Al-Kindi authored works on a number of important mathematical subjects, including arithmetic, geometry, the Hindu 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 Hindu Numerals  ( Kitāb fī Isti`māl al-A`dād al-Hindīyyah') which contributed greatly to diffusion of the Hindu 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.