The ‘Physics is the initial work in Aristotle’s natural philisophy and, to paraphrase the Philosopher, a little clarity in the beginning magnifies as study continues. From the outset of his career, Thomas recognized the importance of the 'Physics’. In the youthful ‘Principles of Nature’, he shows a mastery of Aristotle’s teaching on the constitution of physical objects. But the importance of ‘Physics’ is by no means exhausted by its role within natural philosophy. Without physics there can be no metaphysics. The fundamental affinity betwwen Thomas and Aristotle is nowhere clearer than in the shared conviction that it is only on the basis of knowledge of the things around us, the things that come to be and pass away, physical things, that we can hope to have knowledge of things whose existence is not material. This commentary exhibits the familiar technique of the commentaries on Aristotle, a technique to be found in Thomas’s biblical commentaries as well. The order of the division of the text is the principle of manifestation. The primary purpose is to get clear as to what Aristotle taught and why. As commentator, Thomas is both objective and empathetic. The commentary continues the discourse in the text.