This article examines four medieval views on the subject of theology. Thomas Aquinas, Giles of Rome, John Duns Scotus, and Peter Auriol were all confronted with an idea based on Aristotle’s theory of knowledge according to which any scientific discipline is unified by its proper subject. In defining this subject of theology, however, the theologians had to confront one thorny problem: God, whom they considered to be the subject of theology, cannot be grasped by any concept accessible to the human mind. In their respective discussions, two distinct strategies to solving this puzzle emerged. Aquinas and Giles, on the one hand, argued for a concept proportionate to human cognition. This concept or ratio functioned as a placeholder for the quidditative concept of God. Scotus and Auriol, on the other hand, elaborated on a concept which they believed grasped God’s quiddity, albeit in a somewhat approximative way. Their theories, therefore, figure as attempts to find a concept, that is, the concept of being, that in itself was as boundless as to grasp God’s immensity.