This chapter discusses an illustration that enables us to understand what order is and what it implies. First of all we know that order never exists without a plurality of elements. The simple has no need of being placed in order. Unity is the second element of order. The arrangement of height in a regiment, which from left to right, from right to left, places the soldiers according to an ascending or a descending scale, rests on the principle of unity recognized by the senses: the size of the body. The same holds true in different arts: the Roman or Gothic arch, the magnificent repetitions of which produced the naves of the churches and the cathedrals of France, are linear forms seen with the eyes but perceived in an abstract way as the principle of unity.