(I) There is first of all, at least in great inventors, an inborn quality,--that is, a natural disposition,--that does not originate in experience and owes the latter only its development. This quality is a bent in a practical, useful direction; a tendency to act, not in the realm of dreams or human feeling, not on individuals or social groups, not toward the attainment of theoretical knowledge of nature, but to become master over natural forces, to transform them and adapt them toward an end. Every mechanical invention arises from a need: from the strict necessity for individual preservation in the case of primitive man who wages war against the powers of nature; from the desire for well-being and the necessity for luxury in growing civilization; from the need of creating little engines, imitating instruments and machines, in the child. In a word, _every particular invention, great or small, arises from a particular need_; for, we repeat again, there is no creative instinct in general. A man distinguished for various inventions along practical lines, writes: "As far as my memory allows, I can state that in my case conception always results from a material or mental need.[124] It springs up suddenly. Thus, in 1887, a speech of Bismarck made me so angry that I immediately thought of arming my country with a repeating rifle. I had already made various applications to the ministry of war, when I learned that the Lebel system had just been adopted. My patriotism was fully satisfied, but I still have the design of the gun that I invented." This communication mentions two or three other inventions that arose under analogous circumstances, but have had a chance of being adopted. Among the requisite qualities I mention the natural and necessary preeminence of certain groups of sensations or images (visual, tactile, motor) that may be decisive in determining the direction of the inventor. (II) Mechanical invention grows by successive stratifications and additions, as in the sciences, but more completely. It is a fine verification of the "subsidiary law of growing complexity" previously discussed.[125] If we measure the distance traversed since the distant ages when man was naked and unarmed before nature to the present time of the reign of machinery, we are astonished at the amount of imagination produced and expended, often uselessly lavished, and we ask ourselves how such a work could have been misunderstood or so lightly appreciated. It does not pertain to our subject to make even a summary table of this long development. The reader can consult the special works which, unfortunately, are most often fragmentary and lack a general view. So we should feel grateful to a historian of the useful arts, L. Bourdeau, for having attempted to separate out the philosophy of the subject, and for having fastened it down in the following formulas:[126] (a) The exploitation of the powers of nature is made according to their degree of power.