  If \(K\) is the stock of capital of the economy’s one and only commodity, output is taken to be \(F(K)\), where \(F(0) = 0\) (i.e., output is zero if there is no capital), \(dF(K)/dK \gt 0\) (i.e., the marginal product of capital is positive), and \(d^2 F(K)/dK^2 \le 0\) (i.e., the marginal product of \(K\) does not increase with \(K\)). \(F(K)\) is a flow (production at a moment in time), in contrast to \(K\), which is a stock (quantity of capital, period). Notice also that output depends solely on the stock of capital. No mention is made of possible improvements in the quality of capital or labour. Thus, there is no prospect of technological progress or accumulation of human capital in Ramsey’s model (but see Mirrlees, 1967, for one of the first of many extensions of the Ramsey model that include technological advances in production and human capital formation); nor are there any natural resources in the model (but see Dasgupta and Heal, 1974, for one of the first of many extensions of the Ramsey model that include natural capital in production).