Much have been written about al-Biruni's contribution to the development of astronomy and materia medica that has earned him a distinguished place in the history of science. Yet little is known about his philosophy of mathematics, especially pertaining to the interplay of reason and spirit in his view of the foundation of mathematics. In this article, the author will examine al-Biruni's position regarding the matter based on some of his writings, in particular, his Kitab al-tafhim li-awa'il sina"at al-tanjim (The Book of Instruction in the Elements of the Art of Astrology).