Objection 1. It would seem that divination by auguries, omens, and by like observations of external things is not unlawful. If it were unlawful holy men would not make use thereof. Now we read of Joseph that he paid attention to auguries, for it is related (Genesis 44:5) that Joseph's steward said: "The cup which you have stolen is that in which my lord drinketh and in which he is wont to divine [augurari]": and he himself afterwards said to his brethren (Genesis 44:15): "Know you not that there is no one like me in the science of divining?" Therefore it is not unlawful to make use of this kind of divination.
Objection 2. Further, birds naturally know certain things regarding future occurrences of the seasons, according to Jeremiah 8:7, "The kite in the air hath known her time; the turtle, the swallow, and the stork have observed the time of their coming." Now natural knowledge is infallible and comes from God. Therefore it seems not unlawful to make use of the birds' knowledge in order to know the future, and this is divination by augury.
Objection 3. Further, Gedeon is numbered among the saints (Hebrews 11:32). Yet Gedeon made use of an omen, when he listened to the relation and interpreting of a dream (Judges 7:15): and Eliezer, Abraham's servant, acted in like manner (Genesis 24). Therefore it seems that this kind of divination is not unlawful.
On the contrary, It is written (Deuteronomy 18:10): "Neither let there be found among you anyone... that observeth omens."
I answer that, The movements or cries of birds, and whatever dispositions one may consider in such things, are manifestly not the cause of future events: wherefore the future cannot be known therefrom as from its cause. It follows therefore that if anything future can be known from them, it will be because the causes from which they proceed are also the causes of future occurrences or are cognizant of them. Now the cause of dumb animals' actions is a certain instinct whereby they are inclined by a natural movement, for they are not masters of their actions. This instinct may proceed from a twofold cause. On the first place it may be due to a bodily cause. For since dumb animals have naught but a sensitive soul, every power of which is the act of a bodily organ, their soul is subject to the disposition of surrounding bodies, and primarily to that of the heavenly bodies. Hence nothing prevents some of their actions from being signs of the future, in so far as they are conformed to the dispositions of the heavenly bodies and of the surrounding air, to which certain future events are due. Yet in this matter we must observe two things: first, that such observations must not be applied to the foreknowledge of future things other than those which can be foreknown from the movements of heavenly bodies, as stated above (A5,6): secondly, that they be not applied to other matters than those which in some way may have reference to these animals (since they acquire through the heavenly bodies a certain natural knowledge and instinct about things necessary for their life--such as changes resulting from rain and wind and so forth).
In the second place, this instinct is produced by a spiritual cause, namely, either by God, as may be seen in the dove that descended upon Christ, the raven that fed Elias, and the whale that swallowed and vomited Jonas, or by demons, who make use of these actions of dumb animals in order to entangle our minds with vain opinions. This seems to be true of all such like things; except omens, because human words which are taken for an omen are not subject to the disposition of the stars, yet are they ordered according to divine providence and sometimes according to the action of the demons.
Accordingly we must say that all such like divinations are superstitious and unlawful, if they be extended beyond the limits set according to the order of nature or of divine providence.
Reply to Objection 1. According to Augustine [QQ. in Genes., qu. cxlv], when Joseph said that there was no one like him in the science of divining, he spoke in joke and not seriously, referring perhaps to the common opinion about him: in this sense also spoke his steward.
Reply to Objection 2. The passage quoted refers to the knowledge that birds have about things concerning them; and in order to know these things it is not unlawful to observe their cries and movements: thus from the frequent cawing of crows one might say that it will rain soon.
Reply to Objection 3. Gedeon listened to the recital and interpretation of a dream, seeing therein an omen, ordered by divine providence for his instruction. On like manner Eliezer listened to the damsel's words, having previously prayed to God.
Source: Summa Theologica (New Advent)