Something I've been pondering for a while:
When I attempt to explain the principle of Equal Exchange and why human transmutation never works, the thing I often emphasize is that nothing is equal to the value of a human soul. This often brings up the question "then how did he get Al's soul back by just sacrificing an arm? Was Al's soul only worth an arm?" Then there's also the questions of why Ed lost only his leg when Al did his whole body...
I've thought alot about it, and here's my conclusion:
It is true that nothing can be equally paid to *recreate* a human soul. I believe that what Edward did was retrieve a soul that hadn't completely crossed over or dissipated-it was still within the threshhold. He paid his arm to "reach out" and bring it back.
And as for Al's body:
Trisha Elric was an adult human, and with the exception of her illness, in perfect condition. She was a complete human body. Alphonse Elric was a ten year old boy, still in his growing stages and he had not yet reached the peak of his life, or adulthood. Therefore, he could have been called an incomplete human body. The deformed creature that was birthed from the transmutation was incomplete. The Gate took Al's body, and gave them that.
An incomplete human body for an incomplete human body.
And as for Edward's leg:
The vast amount of information he received from the Gate is seen by so few, it had to have had a hefty price on it. Transmuting without a circle means you've tapped into the flow of the universe itself. Part of yourself to know your place in the cosmos, that is what I belive he and Izumi paid for that knowledge.
His left leg for the knowledge.
And those are my thoughts thusfar. I hope you enjoyed my first entry, I didn't have time to look back through the others at this moment. I would appreciate the feedback, and I hope I've brought something of interest to the table.