St. Thomas raises this objection to the existence of God:
Objection 2. Further, it is superfluous to suppose that what can be accounted for by a few principles has been produced by many. But it seems that everything we see in the world can be accounted for by other principles, supposing God did not exist. For all natural things can be reduced to one principle which is nature; and all voluntary things can be reduced to one principle which is human reason, or will. Therefore there is no need to suppose God’s existence.
He responds to the objection:
Since nature works for a determinate end under the direction of a higher agent, whatever is done by nature must needs be traced back to God, as to its first cause. So also whatever is done voluntarily must also be traced back to some higher cause other than human reason or will, since these can change or fail; for all things that are changeable and capable of defect must be traced back to an immovable and self-necessary first principle, as was shown in the body of the Article.
The explanation here is that things do have their own proper causes, but these proper causes do not have the properties necessary to be a first cause. Likewise, the very distinction of these proper causes from one another shows that they must be reduced to a one single principle.
This response is correct, but it is difficult for people to understand. People tend to assume that the objection is fundamentally valid, given its premises. Thus many atheists believe that they have a very good argument for their atheism, and many theists assume that there must be falsehood in the premises. And the ordinary way to assume this is to say that we do see things in the world that cannot be accounted for by other principles.
This leads to an undue zeal on behalf of God, of the sort mentioned in the previous post. There is the desire to say that something was done by God, and only by God; not by anything else. In this way the premise that “everything we see in the world can be accounted for by other principles” would turn out to be false. The Intelligent Design movement provides an example of this desire. The linked Wikipedia article approaches this with a very polemical point of view, but I am not concerned here with the scientific issues. It is very evident, in any case, that there is the idea here that it would be good to prove that something was done by God alone, and not by any secondary causes. In this way people are jealous on behalf of God: if it turns out that it was done by secondary causes, that takes something from God, and in particular it makes it less likely that God exists.
The truth is mostly the opposite of this. Although nothing can be taken from God, the purposes of creation are better obtained if created things contribute whatever they can to the production of other things. Thus the world is more ordered, and so more perfect simply speaking.
As an example, consider the case of the origin of life. Unlike the process which gave rise to the origin of species, abiogenesis is not an established fact. What would be best, were it the case? I do not speak of the truth of the matter, nor what we might wish to believe about it, but which thing would be better in itself: is it better if life arises from non-living things, or is it better if life is directly created by God? For someone jealous for God in this way, it seems better if life were directly created, in order better to prove that God exists. In reality, however, it is better if life comes to be in a certain order, with a contribution from non-living things, to whatever degree that this is possible.
This is not just a matter of wishful thinking, in one direction or the other, although that can be involved. Rather, in cases of this kind, the fact that one thing is better is an argument, although not a conclusive one, for its reality.
There are many other ways in which this kind of undue zeal influences human opinions, and recognition of the truth of this matter has many consequences. But for the moment we are on another path.