Alchemy's First Law of Equivalent Exchange

The first law of alchemy states that humankind cannot gain anything without first giving something in return; to obtain something of equal value, something must be lost. 

However, there is no equivalent exchange in human lives, as illustrated by these characters:

The Elric brothers

• Edward lost his legs
• Alphonse was close to death

Izumi Curtis

• She lost part of her organs while trying to resurrect her stillborn child

This shows the value of life is priceless. One can try and save a life by alchemy, only to realise nothing could ever be close to a life.