“All perishable is but an allegory.”

Letter to his publisher (31 July 1947); published in The Letters of J.R.R. Tolkien (1981), Letter 109

Guide for the Perplexed (c. 1190), Introduction
Context: You must know that if a person, who has attained a certain degree of perfection, wishes to impart to others, either orally or in writing, any portion of the knowledge which he has acquired of these subjects, he is utterly unable to be as systematic and explicit as he could be in a science of which the method is well known. The same difficulties which he encountered when investigating the subject for himself will attend him when endeavouring to instruct others: viz., at one time the explanation will appear lucid, at another time, obscure: this property of the subject appears to remain the same both to the advanced scholar and to the beginner. For this reason, great theological scholars gave instruction in all such matters only by means of metaphors and allegories.

IV. That the species of myth are five, with examples of each.
On the Gods and the Cosmos
Context: Of myths some are theological, some physical, some psychic, and again some material, and some mixed from these last two. The theological are those myths which use no bodily form but contemplate the very essence of the Gods: e. g., Kronos swallowing his children. Since god is intellectual, and all intellect returns into itself, this myth expresses in allegory the essence of god.
Myths may be regarded physically when they express the activities of the Gods in the world: e. g., people before now have regarded Kronos as time, and calling the divisions of time his sons say that the sons are swallowed by the father.
The psychic way is to regard the activities of the soul itself; the soul's acts of thought, though they pass on to other objects, nevertheless remain inside their begetters.
The material and last is that which the Egyptians have mostly used, owing to their ignorance, believing material objects actually to be Gods, and so calling them: e. g., they call the earth Isis, moisture Osiris, heat Typhon, or again, water Kronos, the fruits of the earth Adonis, and wine Dionysus.
To say that these objects are sacred to the Gods, like various herbs and stones and animals, is possible to sensible men, but to say that they are Gods is the notion of madmen — except, perhaps, in the sense in which both the orb of the sun and the ray which comes from the orb are colloquially called "the sun".

Act III, sc. iii.
The Rivals (1775)

Source: Kafka on the Shore (2002)

Between the Bridge and the River (2006)

Vanna Bonta Talks About Quantum fiction: Author Interview (2007)

Letter to George and Georgiana Keats (February 14 - May 3, 1819)
Letters (1817–1820)
Context: A man's life of any worth is a continual allegory — and very few eyes can see the mystery of life — a life like the Scriptures, figurative... Lord Byron cuts a figure, but he is not figurative. Shakespeare led a life of allegory: his works are the comments on it.