One of the most important points of confusion about the relationship between faith and science
is a philosophical mistake about the difference between God and creatures, and specifically
about how God acts in the world in a very different way from all other things.
Saint Thomas Aquinas refers to this as the distinction between the primary--or first
causality--of God, and the secondary causality of creatures.
Unlike every creature, God simply is.
He is being itself, and therefore He is absolutely uncaused.
He is not simply in our world.
He is the very source of the world itself.
Everything else, the universe and all of the things in it, all of these things radically
depend on God for their very being.
This means that God is a radically different kind of cause than every other thing, because
he gives to other causes their very being.
He makes them to be what they are, and therefore he is the source of their very capacity to
cause and even the source of the precise way in which each thing functions according to
what it is as a cause of other things.
We call this special causality of God God's primary causality.
Now the causality that creatures have is called secondary causality.
That's because they are caused by God.
This creaturely form of causality is real and very important.
It's, in fact, precisely what modern science rightly investigates.
Every cause that we encounter in the world, as long as we're not talking about a miracle,
is a creaturely cause, a secondary cause.
The fact that God gives secondary causality to creatures is a result of two things: The
infinity of his power (that God would give to others, the power to be a cause), and it
also shows God's goodness (because he desires creatures to have the perfection of being
real causes, even causes of goodness in others).
Now, Aquinas's principle of secondary causality (and the distinction between creaturely causality
and God's primary causality) prevents us from falling into two common errors.
The first error says that God is constantly and directly bringing about every effect in
the world so that the causality of things around us is only an illusion.
Such a view makes God too much a part of the world.
This is the kind of mistake that some religious opponents of the theory of evolution fall
into.
They are worried that if we say that something happens in the world due to a natural cause,
for example, that species are diversified through natural selection and genetic mutation,
if we say this, that we thereby deny that God is causing this.
The second error says that God does not have any real causal role in the universe.
We don't need him because this worldly causation can explain everything that we observe.
This kind of view has a contemporary analog in the materialistic Neo-Darwinians who claim
that because we can account for the development of species by natural causes--genetic mutation
and natural selection--therefore God cannot have brought this about through the causality
of creatures in His providential plan.
In fact, Saint Thomas Aquinas's understanding of primary and secondary causality permits
him to explain how God is the first cause of the world, and at the same time, gives
to creatures in the world the power to be true causes.
So it is the foundation of a right understanding of the relationship between God and creatures.
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