One of the fundamental principles of the study of being (how things exist) is expressed by author George Klubertanz as “Act, in the order in which it is act, is unlimited in itself and limited by the potency in which it is received.” While this is quite abstract, especially to those who have not studied metaphysics, the significance of this principle should not be missed.
What is actual is limited only by the potential of the substance in which it is act. Thus the act of seeing is limited by the eyes in which the seeing is realized. Seeing, in itself, would therefore be unlimited, and limited only by the potential of the eyes which the seeing is realized. If we had a thing which was pure existence (pure act), then this thing would be unlimited, since it has no potency.
Further, all potency can only be actualized by something which is already in act. A pile of lumber can only be made into a building by something which is already actual. If we work the chain of all causality backward, we come to the conclusion that a thing of pure act with no potency necessarily exists. This we call God.
But we can make some conclusions about this God who is pure act. The principle stated above says that pure act is unlimited in itself because it is only limited by any potency in which it might be received. Thus God is unlimited in Himself; He has no matter, which would limit the act. Humans, on the other hand, are material, and therefore limited by our potency. We are composed of act and potency, while God is pure act, unlimited in Himself.