People cannot gain anything without sacrificing something. You must present something of equal value in order to gain something. That is the principle of equivalent exchange in alchemy.
"In the early days, we believed that to be the one truth in the world." -Alphonse Elric
Humankind cannot gain anything without first giving something in return. To obtain, something of equal value must be lost. That is Alchemy's First Law of Equivalent Exchange.
"Alchemy: the science of understanding the structure of matter, breaking it down, then reconstructing it as something else. It can even make gold from lead. But alchemy is a science so it must follow the natural laws: to create, something of equal value must be lost. This is the principal of equivalent exchange. But I learned that night that some things cannot be measured on a simple scale. My brother and I knew the laws of science, of equivalent exchange. The game required sacrifice, that something had to be taken from us, but we thought there was nothing more we could loose....We were wrong."
~Edward Elric (Fullmetal Alchemist)