… is from pages 200-201 of the hot-off-the-press collection of previously unpublished (or only obscurely or inaccessibly published) essays by Milton Friedman, The Indispensable Milton Friedman  (Lanny Ebenstein, ed. 2012); specifically, it’s from Friedman’s 1983 Economist essay, “The Keynes Centenary: A Monetarist Reflects”:
Keynes’s flexibility was both a virtue and a vice. It enabled him to adapt his ideas and proposals promptly to changing circumstances. But it also meant that he was something of a fine-tuner, that he tended to neglect the cumulative effect of short-run policies. The truth that in the long run we are all dead needs to be balanced by the equally relevant truth that the long run consists of a succession of short runs.
Keynes’s flexibility and fine-tuning propensities were in accord with his elitist political philosophy, his conception of a society run by an able corps of public-spirited intellectuals entitled to power that they could be counted on to exercise in the interest of the masses.