This book offers a contemporary interpretation and defense of a much neglected proof for the existence of God offered by St Thomas Aquinas in De Ente et Essentia. The book establishes that Thomas is on firm ground when he concludes that there exists a single unique source of being from which all that is comes to be. reading this book is highly recommended to anyone interested in Aquinas's metaphysics, in his philosophical thought broadly conceived, and also in his theological views.