Insofar as a miracle is commonly understood to be worked by God, people frequently understand this to mean that a miracle is a direct effect of the first cause.

This understanding, however, is basically a distraction from anything important about miracles. Due to the order of the world, everything that happens will happen to some extent due to secondary causes. And it is unlikely that one will be able to prove definitively that any effect could not have resulted from a secondary causes. For example, St. Thomas argues that angels cannot directly cause bodies to acquire certain forms:

I answer that, The Platonists [Phaedo. xlix: Tim. (Did.) vol. ii, p. 218 asserted that the forms which are in matter are caused by immaterial forms, because they said that the material forms are participations of immaterial forms. Avicenna followed them in this opinion to some extent, for he said that all forms which are in matter proceed from the concept of the “intellect”; and that corporeal agents only dispose [matter] for the forms. They seem to have been deceived on this point, through supposing a form to be something made “per se,” so that it would be the effect of a formal principle. But, as the Philosopher proves (Metaph. vii, Did. vi, 8), what is made, properly speaking, is the “composite”: for this properly speaking, is, as it were, what subsists. Whereas the form is called a being, not as that which is, but as that by which something is; and consequently neither is a form, properly speaking, made; for that is made which is; since to be is nothing but the way to existence.

Now it is manifest that what is made is like to the maker, forasmuch as every agent makes its like. So whatever makes natural things, has a likeness to the composite; either because it is composite itself, as when fire begets fire, or because the whole “composite” as to both matter and form is within its power; and this belongs to God alone. Therefore every informing of matter is either immediately from God, or from some corporeal agent; but not immediately from an angel.

It would be a mistake to suppose that St. Thomas has proven his point here. And most likely it would be a mistake to suppose that he believes that he has. When St. Thomas discusses matters that cannot humanly be known one way or another, he frequently picks the position that seems most plausible (or perhaps the position which is most desirable for another reason), and then constructs an argument for it. In this case, it is reasonable to say that a cause has a likeness to its effect. But when St. Thomas says, “either because it is composite itself, as when fire begets fire, or because the whole composite as to both matter and form is within its power,” he is merely speculating. One could simply respond that an angel can cause a body immediately to become informed through some other remote likeness which it possesses.

He may have adopted this position because it seemed most likely to him to be true; or he may have adopted it because it was suited to arguing that certain miracles could not be worked by angels. Either way, in reality one cannot prove such a thing. If a blind man instantly acquires his sight, or even if a dead man comes to life, one cannot prove that secondary causes did not produce these things. And for reasons relating to the order of the world, one would reasonably assume that secondary causes contributed as much as possible to these effects.