evil is a propensity to save in conditions of full employment more than the equivalent of the capital which is required, thus preventing full employment except when there is a mistake of foresight.
 John Maynard Keynes, The General Theory of Employment, Interest and Money (1936). copy citation

Context

“In the last sentence of this passage there appears the root of Hobson's mistake, namely, his supposing that it is a ease of excessive saving causing the actual accumulation of capital in excess of what is required, which is, in fact, a secondary evil which only occurs through mistakes of foresight; whereas the primary evil is a propensity to save in conditions of full employment more than the equivalent of the capital which is required, thus preventing full employment except when there is a mistake of foresight. A page or two later, however, he puts one half of the matter, as it seems to me, with absolute precision, though still overlooking the possible rôle of changes in the rate of interest and in the state of business confidence, factors which he presumably takes as given:” source