Arg. 2. When an action can be sufficiently done by one, it is superfluous to have it done by more: we see that nature does not do through two instruments what she can do through one. Since then the divine power is sufficient to produce natural effects, it is superfluous to employ also natural powers for the production of those same effects. Or if the natural power sufficiently produces its own effect, it is superfluous for the divine power to act to the same effect.
Arg. 3. If God produces the whole natural effect, nothing of the effect is left for the natural agent to produce.
Upon consideration, these arguments are not difficult.
Reply 1. The power of the inferior agent depends upon the power of the superior agent, inasmuch as the superior agent gives to the inferior the power whereby it acts, or preserves that power, or applies it to action; as a workman applies a tool to its proper effect, frequently however without giving the tool the form whereby it acts,* nor preserving it, but merely giving it motion. The action therefore of the inferior agent must proceed from that agent not merely through its own power, but through the power of all superior agents, for it acts in virtue of them all. And as the ultimate and lowest agent acts immediately, so is the power of the prime agent immediate in the production of the effect. For the power of the lowest agent is not competent to produce the effect of itself, but in power of the agent next above it; and the power of that agent is competent in virtue of the agent above it; and thus the power of the highest agent proves to be of itself productive of the effect, as the immediate cause, as we see in the principles of mathematical demonstrations, of which the first principle is inimediate.* As then it is not absurd for the same action to be produced by an agent and the power of that agent, so neither is it absurd for the same effect to be produced by an inferior agent and by God, by both immediately, although in different manners.
Reply 2. Though a natural thing produces its own effect, it is not superfluous for God to produce it, because the natural thing does not produce it except in the power of God. Nor is it superfluous, while God can of Himself produce all natural effects, for them to be produced by other causes: this is not from the insufficiency of God's power, but from the immensity of His goodness, whereby He has wished to communicate His likeness to creatures, not only in point of their being, but likewise in point of their being causes of other things (Chap. XXI).
Reply 3. When the same effect is attributed to a natural cause and to the divine power, it is not as though the effect were produced partly by God and partly by the natural agent: but the whole effect is produced by both, though in different ways, as the same effect is attributed wholly to the instrument, and wholly also to the principal agent.*
3.69 : Of the Opinion of those who withdraw from Natural Things their Proper Actions
3.71 : That the Divine Providence is not wholly inconsistent with the presence of Evil in Creation