The author looks at Keynes's concept of the marginal effiiency of capital in Book IV of The General Theory, which is meant to analyse the factors determining 'The Inducement to Invest'. Keynes singles out two major sources of inducement. First : the state of long-term expectations' - the result of entrepreneurs' 'animal spirits - and a spontaneous urge to action rather than inaction'. It is 'autonomous', with respect to formal economic analysis. Second: a careful economic calculus. Let us call it 'endogenous' investment. It is with reference to the endogenous source of investment that Keynes coins the analytical concept of the 'marginal efficiency of capital'.