This fundamental resemblance between two such apparently incommensurable things as the buying of material commodities and the borrowing of capital is highly significant; it is another instance of that order in the economic world, of which the reader may now be growing weary.  But so difficult is it to see clearly and fully something which one sees, as it were, every day of one’s life, that a few more moments of reflection on the special case of capital will be time well spent.  Let us revert then to our fantasy of a world socialist commonwealth; and humbly submit another poser to its supreme executive.  The question this time will be whether some great constructional work, such, let us say, as the recently mooted Severn barrage scheme, should or should not be undertaken.  Let us suppose that the costs and future benefits of the undertaking can be estimated accurately; and that the problem reduces itself to one of expending now a sum, let us say, of $100,000,000, with the prospects of obtaining in the future an income of power, or whatever it may be, worth $5,000,000 per annum.  I have assumed for the sake of simplicity that we shall still be reckoning in terms of money, though possibly the executive may have substituted Marxian labor units; but it is quite immaterial to the present argument what the measuring rod may be.  The point to be observed is, that it is impossible to tackle the problem at all without the conception of a rate of interest.  For suppose that you tried to do without it, and said, “We shall take a long view.  The interests of the future are no less our concern than those of the present; we shall not discriminate between them.  We shall regard as an enterprise worthy to be undertaken whatever promises to yield in the course of time a return larger than the outlay.”  Where will this lead you?  The particular proposal set out above would clearly pass the test; for in twenty years the resultant benefits would have added up to a figure equivalent to the initial cost.  But equally clearly, the cost might have been more than $100,000,000; it might have been $250,000,000, $500,000,000, whatever figure you care to take, and if you extend the period similarly to fifty or one hundred years, sooner or later the gains would top the cost.  Now there is no limit to the enterprises which would pay their way on this basis; and it would be quite impossible to undertake them all.  For they would swallow up all and more than all your labor and your materials, and would leave you with no resources with which to meet the recurrent daily wants of men.  Clearly, then, in some way or other, you must pick and choose, you must reject some enterprises as insufficiently worth while.  But how would you proceed to choose?  Without a clear principle, a simple criterion to guide you, you would be plunged in utter chaos.  You could not say, “Let all proposals involving capital expenditure be submitted to a central committee, who shall compare them with one another in a sort