In the Physics, Aristotle’s object of study is the world of nature. Nature, in this sense, means that which is the primary and internal principle of change or motion, and it is the subject of motion that occupies much of the treatise since it is most characteristic of the natural world as such. In this way, the treatise is an attempt to arrive at a science of motion and, therefore, nature. At the outset, though, there is a possibility for confusion in the understanding of the modern reader. Modern science also speaks of nature, motion, and change, yet Aristotle’s treatise does not appear to concern itself with the same type of study that modern science does. In fact, it would not be too far of an exaggeration to say that they do not appear to study the same subject at all.
Aristotle does not fill his work with equations or any numbers or measurements and it does not contain quantifiable empirical data. The reason for this disparity is not obvious. It could be because Aristotle’s knowledge of science is so primitive and mistaken that it is different simply because it is false. Another reason, the true reason, is that Aristotle is considering nature in a more general way than any particular modern science does, and although it is certainly a different way of speaking about nature it is not wrong because of its difference, rather, Aristotle’ physics is indeed prior to any modern account of science.
In Book III of the Physics, Aristotle arrives at a definition of motion simply. Motion is the actuality of some potentiality inasmuch as it is potential. A man who is pale may become tan only if he is first potentially tan and actually pale. As something which is actually pale, a man undergoes a process whereby his being-potentially-tan becomes his being-actually-tan. This process is called motion or change, and in this particular instance we would call it the act of tanning. Notice that the words ‘act’ is even present in a conventional description of the motion. This is a definition of motion per se, meaning that it applies to all motions inasmuch as they are motions.
Aristotle also makes a second distinction that is related to the act/potency distinction. Natural substances are also considered composites of matter and form. Matter is a potency which becomes an actual substance through the possession of some form. This form, then, is the act in relation to the matter which is in potency. Applying the act/potency distinction specifically to this matter/form distinction, we can say that a man’s skin, which would be the matter in this case, is informed by paleness. Motion is just the process by which the matter, skin, looses the form it currently possesses, paleness, and acquires the form of tanness. This is Aristotle’s way of describing how someone becomes tan.
Aristotle’s account is not a scientific account in the modern sense, but difference does not necessarily imply incompatibility. Because he is seeking to define motion as such, Aristotle is not attempting to describe the particular motion of any one thing. He is also not attempting to exhaustively define the motions of any particular things. What he is doing is demonstrating what all motion must essentially be at the most general level. There is a sense in which modern science fills in the blanks of Aristotle’s conceptual schema by attempting to discover the particulars of individual natural motions. Aristotle’s account of motion is formal, while modern science’s account provides the material. Ultimately, the modern empirical science that is studied today is conceptually dependent upon Aristotle’s science, which is prior to it and presupposed by it.