Anthony Kenny offers a critical examination of a central metaphysical doctrine of Thomas Aquinas, the greatest of the medieval philosophers. Aquinas's account of being is famous and influential: but Kenny argues that it in fact suffers from systematic confusion. Because of the centrality of the doctrine, this has implications for other parts of Aquinas's philosophical system: in particular, Kenny shows that the idea that God is pure being is a hindrance, not a help, to Aquinas's natural theology. Kenny's clear and incisive study, drawing on the scholastic as well as the analytic tradition, dispels the confusion and offers philosophers and theologians a guide through the labyrinth of Aquinas's ontology.