Saint Thomas (Aquinas), Peter Kreeft (1990). “A Summa of the Summa: The Essential Philosophical Passages of St. Thomas Aquinas' Summa Theologica”, p.41, Ignatius Press
Joseph Bobik, Saint Thomas (Aquinas) (1998). “Aquinas on matter and form and the elements: a translation and interpretation of the De principiis naturae and the De mixtione elementorum of St. Thomas Aquinas”, Univ of Notre Dame Pr