Fullmetal Alchemist: Brotherhood is routinely classified as one of the best anime ever made, and for good reason. Not only is it an exciting, action-packed shonen series with fabulous character development, it also has some seriously deep symbolism that adds an extra layer of intrigue.
Some symbolism seems obvious; most viewers understand the Homunculi are named after the Seven Cardinal Sins. However, it's easier to miss the way all Homunculi (aside from Gluttony) punish people who commit the sin they represent but are ultimately wiped out by the same sin.
Other bits of symbolism are more subtle, like the connection between Truth, the Gate, and a religious philosophical movement called theosophy.
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Truth And The Gate Are Concepts From Theosophy