Joseph Bobik offers a translation of Aquinas’s De Principiis Naturae (circa 1252) and De Mixtione Elementorum (1273) accompanied by a continuous commentary, followed by two essays: “Elements in the Composition of Physical Substances” and “The Elements in Aquinas and the Elements Today.” The Principles of Nature introduces the reader to the basic Aristotelian principles such as matter and form, the four causes so fundamental to Aquinas’s philosophy. On Mixture of the Elements examines the question of how the four elements (earth, air, fire, and water) remain within the physical things composed from them.