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 and καθ΄ ἕκαστα are not used in their usual meaning. This paper examines the Physics I.1 in comparison with the Posterior Analytics II. 19 in order to elucidate the meaning of καθόλου in the first chapter of Aristotle’s Physics. Furthermore, it reaches the conclusion that the structure of the natural world to which we belong determines the structure and the form of our knowledge. On the one hand, natural things are composite and, on the other hand, perception is involved in the initial grasping of natural things as composites. Thus, since perceptual knowledge is more accessible to us than any other kind of knowledge it is natural to reach knowledge of simple things, i.e., of the principles, starting our inquiry with the composites.