Law of Equivalent Exchange

This is from a TV show called Fullmetal Alchemist. I'm not even sure that it belongs in this category, but in the beginning of every episode it says: "Humankind cannot gain anything without first giving something
in return... To obtain, something of equal value must be lost." I don't mean, I can get this for free, ha ha, law broken. I know it's just a TV show, but is there any merit to this?

 

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By what objective methods would one measure that which is obtained versus lost and come to conclude that they are "equal" in value?

 

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Physics?

Isn't that just a mystical rephrasing of physical law?

 

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As the show uses it, it is more mystical (though it also has specific quasi-scientific applications in the show as well). On the mystical side, for example, the main protagonist is able to restore his brother's body to full health because of the many hardships they endure and good deeds they perform over the course of several years. The self-sacrifice and hardships are deemed to be an equivalent exchange for the fulfillment of a particularly "expensive" wish.

On the more natural side, the show assumes that, if you have the right amount of the right elements, then alchemy can transform them into some other equivalent form composed of roughly the same materials (or in some cases, materials of different elements with similar properties, akin to the fabled changing lead into gold paradigm).

In that more specific sense it is akin to a law of conservation of matter. In the broader story arc though, the characters refuse to ask their friends and colleagues for favors for fear of what "equivalent exchange," might require from them in return. In that broader sense it is more like a principle of cosmic fairness.