[mhagain]

Originally Posted by denethorthefirst

That Tom Bombadil is a Maia (or more accurately: an unaffiliated Ainu) is, in my opinion, the only logical in-universe explanation. How is that theory refuted?

What you're essentially asking here is to prove a negative: it can't be disproven that he's a Maia but that does not constitute proof that he is one.

There are actually plenty of logical in-universe explanations for what he could be, and none of them require him being a Vala or Maia. First of all let's take a quote from the Valaquenta:

...in majesty they are peers, surpassing beyond compare all others, whether of the Valar and the Maiar, or of any other order that Ilúvatar has sent into Ea.

So now that we've established that the Valar and the Maiar weren't the only spirits that entered into Ea, let's look at some potential examples of other spirits. Here's another quote, this time from the Ainulindale:

But Manwe was the brother of Melkor in the mind of Ilúvatar, and he was the chief instrument of the second theme that Ilúvatar had raised up against the discord of Melkor; and he called unto himself many spirits both greater and less, and they came down into the fields of Arda and aided Manwe...

That's example one; here's example two, from Of Aule and Yavanna:

When the Children awake, then the thought of Yavanna will awake also, and it will summon spirits from afar, and they will go among the kelvar and the olvar, and some will dwell therein, and be held in reverence, and their just anger shall be feared.

So right now we have two in-universe examples of candidates for what Tom Bombadil could be that don't necessarily involve him being either a Vala or a Maia. Therefore he doesn't have to be a Maia (and nor do any of the many other beings which it's normally assumed must be one).