Abū Yūsuf Yaʻqūb ibn Isḥāq al-Kindī (Arabic: أبو يوسف يعقوب إبن إسحاق الكندي) (c. 801–873 CE), also known to the West by the Latinized version of his name Alkindus, was an Arab Iraqi polymath: an Islamic philosopher, scientist, astrologer, astronomer, cosmologist, chemist, logician, mathematician, musician, physician, physicist, psychologist, and meteorologist. Al-Kindi was the first of the Muslim Peripatetic philosophers, and is known for his efforts to introduce Greek and Hellenistic philosophy to the Arab world, and as a pioneer in chemistry, cryptography, medicine, music theory, physics, psychology, and the philosophy of science.
Al-Kindi was a descendant of the Kinda tribe which is a well known Arabic tribe native of Najd (present day Saudi Arabia). He was born and educated in Kufa, before pursuing further studies in Baghdad. Al-Kindi became a prominent figure in the House of Wisdom, and a number of Abbasid Caliphs appointed him to oversee the translation of Greek scientific and philosophical texts into the Arabic language. This contact with “the philosophy of the ancients” (as Greek and Hellenistic philosophy was often referred to by Muslim scholars) had a profound effect on his intellectual development, and led him to write original treatises on subjects ranging from Islamic ethics and metaphysics to Islamic mathematics and pharmacology.
In mathematics, al-Kindi played an important role in introducing Indian numerals to the Islamic and Christian world. He was a pioneer in cryptanalysis and cryptology, and devised new methods of breaking ciphers, including the frequency analysis method. Using his mathematical and medical expertise, he developed a scale to allow doctors to quantify the potency of their medication. He also experimented with music therapy.
The central theme underpinning al-Kindi’s philosophical writings is the compatibility between philosophy and other orthodox Islamic sciences, particularly theology. Many of his works deal with subjects that concerned theology, including the nature of God, the soul, and prophetic knowledge. However, despite the important role he played in making philosophy accessible to Muslim intellectuals, his own philosophical output was largely overshadowed by that of al-Farabi and very few of his texts are available for modern scholars to examine. However, he is still considered one of the greatest philosophers of Arab descent, and for this reason is known simply as “The Arab Philosopher”.
Al-Kindi was a pioneer in cryptography, especially cryptanalysis. He gave the first known recorded explanation of cryptanalysis in A Manuscript on Deciphering Cryptographic Messages. In particular, he is credited with developing the frequency analysis method whereby variations in the frequency of the occurrence of letters could be analyzed and exploited to break ciphers (i.e. cryptanalysis by frequency analysis). This was detailed in a text recently rediscovered in the Ottoman archives in Istanbul, A Manuscript on Deciphering Cryptographic Messages, which also covers methods of cryptanalysis, encipherments, cryptanalysis of certain encipherments, and statistical analysis of letters and letter combinations in Arabic. Al-Kindi also had knowledge of polyalphabetic ciphers centuries before Leon Battista Alberti. Al-Kindi’s book also introduced the classification of ciphers, developed Arabic phonetics and syntax, and described the use of several statistical techniques for cryptoanalysis. This book apparently antedates other cryptology references by several centuries, and it also predates writings on probability and statistics by Pascal and Fermat by nearly eight centuries.
Al-Kindi authored works on a number of other important mathematical subjects, including arithmetic, geometry, the Indian numbers, the harmony of numbers, lines and multiplication with numbers, relative quantities, measuring proportion and time, and numerical procedures and cancellation. He also wrote four volumes, On the Use of the Indian Numerals (Ketab fi Isti’mal al-‘Adad al-Hindi) which contributed greatly to diffusion of the Indian system of numeration in the Middle East and the West. In geometry, among other works, he wrote on the theory of parallels. Also related to geometry were two works on optics. One of the ways in which he made use of mathematics as a philosopher was to attempt to disprove the eternity of the world by demonstrating that actual infinity is a mathematical and logical absurdity.