Al-Kindi worked at the centre of the great ninth century effort, patronised by the Abbasid caliphs in Baghdad, to carry Greek science and philosophy into Arabic. He was not himself a translator; his role was to commission, correct and digest, standing at the head of a circle that produced Arabic versions of Aristotle, of Euclid and Ptolemy, and of the paraphrase of Plotinus that circulated, fatefully, as the Theology of Aristotle. On that foundation he wrote in his own voice across an astonishing range, and two threads matter most to his afterlife. The first is method: his treatise on deciphering encrypted messages describes counting the frequency of letters in a known language and matching the counts against a ciphertext, the earliest surviving statement of frequency analysis and the founding move of cryptanalysis. The second is the physics of influence.
In the work Latin readers knew as De radiis, everything in the world emits rays that act on everything else, and the rays of the stars shape events below; words, images and figures emit rays of their own, which is why, on this account, talismans and incantations can work. Medieval Europe read the book nervously and sometimes condemned it, precisely because it offered magic a natural mechanism rather than a demonic one. Astrologer, music theorist, optician and philosopher of the One, al-Kindi made the case that none of these was a separate pursuit.