Every aspect of Thomas's philosophy has called forth a massive amount of commentary, but the amount of commentary on God's existence perhaps outdoes all others, and the author's treatment of the literature is necessarily be partial. This chapter looks at Thomas's much discussed arguments on God's existence. When we consider Aquinas on God's existence, two things stand out. One is the clear rejection of ontological arguments, and Aquinas's interesting reason for that rejection. The second is the fact that ST 1a 2.3, containing the five ways, is the opening article in the proof of God's existence, and should be supplemented by the considerations which Aquinas immediately offers concerning God's nature. The five ways are really the beginning of a proof of God's existence. In the case of God, we have: "If God has necessary existence then, necessarily God exists", where the necessity in the consequent is logical necessity.