Joseph Bobik offers a translation of Aquinas's <i>De Principiis Naturae</i> (circa 1252) and <i>De Mixtione Elementorum</i> (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." <i>The Principles of Nature</i> introduces the reader to the basic Aristotelian principles such as matter and form, the four causes so fundamental to Aquinas's philosophy. <i>On Mixture of the Elements</i> examines the question of how the four elements (earth, air, fire, and water) remain within the physical things composed from them.