From Robert Skidelsky:
"Fundamental to Keynes are the concepts of uncertainty and under- employment equilibrium. From Hobson we get an understanding of how inequality of wealth and income makes crisis more likely and recovery more difficult. From Marx we get an explanation of why inequality of wealth and income is inherent in an unmodified capitalist system. We need to put together the three accounts in order to achieve a fuller understanding of the events through which we have recently been living." (the rest: see here).PS: A detailed presentation of classical-Keynesian political economy, which is not Skidlesky's view of Keynes, by the way, is provided by Henrich Bortis - see here.