There is reason to believe that Keynes himself did not truly understand his own analysis

From The General Theory [1946]. In: Lekachman R. (eds) Keynes’ General Theory. Palgrave Macmillan, London, chapter by Paul Samuelson, starting page 315.

I have always considered it a priceless advantage to have been born as an economist prior to 1936 and to have received a thorough grounding in classical economics. It is quite impossible for modern students to realize the full effect of what has been advisably called "The Keynesian Revolution" upon those of us brought up in the orthodox tradition. What beginners today often regard as trite and obvious was to us puzzling, novel, and heretical.

To have been born as an economist before 1936 was a boon - yes. But not to have been born too long before!

Bliss was it in that dawn to be alive,
But to be young was very heaven!

The General Theory caught most economists under the age of thirty-five with the unexpected virulence of a disease first attacking and decimating an isolated tribe of south sea islanders. Economists beyond fifty turned out to be quite immune to the ailment. With time, most economists in-between began to run the fever, often without knowing or admitting their condition.

I must confess that my own first reaction to the General Theory was not at all like that of Keats on first looking into Chapman's Homer. No silent watcher, I, upon a peak in Darien. My rebellion against its pretensions would have been complete, except for an uneasy realization that I did not at all understand what it was about. And I think I am giving away no secrets when I solemnly aver upon the basis of vivid personal recollection-that no one else in Cambridge, Massachusetts, really knew what it was about for some twelve to eighteen months after its publication. Indeed, until the appearance of the mathematical models of Meade, Lange, Hicks, and Harrod, there is reason to believe that Keynes himself did not truly understand his own analysis.