Several important themes are developed in this rich collection of stories. One is the theme of idealism, revealing Borges as a disciple of the philosophical systems constructed by Berkeley and Hume — among others — that assert that ideas are the only true measure of reality and that objects are nothing but imperfect and transitory images. In the universe of "Tlon," a perfect idealist construct, objects do not exist outside the idea of their existence.
Another theme, that of the interplay between reason and the absurd, appears in several stories in which man attempts to understand reality around him by conceiving of a world according to his own sense of the perfect form. Invariably this universe, as in "The Library of Babel," is just as — if not more — paradoxical and labyrinthian as reality: its mysteries as secret and obscure, its custodians as ignorant, and its limits as endless. The quest for full understanding and perfection therefore, leads one into the world of the absurd.
A third related theme is that of the nature of knowledge, exemplified by the omniscient Ireneo, the protagonist of "Funes the Memorious." When a hapless farm boy falls off a horse, the fall causes him to develop extraordinary mental powers; he is able to recall all his experiences to the slightest, most irrelevant detail. The fall paralyzes him, literally and figuratively. He is injured and loses all mobility. At the same time his prodigious memory leads to excessive knowledge which, in turn, traps him, engaging his mind in trivial endeavors and making it as useless as his body.
One last theme is time, its confusing nature, and the difficulty of drawing clear lines between present, past and future. Its mysteries are eloquently portrayed in "The Secret Miracle" when Hladik the Jewish protagonist, about to be shot by a Nazi firing squad, receives a one-year reprieve from God, time he requests to finish a play. His wish is granted the instant the final order to shoot is given. Happily, Hladik has enough time to finish his work.