Durability seems at the outset a very simple element in the character of a tool. Yet all the complexities of investment centre on it. If the values of the services that a tool may be able to render in the distant future are highly uncertain and unknowable, why trouble to make tools durable? Why go in for durability, when it is no more than a road lost in the abyss of time? Why not use non-durable tools? The reason lies, we may fairly say, in the nature of physical things, in the unarguable facts of technology. A tool can be built with nearmiraculous powers, effecting huge economies of human effort or doing what no number of men without its aid could ever do; fabricating, shaping or assembling with humanly incomparable precision, force and speed; computing beyond anything the human brain can compass in a lifetime; condensing and concentrating vast energies into infinitesimal spaces and moments of time. But such a tool is so expensive to make, that if it could only be used on a single occasion, or only for a day or a week, even such transcendent capabilities could not enable it to earn its first cost. Durability gives us the only hope that the most expensive tools can pay for themselves. To say that the prospect of their paying for themselves depends on a conjectural future (which is true) is simply to say that we either accept this uncertainty or totally renounce the help of such tools. Thus, then, the problem of fitting an act of investment into a policy which can commend itself, is the problem of finding for such acts a frame of thought which can relate them to the visible outlines of a firm's circumstances.