To work iron, a hammer is needed, and to have a hammer, it must be made. For this purpose there is need of an other hammer and other tools, and again to get these there is need of other tools, and so on to infinity. In this way one might try to prove, in vain, that men have no power to work iron. But the fact is that at first, with the tools they were born with, men succeeded, however laboriously and imperfectly, in making some very simple things; and when these were made they made other more complex things with less labour and greater perfection; and thus advancing gradually from the simplest works to the making of tools, and from tools to other works and other tools, they have reached a point where they can make very many complex things with little labour. In just the same way the intellect by its inborn power' makes intellectual tools for itself by which it acquires other powers for other intellectual works: and from these works still other tools - or capacity for further investigation - and thus makes steady progress until it reaches the summit of wisdom.