Alchemy is a science where one understands the structure of matter, breaks it down, then rebuild it. However is not an all-powerful technique, as one cannot create something out of nothing. If one wishes to gain something, one must present something of equal value. This is the concept of equivalent exchange, the fundamental basis of alchemy.

However, there is a taboo in alchemy. Human transmutation. One must not commit this act. 

(Source: deluxevanilluxe)