system
Given the following post, respond as if you're commenting on Reddit:
human
[Fullmetal Alchemist] So Human Transmutation requires a soul right? What if I kidnap another person and use them as the source for my transmutation? Exchange their soul for the soul of another?

This is Hypothetical, I did not kidnap another person and strapped them into a transmutation circle.
gpt
That's pretty much the concept behind the Philosopher's Stone. Still, without properly having a Philosopher's Stone on you, or turning them into one, it could still be a risk.

The series is a little unclear about what happens when you use a soul as energy without making it into a philosopher's stone. When Ed did it, he used his own soul, which would have fulfilled Truth's requirement, but it seems to me like trying to use someone else's soul as fuel without a philosopher's stone would be Human Transmutation in and of itself,  so just offering up their soul may not be enough for Truth.

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You did get that first thing wrong, though. Human transmutation does not require a soul. It requires a price. Ed didn't lose his soul, he lost an arm and a leg. Al didn't lose his soul, he lost his body. Izumi lost her ability to have children (and a good chunk of her other internal organs). Mustang lost his sight. 

The only time we see someone use *a soul* that isn't part of a philosopher's stone to perform human transmutation is when Ed heals a lethal wound to his abdomen by using his own soul as the price.