This paper presents Aristotle’s method of understanding the first principles of natural things in the Physics I.1 and analyzes the three stages of which this method consists. In the Physics I.1, Aristotle suggests that the natural proper route which one has to follow in order to find out the first principles of natural things is to proceed from what is clearer and more knowable to us to what is more knowable and clear by nature. In the Physics I.1, the terms καθόλου (universal) and καθ΄ ἕκαστα (particular) are not used in their usual meaning (e.g., the meaning which the same terms have in the Posterior Analytics I. 2). This paper examines the Physics I.1 in comparison with the Posterior Analytics II. 19 in order to elucidate the mea...