There are many themes that we can find in Tolkien's books. This has to do with the principle of applicability and the fact that his work encompasses so much that is of general interest to all of us. One of the themes that I followed when last reading LotR was the theme of
"Hope and Hopelessness" - I presented it for discussion on that thread.
However, I would not say that any theme that we can find could be considered Tolkien's own main theme. Only he himself can tell us that, and he did, in his letters. His main theme is death and immortality. I will try to find a quote to back that reference up, or perhaps someone else knows exactly where it is...
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'Mercy!' cried Gandalf. 'If the giving of information is to be the cure of your inquisitiveness, I shall spend all the rest of my days in answering you. What more do you want to know?' 'The whole history of Middle-earth..
.'