Avicenna and Rhazes are two significant initiative physicians in the golden age of the Islamic period. Although both are raised from the Galenic tradition, we see the controversy and their different methods. Generally, it is said that Rhazes valued experiences, while Avicenna rather more theoretical approaches. Nonetheless, both physicians likewise their pioneer, Galen, were also philosophers. In this paper, I aim to clarify that their contrasting views are due to their different natural philosophy. In this paper, I investigate the basis of their ideas in natural philosophy, according to the existent sources (i. e. their books and treatises), in an analytical approach. And then, in their medical texts, I examine the hypothesis derived from their natural philosophy books. As Rhazes was an atomist in natural philosophy, this view rejects determinism and essentialism in Physics. As a result of this approach, the analysis of questions of nature turned less to metaphysics and more to empiricism. This view contrasts with Avicenna, who believed in Continuism and Hylomorphism and consequently he had an essentialist and teleologist view of nature. These led him to metaphysical analysis in the exploration of natural phenomena. I show that their different methods of writing medical books originate from their different natural philosophies and views on the principles of nature.
