This book provides scholars and students of medieval philosophy and theology with a readable and accurate English language version of St. Thomas Aquinas's earliest major treatment of God's simplicity, along with other themes characteristic of his metaphysics. By including the original Latin, this edition retains the richness of the text and offers an intellectual opportunity for serious students who desire a careful reading of Aquinas. The philosophical importance of explanations given to the metaphysical themes in Aquinas's work and the problems regarding essence continue to play a crucial role in discussions today.