"He is a strange creature"
? Elrond, The Council of Elrond
Tom Bombadil's mythological origins in the internal cosmology of Middle-earth have puzzled even erudite fans. For example the fact that the Ring had no power on Tom would either suggest that Tom existed in both the Seen and the Unseen realms, or that the Unseen had no effect on him since "He is his own master".
Speculative ideas about his true nature range from simply a wise Elven hermit to an angelic being (a Maia or Vala), to the creator God, Eru Ilúvatar.
Tom as Eru
When Goldberry was asked by Frodo Baggins who he was, she simply said "He is" which parallels the scriptural name of God in Christianity, "ho on" (The Being - the One who is). Goldberry also said that Tom is simply "the Master", and the theory that Tom is an incarnation, embodiment or "avatar" of Eru has been favored by the fans.
However Tolkien later remarked that this dialogue was not a reference to God, the way that priests called "Father" does not allude to God.
In the Council of Elrond it was mentioned that Tom has limited knowledge and understanding of the powers of the Ring, a limitation that the creator God arguably would not have.
It must be added, that Tolkien was a devout Roman Catholic, so the idea of an incarnation of the Creator being married may not have went well with him; he explicitly said that the idea of God's Incarnation was too large to fit in his work.
In any case however, Tolkien experimented with this idea in the Athrabeth Finrod ah Andreth about a future incarnation or Eru into the world.
Tom as an Ainu
If we reject the Eru theory, the most common possible explanation is that Tom is an Ainu such as a Vala. Tom seems to have unlimited power inside the boundaries that he set for himself and perhaps the reason of why he has such powers might be the fact that he set himself limits in which he is master.
However since all seven Valar are accounted and known by their names, one has to associate one with Tom. For instance, a theory identifies Tom with Aulë (and Goldberry with Yavanna). Furthermore, it is nowhere referred that Aulë or any other Vala abandoned Valinor to live on Middle-earth and such an explanation becomes risky and radical; The most common theory is therefore that Bombadil is just a "rogue" Maia who perhaps stayed behind and did not follow the other Ainur at the Breaking of the World. In contrast to the seven Valar, the Maiar are an unknown number, so it's easy to associate Tom with them.
Tom remembers to be the "Eldest, that's what I am... Tom remembers the first raindrop and the first acorn... He knew the dark under the stars when it was fearless – before the Dark Lord came from Outside". Eldest is also a title given to Treebeard.
If Tom refers to Melkor, who was the first Vala to enter Arda, Tom was already there even before all the Valar, dismissing the theory that he is a Vala or a Maia. If Tom was refering to Sauron, however, then being a Maia may not be as far fetched.
One possible drawback was his relationship to the Good and Evil and the Unseen realm, which is evident on the effect of the One Ring on him. The Ring (containing Sauron's essence) had no effect on him although it had some effect on Saruman and Gandalf. Also, Gandalf in the Council of Elrond said that Tom is "neutral" and would not understand the cause of the Free peoples, while all other known Maiar had their sides.
Robert Foster in The Complete Guide to Middle-earth indeed identifies Tom, Goldberry and the River-woman as Maiar.
Tom as a nature sprite
A more radical and abstract theory is that he is possibly the embodiment of Arda itself, a "Father Nature", or some kind of 'sprite' which (unlike the greater Ainu spirits) were of non-divine nature. Not only does the Ring have no effect on him, Tom himself seems unable to affect the Ring in return which shows that Tom was outside the divine plan and Good vs Evil struggle and had no position in it. Bombadil could have been created as a side-effect of the Music of the Ainur and that would explain why he was there in the beginning.
His Elvish name "Eldest Fatherless" can support this notion: since he is only a part of creation, he has no "father", while the Ainur have (Eru).
This also would be consistent with his neutrality: Nature is neutral and has no morality.
One problem with this theory is the Ring not having power over him. It is clear that the three elvish rings possess power over nature so one has to wonder if Tom is some spirit of nature why would the One Ring not have power over him?
Another problem with this explanation is that no such beings are mentioned elsewhere in late writings such as The Lord of the Rings (although one could argue that the sprites and fays, found in such early writings as the Book of Lost Tales, would fit this description).
Tom as the Spirit of the Music of the Ainur
This theory is a variant of the nature spirit theory, asserting that in his essence Tom is the Spirit of the Music of the Ainur. According to its proponent, this would explain his unique power and its limitations, his timelessness, his disposition, his affinity to song, his power via song over trees and barrow-wights and many of the other oddities found in his character.
This theory operates with the understanding that any spirit must be understood to be what they are personally most related to. The argument is that Tom, though close to nature, ultimately separates himself from the Forest by battling against Old Man Willow and by having a different disposition than that of the Old Forest which is described as dark and full of hate for everything that goes about freely. Tom on the other hand, points to song constantly even when fighting the barrow-wight: "None has ever caught him yet, for Tom, he is the Master: His songs are stronger songs,and his feet are faster".
Beings like him
Whatever his nature, another question about Bombadil is whether he is one-of-a-kind being, or if he has other colleagues in other parts of Arda. Bombadil could be for example the one and single "spirit of all Arda", or just the "spirit of the Old Forest" with other such beings in other forests.
The nature of Goldberry (and River-woman) is also obscure. They could be the same kind of being like himself, or his female counterpart(s). If we had more knowledge about them, we could partially answer the question about Tom as well.
Gandalf, when narrating his fight with Durin's Bane, mentions tunnels made by dark nameless things whose existence Sauron knows not, since they are "older than him".
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