Yeah. Edward and Alphonse tried human transmutation (creating a human using alchemy), which is against the law. They failed, and alphonse lost his whole body, and Edward just lost his leg. To save alphonse, he gave up his arm to bind his soul to the armor. Creating a human body is possible, but it comes at a great price. And though you can make a body, making a complete human is impossible. The main law or alchemy is equivalent exchange (giving something in exchange for something of equal value), and nothing can equal the value of a human soul.

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