Alchemy, the science of understanding deconstructing and reconstructing matter, however, it is not an all-powerful art.[.....] If one wishes to obtain something, something of equal value must be given. This is the law of equivalent exchange,[...]. In accordance with this law, there is a tabu among alchemists: Human transmutation is strictly forbidden, for what could equal the value of a human soul?
Fullmetal Alchemist Brotherhood - Pre-Opening
([...] = Stückchen ausgelassen, nicht wichtig, damits nicht zu lang wird)