Keynes was undeniably a genius of his time, but he wasn’t infallible. We should not just assume that he was always correct. As with all prolific writers we can cherry-pick quotations to suit our own political purposes. If we want to argue for more government spending, we can use this:
For the proposition that supply creates its own demand, I shall substitute the proposition that expenditure creates its own income.
(Collected Writings of John Maynard Keynes, Volume XXIX, pp 80-81)
Keynes meant that the mere supply of a commodity is not enough to ensure the sale of that commodity, but money from all government spending inevitably ends up in someone’s pocket. This is a statement of the obvious, maybe, but he evidently felt it needed making anyway. On the other hand, if we are suspicious of what sounds like “magic money tree” economics, as many scathingly describe any deviation from their understanding of ‘sound money’, we can find this quotation: