… is from page 249 of Vol. 9 of Hayek’s Collected Works; here – in an essay written in 1983 and originally published in The Economist – Hayek is discussing J.M. Keynes:
In these theoretical efforts he [Keynes] was guided by one central idea – which in conversation he once described to me as an “axiom which only half-wits could question” – namely, that general employment was always positively correlated with the aggregate demand for consumer goods. This made him feel that there was more truth in that underconsumption theory preached by a long row of radicals and cranks for generations but by relatively few academic economists. It was his revival of this underconsumption approach which made his theories so attractive to the Left.