The reason (理 kotowari) is the absolute fundamental of the world. It consists of the four elements: fire, earth, water and wind and the Two Great Origins (二大起源 Ni Dai Kigen), light and darkness.
There exists one known mortal being who is outside of the reason: Valkenhayn R. Hellsing, the implied reason behind this is that he was created from sorcery. Another being who existed outside of the reason before its death was the Black Beast; because it existed outside of the fundamentals, it was close to invincible, as not even nuclear weapons could dent its hide. To counter it, weapons had to be created outside of the reason to fight back; Magic Formula, Magic Formula Armament and the Arch-Enemy Event Weapons were the eventual products. Rachel Alucard also exists outside of the reason, although it is implied that all vampires are.
In the ancient past, humanity created magic in order to combat beings that existed outside of the reason. Sorcery is actually slightly related to the reason, unlike magic, which utilizes only magic element. An example being that a flame created from the practice will still burn a piece of paper like an ordinary flame would; what separates the average flame and the one created via sorcery, however, is that a flame created through sorcery can harm those outside of the reason.