Objection 1. It would seem unlawful to adjure an irrational creature. An adjuration consists of spoken words. But it is useless to speak to one that understands not, such as an irrational creature. Therefore it is vain and unlawful to adjure an irrational creature.
Objection 2. Further, seemingly wherever adjuration is admissible, swearing is also admissible. But swearing is not consistent with an irrational creature. Therefore it would seem unlawful to employ adjuration towards one.
Objection 3. Further, there are two ways of adjuring, as explained above (1 and 2). One is by way of appeal; and this cannot be employed towards irrational creatures, since they are not masters of their own actions. The other kind of adjuration is by way of compulsion: and, seemingly, neither is it lawful to use this towards them, because we have not the power to command irrational creatures, but only He of Whom it was said (Matthew 8:27): "For the winds and the sea obey Him." Therefore in no way, apparently, is it lawful to adjure irrational creatures.
On the contrary, Simon and Jude are related to have adjured dragons and to have commanded them to withdraw into the desert. [From the apocryphal Historiae Certam. Apost. vi. 19.]
I answer that, Irrational creatures are directed to their own actions by some other agent. Now the action of what is directed and moved is also the action of the director and mover: thus the movement of the arrow is an operation of the archer. Wherefore the operation of the irrational creature is ascribed not only to it, but also and chiefly to God, Who disposes the movements of all things. It is also ascribed to the devil, who, by God's permission, makes use of irrational creatures in order to inflict harm on man.
Accordingly the adjuration of an irrational creature may be of two kinds. First, so that the adjuration is referred to the irrational creature in itself: and in this way it would be vain to adjure an irrational creature. Secondly, so that it be referred to the director and mover of the irrational creature, and in this sense a creature of this kind may be adjured in two ways. First, by way of appeal made to God, and this relates to those who work miracles by calling on God: secondly, by way of compulsion, which relates to the devil, who uses the irrational creature for our harm. This is the kind of adjuration used in the exorcisms of the Church, whereby the power of the demons is expelled from an irrational creature. But it is not lawful to adjure the demons by beseeching them to help us.
This suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)