Part I treats of God, who is the "first cause, himself uncaused" ("primum movens immobile") and as such existent only in act ("actu")—i.e. pure actuality without potentiality, and therefore without corporeality. His essence is "actus purus et perfectus". This follows from the fivefold proof for the existence of God; namely, there must be a first mover, unmoved, a first cause in the chain of causes, an absolutely necessary being, an absolutely perfect being, and a rational designer. In this connection the thoughts of the unity, infinity, unchangeability, and goodness of the highest being are deduced.