By self-existence we mean
(a) That God is “causa sui,” having the ground of his existence in himself. Every being must have the ground of its existence either in or out of itself. We have the ground of our existence outside of us. God is not thus dependent. He is a se; hence we speak of the aseity of God.
God's self-existence is implied in the name “Jehovah” (Ex. 6:3) and in the declaration “I am that I am” (Ex. 3:14), both of which signify that it is God's nature to be. Self-existence is certainly incomprehensible to us, yet a self-existent person is no greater mystery than a self-existent thing, such as Herbert Spencer supposes the universe to be; indeed it is not so great a mystery, for it is easier to derive matter from mind than to derive mind from matter. See Porter, Human Intellect, 661. Joh. Angelus Silesius: “Gott ist das was Er ist; Ich was Ich durch Ihn bin; Doch kennst du Einen wohl, So kennst du mich und Ihn.” Martineau, Types, 1:302—“A cause may be eternal, but nothing that is caused can be so.” He protests against the phrase “causa sui.” So Shedd, Dogm. Theol., 1:338, objects to the phrase “God is his own cause,” because God is the uncaused Being. But when we speak of God as “causa sui,” we do not attribute to him beginning of existence. The phrase means rather that the ground of his existence is not outside of himself, but that he himself is the living spring of all energy and of all being.
But lest this should be misconstrued, we add
(b) That God exists by the necessity of his own being. It is his nature to be. Hence the existence of God is not a contingent but a necessary existence. It is grounded, not in his volitions, but in his nature.
Julius Müller, Doctrine of Sin, 2:126, 130, 170, seems to hold that God is primarily will, so that the essence of God is his act: “God's essence does not precede his freedom”; “if the essence of God were for him something given, something already present, the question ‘from whence it was given?’ could not be evaded; God's essence must in this case have its origin in something apart from him, and thus the true conception of God would be entirely swept away.” But this implies that truth, reason, love, holiness, equally with God's essence, are all products of will. If God's essence, moreover, were his act, it would be in the power of God to annihilate himself. Act presupposes essence; else there is no God to act. The will by which God exists, and in virtue of which he is causa sui, is therefore not will in the sense of volition, but will in the sense of the whole movement of his active being. With Müller's view Thomasius and Delitzsch are agreed. For refutation of it, see Philippi, Glaubenslehre, 2:63.
God's essence is not his act, not only because this would imply that he could destroy himself, but also because before willing there must be being. Those who hold God's essence to be simple activity are impelled to this view by the fear of postulating some dead thing in God which precedes all exercise of faculty. So Miller, Evolution of Love, 43—“Perfect action, conscious and volitional, is the highest generalization, the ultimate unit, the unconditioned nature, of infinite Being”; i. e., God's nature is subjective action, while external nature is his objective action. A better statement, however, is that of Bowne, Philos. of Theism, 170—“While there is a necessity in the soul, it becomes controlling only through freedom; and we may say that everyone must constitute himself a rational soul.... This is absolutely true of God.”
2. Immutability.
By this we mean that the nature, attributes, and will of God are exempt from all change. Reason teaches us that no change is possible in God, whether of increase or decrease, progress or deterioration, contraction or development. All change must be to better or to worse. But God is absolute perfection, and no change to better is possible. Change to worse would be equally inconsistent with perfection. No cause for such change exists, either outside of God or in God himself.