We shall only speak of the three principal arms: Infantry, Cavalry, and Artillery.
We must be excused for making the following analysis which belongs more to tactics, but is necessary to give distinctness to our ideas.
The combat is of two kinds, which are essentially different: the destructive principle of fire, and the hand to hand or personal combat. This latter, again, is either attack or defence. (As we here speak of elements, attack and defence are to be understood in a perfectly absolute sense.) Artillery, obviously, acts only with the destructive principle of fire. Cavalry only with personal combat. Infantry with both.
In close combat the essence of defence consists in standing firm, as if rooted to the ground; the essence of the attack is movement. Cavalry is entirely deficient in the first quality; on the other hand, it possesses the latter in an especial manner. It is therefore only suited for attack. Infantry has especially the property of standing firm, but is not altogether without mobility.
From this division of the elementary forces of war into different arms, we have as a result, the superiority and general utility of Infantry as compared with the other two arms, from its being the only arm which unites in itself all the three elementary forces. A further deduction to be drawn is, that the combination of the three arms leads to a more perfect use of the forces, by affording the means of strengthening at pleasure either the one or the other of the principles which are united in an unalterable manner in Infantry.