Here’s a more scholarly way of expressing how (and why) Tolkien saw himself as a discoverer of legends rather than an inventor of stories.
But Tolkien also thought – and this takes us back to the roots of his inventions – that philology could take you back even beyond the ancient texts it studied. He believed that it was possible sometimes to feel one’s way back from words as they survived in later periods to concepts which had long since vanished, but which had surely existed, or else the word would not exist. (…) However fanciful Tolkien’s creation of Middle-Earth was, he did not think he was ENTIRELY making it up. He was ‘reconstructing’, he was harmonizing contradictions in his source-texts, sometimes he was supplying entirely new concepts (like hobbits), but he was also reaching back to an imaginative world which he believed had once really existed, at least in a collective imagination: and for this he had very great deal of admittedly scattered evidence.
~ Tom Shippey: J.R.R. Tolkien. Author of the Century.
“Tolkien was almost excessively ‘tough-minded’: (…) all drove him to work out from single words, or cruxes, or kernels, or nuggets.”
[Philological curiosity] was the natural state, [and Tolkien’s success went] far to proving his point about the naturalness of philology and the appeal of names, words and linguistic styles; and in the wider sense of philology as that branch of learning which ‘presented to lovers of poetry and history fragments of a noble past that without it would have remained for ever dead and dark’ (from Tolkien’s Essays), he showed that its appeal too was not confined to antiquity. I do not see how Tolkien can be denied the tribute of having enlarged his readers’ apprehensions (of language), or their human sympathies (with the disciplined, or the heroic, or the addicted, or the self-sacrificing). But most of all I think his utility for the lover of literature lies in the way he showed creativity arising from the ramifications of words: unpredictable, certainly, but not chaotic or senseless, and carrying within themselves very strong suggestions of ‘the reality of history’ and ‘the reality of human nature’, and how people react to their world. (…) Words, ancient words, do not have to be hooked together to make something. They have their own energy and struggle towards their own connections. Observing this impulse and co-operating with it is as good a guide for an artist as turning within oneself to the inarticulate.