The Truth

Truth is a metaphysical being who appears when a person attempts    Human Transmutation. Seeing Human Transmutation as an unfair exchange according to the equivalent exchange laws of Alchemy, Truth will bring the alchemist into the   Gate   and forceably take their most valued attribute, or something of value that represents their anticipated relationship to the person they are trying to create or resurrect. Truth will accept otherwise impossible exchanges when a  Philosopher's Stone  is used, as it or part of it will be used for the exchange instead.

As a negative version or "conscience" of the alchemist who performed Human Transmutation, Truth is perceived by the finite human mind as punishing them for "Playing God". As a "God", or "the Universe", Truth embodies the universe, serves as both a deity and Monad for it, and also represents the consciousnesses and consciences of living things. Truth oversees everything in the universe, and regulates all Alchemic exchanges in the world.

Aside from humans, all living things possess their own Gate of Truth, through which they can see Truth (or, speculatively, their own cognitive perceptions of the actual Truth that their limited 3-dimensional intelligences make up in a vain attempt to comprehend Truth). In the series, the Gaea hypothesis is proven by Father, and Father subsequently reveals that heavenly bodies, like planets and stars, also have their own Gates since they are "life forms".

Depiction
Truth is depicted as a white, featureless version of whomever passes through the Gate; this is because truth serves as one's own conscience. Truth can be seen being in possession of items that it took from the Alchemist when they opened the Gate. In Edward's case, Truth appears as a featureless young boy, but later possesses Edward's arm and leg which were paid as an equivalent exchange for seeing Truth, and getting Alphonse's soul back. In Alphonse's case, Truth possesses his entire body, and appears as such.

Due to their relative appearance, when Edward and Alphonse Elric each appeared before the Gate, Truth vaguely resembled a young boy and when Izumi Curtis appeared, Truth's shape reflected that of a young woman. It appeared as a vigorous man to Roy Mustang, and when the Dwarf in the Flask entered the Gate, he appeared as the Dwarf's original form when he lived in the flask.

Manga and 2009 Anime
Truth introduces themselves as the being some people call God, the World or the Universe, at first seeming welcoming and jovial to its guests before calling attention to their hubris and opening the Gate, which has forced them inside earlier and now forces them outside.

The Gate is always opened through human transmutation, and Truth takes a toll.

Due to Edward Elric opening the gate in an attempt to bring his mother back through human transmutation, Truth takes Edward's left leg and his brother Alphonse. Both had encountered him as they were forced to pay their tolls.

Though Truth appears in Edward's dreams, mocking him, Truth's next true appearance is in Chapter 53, when Edward crosses to the true Gate from inside Gluttony's internal imitation. Truth appears amused that Edward has not come to retrieve anything that he lost on that day four years ago. Interestingly, Truth does not seem to appear before Edward as he discovers Alphonse's Gate and the body that sits before it, but - as the body speaks to him even while its soul is in the mortal plane - it is likely that what Edward sees and speaks to is Truth speaking through Alphonse's body, especially since the body is seated in the same manner as Truth whenever it appears.

Izumi Curtis attempted to bring back her stillborn child through human transmutation, and Truth took some of her internal organs, presumably including part of her reproductive system, leaving her incapable of ever becoming pregnant again and with chronic haematemesis.

It is unconfirmed if Hohenheim ever saw the Gate, but it can be presumed that his alchemy technique that doesn't even require the normal hand gesture of clapping one's hands together is obtainable by paying an even greater toll upon reaching the gate, letting him see even more of the Truth beyond it.

Roy Mustang, who was forced to open the gate by Pride (with the aid of Wrath). Regardless of this conscious decision to not perform human transmutation, Truth still had him pay the toll by taking his sight. It is imperative to note, however, that Truth also took away most of Pride's strength (or souls that make up his Stone), as Pride used himself as the human transmutation circle, and this may have contributed towards the toll that was paid for opening Mustang's Gate. Unlike the Elric brothers, Mustang went through with the choice to use a Stone as a replacement for his toll.

In Chapter 102, Father discusses the irony behind the works of the 'Truth', who took Ed's way to 'stand by himself' and his 'only family', Al's body so he 'cannot feel the mother's warmth as he craved', Izumi's 'capacity to nurture the seed of life' and, now, with Mustang 'depriving the man who had a grand vision to save his country of his eyesight, denying him to see what his beloved nation will become'. In Father's perspective, the Truth is cruel and sadistic, whose only ambition is to make people suffer.

In Chapter 107, Truth appears once more before Alphonse once his body and soul are finally reunited. Truth returns Edward's arm in exchange for Alphonse's soul, and questions the younger Elric if his brother will return for him and what he'll sacrifice.

In Chapter 108, Truth appears before Father and mocks him for trying to claim the power of God for himself. It then banishes Father to depths of the Gate, where he had presumably come from. Truth also appears before Ed and asks what he'll give up in order to bring back Alphonse. Edward confidently declares the loss of his Alchemic powers as his toll. Truth seems a little startled by this and asks if Ed will be all right without alchemy. When Ed says he will because he still has his friends, Truth gives him a friendly smile, declaring Ed's victory over it, and fades away along with Ed's gate, rendering Ed unable to use Alchemic powers for the rest of his life.

Concept & Symbolism
According to Father, Truth's sense of toll payment is often based on dramatic irony, oddly unfitting of Equivalent Exchange, which Truth upholds. Edward had to pay the leg he stood on and the only family he recognized as having left. He then paid his right arm to get Alphonse's soul 'back', which Truth recognized as an equivalent exchange, as one could consider Alphonse being Ed's 'right-hand man', if you will. Alphonse's soul was not used as payment, however, as it was affixed to the result of their human transmutation, which means that his body contributed to the toll. This can be interpreted as its own punishment in multiple ways, such as taking away his sense of touch and cutting him off from other peoples' warmth, or thinking along the same lines, making him become their failed creation and thereby an abomination no one wants to associate with. Izumi lost the ability to conceive at all through permanent scarring of her insides, depriving her even further of her 'motherhood'. The irony in Mustang's case would be that he would not be able to 'see' the future he would make for Amestris if he were to become Führer.

It would appear that Truth's goal is to discourage humans from treading in God's domain when they do not deserve it. It is implied that this done to evoke growth and evolution of that alchemist through an ordeal that makes them lose faith in higher beings and powers such as 'God' and 'alchemy'; so that they learn to have faith in themselves and those close to them. This is most evidenced by Ed's final encounter with Truth, as Truth amiably admits defeat as Ed proclaims that he doesn't need alchemy, he only needs his friends. Such an interpretation explains some events such as the final talk between the Truth and Father, when Truth says that the Homunculus hasn't grown and evolved through what he experienced, and that he "already knows" what he should have done.

Truth's presence, influence, and implications closely tie in with concept on the 'meaning of life'. It also possesses some parallels with the River Styx.

Trivia

 * In a late interview with the author, Arakawa said the Truth was somewhat a 'hollow' version of oneself (as a sort of 'internal God', or conscience), a sort of 'negative' of that alchemist, which completed itself with the tolls taken by the alchemist upon seeing the Truth.
 * The Truth shares the same English voice actress as the 2003 anime's version of Wrath, who was also responsible for taking Ed's limbs in that series. In the Japanese version, however, is depicted as having the voice of whoever encounters him with a slight effect added to differentiate, which is more in line to its role as a literal 'part' of whomever it is engaging with.
 * In the English dub, when Ed offers up his alchemy to Truth in order to bring Al back, Truth takes on a legion-like crossover of Ed and Al's voices in its shout of approval in a similar vein to the Japanese version, likely for emphasis of the scene.
 * For some reason, in the 5th Fullmetal Alchemist: Brotherhood Opening "Rain", Winry appears as Truth. This is in theory because Winry is what Ed needed most at the time of the episode in which the opening first aired.
 * Another possible reason for Winry's appearance in the opening "Rain":  at this point, the scene is playing a review of Edward's review of his journey to this point.  When he faces Truth originally, and is then dragged backward through his Door (his ten year old self), he glimpses an outline of Trisha, the object of his sacrifice that brought him to the Portal of Truth, and also the most important female in his life at that time.  But in "Rain", he sees Winry instead of Trisha, the current object of his affections and, after regaining Alphonse's body, his goal.
 * A third possibility, is that Winry-as-Truth, was meant to show what he desired as a trap, fitting the next scene: of him screaming within a philosophers stone, before images of every homunculus except Greed. Winry, his desire that tempts him away form what must be done, and the homuncili, whom he must face to succeed.
 * The ethereal being of Truth can be connected to the teachings of Theosophy (see Gate Trivia for connection between the Gate and Theosophy) wherein it is believed that "spirituality, philosophy, science, the arts, commerce, philanthropy and among other virtues, lead people closer to "the Absolute". Planets, solar systems, galaxies, and the cosmos itself are regarded as conscious entities, fulfilling their own evolutionary paths. The spiritual units of consciousness in the Universe are the Monads, which may manifest as angels, human beings, or in various other forms. According to Blavatsky, the Monad is the reincarnating unit of the human soul, consisting of the highest of the seven constituent parts of the human soul. All beings, regardless of stature and complexity, are informed by such a Monad." From this, we can assume that, if Hiromu Arakawa intended the gate of truth to be connected to the teachings of Theosophy, then the Truth would be the Monad.
 * This can be justified by the motto of Theosophy, which is "There is no religion higher than Truth". The emblem of Theosophy also depicts the Ouroboros in it; the Homunculi are a reference to this, as they are the children of Father, who by extension came from Truth.
 * It is also interesting to note that, in the same topic of Theosophy, they teach of the root races of humanity (of which we are in the fifth, the Aryan race,) where they teach that the Aryans established "The City of the Bridge", which is said to be below the city called Shamballa. This could be where the first theatrical film idea "Conquer of Shamballa" came from. This could also be interconnected with the Theosophic ideas of The Gate of Truth, as Dietlinde Eckhart had to pass through the Gate in the 2005 film in order to get to the supposed Shamballa that was Amestris.
 * One could also make a parallel between the meaning of Truth within FMA and the role of some deities in myths, such as Hera, a Greek goddess, in myths involving heroes - which are usually sons of her husband, Zeus. Hera is usually a deity considered to be antagonistic, posing all sorts of difficulties to the hero in question, but the utmost trait associated with Hera is the one of Pedagogy, as well as Pacts and her role in most myths is associated to a sort of "testing" of the hero and his determination. Other deities associated with this symbolic trait are Juno (roman mythology) and Morrigan (Celtic mythology),
 * Another interesting relation with Hera is that of Hera's meaning in Alchemical symbolism, by representing a stage in the Alchemic development, called "Cauda Pavonis" (peacock's tail), set somewhere in the stage of distillation, representing the moment when the alchemist comes to "understand all things for what they truly are". The peacock is also Hera's symbol animal.
 * While most alchemists see a "hollow" version of themselves while in the Portal of Truth, there is at least one case where Alphonse sees something else.  After Al works with May to offer his soul in exchange for Ed's real arm, Al re-enters the Portal to rejoin with his human body.  But the version of Truth he sees has Ed's arm, which disappears and is replaced by a "hollow" version, and also has Ed's human left leg.  This may be because Ed and Al are still "joined" through their original transmutation where they attempted to bring Trisha back from the dead
 * Truth's ultimate punishment for Father seems poetic: by letting the Eye of God drag him back into the Eye, they let Father become as high as God in some senses whilst stripping Father completely of the freedom he cherished.
 * Like most of the manga's major characters, The Truth also makes appearances in omakes. For volume 7 of the manga, The Truth takes Edward's boxer shorts, as opposed to his left leg, leaving the alchemist to feel a draft. In volume 11, The Truth is apparently studying for college entrance exams, but takes a break in order to read from an obscene magazine. He is interrupted by a his mother, also a white human hollow, who comes through The Gate to give her son a snack. The panel ends with the question: "He has a mother?!"
 * The magazine which The Truth is reading, "Naughty Book", is the exact same book Hohenheim gives to Edward as a present in a previous omake of the same volume.
 * The Truth appeared in an extra in volume 18 of the manga where it showed Yoki attempting human transmutation and having his hair taken as a toll.
 * In the 2013 anime-styled web series RWBY, The Truth appears as an easter egg in the first episode on the cover of a magazine (also titled 'Truth').