Thomas Aquinas, Compendium of Theology

The entire order of secondary causes, as well as their power, comes from God. He Himself, however, produces His effects not out of necessity, but by free will, as was shown above. Clearly, then, He can act outside the order of secondary causes, as when He cures those who are incurable from the standpoint of natural causality, or when He does something else of this kind that is not within the sphere of natural causes but is nevertheless consonant with the order of divine providence. What God occasionally does in this way, independently of the order of natural causes, is designed by Him for a definite end. When effects are thus wrought by divine power outside the order of secondary causes, they are called miracles; for when we perceive an effect without knowing its cause, our wonder is excited (mirum est). God is a cause that is completely hidden from us. Therefore, when some effect is wrought by Him outside the order of secondary causes known to us, it is called simply a miracle. But if an effect is produced by some other cause that is unknown to this or that person, it is not a miracle simply as such, but only with regard to him who is ignorant of the cause. Thus an event may appear marvelous to one person without seeming marvelous to another who is acquainted with its cause.

To act in this way, outside the order of secondary causes, is possible for God alone, who is the founder of this order and is not confined to it. All other beings are subject to this order; and so God alone can work miracles, as the Psalmist says: "Who alone doth wonderful things" (Ps. 71:18). Therefore, when miracles are apparently worked by some creature, either they are not true miracles, but are effects produced by the power of natural agents, which may be concealed from us, as happens in the case of miracles wrought by demons with their magical arts; or else, if they are true miracles, someone obtains the power to work them by praying to God. Since such miracles are wrought exclusively by divine power, they are rightly appealed to in proof of the faith, which has God alone as its author. For a pronouncement issued by a man with a claim to divine authority, is never more fittingly attested than by works which God alone can perform.

Although such miracles occur outside the order of secondary causes, we should not simply say that they are against nature. The natural order makes provision for the subjection of the lower to the activity of the higher. Thus effects brought about in lower bodies in consequence of the influence emanating from the heavenly bodies, are not said to be simply against nature, although they may at times be against the particular nature of this or that thing, as we observe in the movement of water in the ebb and flow of the tide, which is produced by the action of the moon. In the same way, effects produced in creatures by the action of God may seem to be against some particular order of secondary causes; yet they are in accord with the universal order of nature. Therefore miracles are not contrary to nature.