Since the subject of divine simplicity has come up in a recent conversation (and I have added a label for it on my blog now), here's Dabney on the subject in his Systematic Theology:
"Simplicity of God’s Substance.Divines are accustomed to assert of the divine substance an absolute simplicity. If by this it is meant that He is uncompounded, that His substance is ineffably homogeneous, that it does not exist by assemblage of atoms, and is not discerptible, it is true. For all this is clear from His true spirituality and eternity. We must conceive of spiritual substance as existing because all the acts, states, and consciousnesses of spirits, demand a simple, uncompounded substance. The same view is probably drawn from His eternity and independence. For the only sort of construction or creation, of which we see anything in our experience, is that made by some aggregation of parts, or composition of substance; and the only kind of death we know is by disintegration. Hence, that which has neither beginning nor end is uncompounded.But that God is more simple than finite spirits in this, that in Him substance and attribute are one and the same, as they are not in them, I know nothing. The argument is, that as God is immutably what He is, without succession, His essence does not like ours pass from mode to mode of being, and from act to act, but is always all modes, and exerting all acts; His modes and His acts are Himself. God’s thought is God. He is not active, but activity. I reply, that if this means more than is true of a man’s soul, viz: that its thought is no entity, save the soul thinking; that its thought, as abstracted from the soul that thinks it, is only an abstraction and not a thing; it is undoubtedly false. For then we should have reached the pantheistic notion, that God has no other being than the infinite series of His own consciousnesses and Nor would we be far off from the other result of this fell theory; that all that is, is God. For he who has identified God’s acts hence with His being, will next identify the effects thereof, the existence of the creatures therewith."