Let us begin by investigating the classical idea of instrumental causality. In Unlocking Divine Action, Dominican Father Michael Dodds refers to chalk as a writing instrument:
Sometimes a creature, even while exercising its own proper causality, achieves something beyond its natural capacity. Chalk, for instance, by its own nature, may be able to leave marks when it is drawn across a surface. If it [leave] intelligible marks in the form of letters and words, however, this effect must be due not just to the chalk but to some intelligent agent employing the chalk as an instrument. The chalk is an "instrumental cause", and the one who uses it is a "principal cause".The instrumental cause operates in one way, and the principal cause operates in a deeper way. Let us consider two cases, in each of which a pair of causes brings about the same effect.
- In the first case, consider two instrumental causes of the same effect. These causes interfere. For example, each of two men carrying a table is a cause of the table's motion. If one man carry more of the table's weight, then the other carries less of the table's weight. Some of the table's motion is due to one man, and the rest is due to the other. One man's causal influence interferes with the other's because the more one man causes the table to move, the less the other man causes the table to move.
- In the second case, consider a principal cause and an instrumental cause of the same effect. These causes do not interfere. For example, each of the chalk and the person who wields it is entirely the cause of a writing. The writing is not divided up as though part of the writing is caused by the chalk and another part is caused by the wielder. Every bit of the writing is caused by the chalk, and every bit of the writing is caused by the wielder. The chalk's causal influence does not interfere with that of the wielder because the wielder's causal influence is what allows the chalk to have its influence.
Similar to instrumental causality is secondary causality. St. Thomas Aquinas calls every created thing a "secondary cause", and God is the "primary cause". A piece of chalk causes a mark on a chalkboard. A gust of wind causes leaves to rustle. The primary cause, however, is what gives every created thing the power to cause something else. The effect of the primary cause is for the secondary cause to have its particular effect. Just as the principal cause and the instrumental cause do not interfere, so too the primary cause and the secondary cause do not interfere. Just as each of the chalk and its wielder is entirely the cause of the written word, so too each of the gust and the One Who gives every gust the power to rustle leaves is entirely the cause of the rustling of the leaves. Instrumental causality and secondary causality are similar in that each involves causes that do not interfere; one cause, operating at a deeper level than the other, enables the other cause to have its effect.
The point is that, when God is considered as the primary cause, God's action in the world, unlike the actions of created things, is of a completely different category. A logically consistent theory of evolution does not for the generation of new species force a choice between God's intervention on the one hand and random variation on the other. In a Catholic view of evolution, God is the cause of the randomness that brings about new species. In other words, the Catholic can admit that randomness is real and really is random. God knows what will result from it, but God's knowledge, transcendent of secondary causes, does not deprive randomness of its nature relative to other secondary causes. The randomness is just another created thing, another secondary cause.
Perhaps the best popular exponent of the relationship between primary and secondary causality is Edward Feser, a Catholic professor of philosophy and expert on Aquinas. In the following quotation, Feser is responding to Alex Rosenberg, author of The Atheist's Guide to Reality. Feser writes:
Rosenberg’s entire argument rests on a crude misunderstanding of the nature of divine causality. In particular, he evidently knows nothing about the traditional distinction between primary causality and secondary causality, and operates with a crudely anthropomorphic conception of deity. For he assumes that making evolution compatible with theism would require supposing that God intervenes in biological history at various points in order to alter the course of events so as to ensure that homo sapiens ... comes about, but ... in so subtle a way that it [only] looks like the product of random variation and natural selection....See 'http://edwardfeser.blogspot.com/2012/01/reading-rosenberg-part-vi.html'.
But this ... is like saying that the author of a science fiction novel in which ... a species comes about via natural selection has to “intervene” at key points so as to make sure that the evolutionary process comes out the way he needs it to ... for the story to work -- but at the same time has to do so subtly, so that none of the characters would guess that he had intervened in this way. The very suggestion is silly, for the author isn’t one causal factor in the story among others. His causality relative to the story is not at all like the causality of either the characters or the impersonal processes operating within the story.
Similarly, on the classical [theist's] conception of God, God is not one causal factor in the universe among others, not even an especially grand and powerful causal factor. He is not a “first” cause in the sense of being followed in a temporal series by a second cause, a third cause, a fourth cause, etc. Rather, He is “first” or primary in the sense of being the fundamental cause, the necessary precondition of there being any causality within the universe at all, just as the author of a story is the “first cause” of what happens in the story, not in the sense of generating effects in the way the characters and processes described in the story do, but rather in the sense of being the necessary precondition of there being any characters or processes in the story at all. Things in the world are “secondary” causes, then, in the sense of deriving their being and causal power from God, just as the characters in the story have any reality and causality at all only because the author of the story has imparted it to them by virtue of writing the story....
Now, it would be absurd to suggest that Macbeth did not really murder Duncan, but that it was Shakespeare who committed the murder and merely made it look like Macbeth had done it. This would be to treat the author as if he were a character in the story. For the same reason, it would be absurd to suggest that in a science fiction novel in which ... a species evolves, it is not really Darwinian processes that generate the species, but rather the author of the story who does so and merely made it seem as if Darwinian processes had done it. But by the same token, it is absurd to suggest that if God [create] a world in which human [bodies] come about by natural selection, He would have to intervene in order to make the Darwinian processes come out the way He wants them to, in which case they would not be truly Darwinian. This is to confuse primary with secondary causality, to think of God as if He were merely one causal factor in the world among others, like treating an author as if he were merely one character in his story among the others....
Feser goes on to point out that the advocate of Intelligent Design theory makes the same mistake as the atheist. Each confuses primary and secondary causality. However, there is an interesting sense in which the atheist is closer to the Catholic than is the proponent of Intelligent Design. Neither the atheist nor the Catholic sees the need to invoke God as acting in the same way as causes that can be studied by way of science.