A central and unifying theme in Aquinas’s metaphysical thought is the act of existence. This signifies an actuality more fundamental than form. In exploring the act of existence and its relation to the thing whose act it is, Thomas is led to consider the causality of that act. It is through considering the causality of the act of existence that Aquinas is led to affirm God’s existence, and in turn is able to anaylse God’s causality in the act of creation.