This mutuality between civilization and the gun is evident in the relations between civilization and every other great invention. It is very clearly evident in the case of material mechanism; for it has been plainly impossible for any material invention to exist without directly and indirectly contributing to the improvement, and even to the birth, of others. Any improvement in the process of making any metal or any compound has always been of assistance to every mechanism using that metal or that compound; and it seems impossible to name any mechanism or process whose invention has not helped some other mechanism or process. In the matter of the invention of immaterial things, the effect may not be quite so obvious; and yet it is plain that most of those inventions have contributed to the safety, intelligence and stabilization of peoples, and therefore to a condition of mentality and of tranquillity that permitted and often encouraged the improvement of existing appliances, and the invention of new ones. Of one class of immaterial invention, such as new books on the physical and engineering sciences, the influence on material inventions is, of course, as obvious as it is profound. The boom of the gun may be said, by a not forced figure of speech, to have ushered in the new civilization that rose from the mental lethargy of the Middle Ages; for it was the first great invention of all in the long line that have followed since. As it was the first, and because without it the others would have been impossible, we can hardly avoid the conclusion that it was the most important.