an otiose architecture. The very idea of there being an intangible social essence - value - behind the manifest appearance of relations of exchange in prices would have struck Keynes as an unnecessary fiction.
Modern readers find Marx tough, because they are trained in reading habits that are closer to Moore's anti-essentialism than to Marx's Hegelianism. Keynes irritation is repeated by thousands of undergraduates every year, who quickly move on, relieved to Weber, and forget all about Marx's method, hanging onto a few slogans about 'class struggle' and 'gravediggers' etc..