Keynes’ General Theory was a huge step forward relative to classical economics, but it was also a step backward in its denial of the conflictual nature of capitalism. There is need to understand Keynes’ technical contributions regarding the workings of monetary economies, but also need to understand the flaws within his thinking and the consequences thereof. Keynes made a fundamental contribution elucidating the mechanism of effective demand, and he also has claim to be the preeminent monetary theorist. However, his critique of classical economics was technocratic and focused on the interest rate mechanism. Reflecting his own liberal political economic disposition, he denied the conflictual nature of capitalism. That is the “original sin” in Keynesian economics, and it has far-reaching implications. It explains why establishment Keynesian economics struggles to explain the current tendency to stagnation. Even more importantly, it has kept economics locked into a false conception of capitalism that undermines the case for Social Democracy.