Tobin takes the concept of the required rate of profits as the key variable in modern Keynesian growth models.3 In order to forge a place for portfolio balance in the analysis, money is introduced to compete with the required rate of profit on physical capital for a place in households’ portfolio holdings. ‘In a closed economy clearly the important stores of value are monetary assets. It is their yields which set limits on the accept­ able rates of return on real capital and on the acceptable degree of capital intensity’.4