This course aims to answer fundamental questions about physical nature. What are the common principles and causes underlying all natural things? It emphasizes the perennial notions of natural philosophy in the Aristotelian-Thomistic tradition, while engaging with issues and concerns from contemporary science. It will analyze the phenomenon of change in mobile beings using concepts of act and potency. Next it studies the notions of substance and accidents. Lastly, we examine the notions of form and matter more generally and the hylomorphic synthesis.