When an express pact is present, the matter has no difficulty. In a tacit pact, judgment is more difficult. Nevertheless, three things can be observed: namely, the proportion of the thing to be known and the cause through which knowledge is sought; also the nature with which the knowledge sought is endowed; and the intention which the one seeking it has.
First, if there is such a proportion that one thing is sufficiently apprehended from another, this inquiry will not be divinatory or to be condemned. This proportion is threefold. One is such as effects have to their cause; thus an eclipse is known from the motion of the heavens. Another is such as a cause has to its effect; thus fire is known from smoke. The third is such as different effects have to a common cause with which each effect is connected, and thus they are also in some way proportioned to one another by reason of origin. Thus, because the cause of the heavenly rainbow is also the cause of fair weather, we rightly gather fair weather from the rainbow. But if from none of these three heads one thing can be known from another, the investigation should be judged joined with a tacit pact with a demon.
Second, as regards the cognition itself, or the mode of knowing, consider whether certain and infallible cognition is being sought, or at least cognition firmer than it is right to hope for, once the freedom of human choice and of other similar concurrent causes has been considered. Then a tacit pact will be judged present. From this there arises a very strong suspicion against astrologers, interpreters of dreams, and the like, who assert something absolutely about future contingents of this sort, even if they say that they wish to affirm it only probably. On the contrary, from the same head astrologers, dream-interpreters, and the like are excused if in reality they only conjecture certain things about human events as probable and indefinite, from heavenly impressions and bodily affections which human beings usually follow of their own accord.
Third, one must attend whether someone wishes seriously to seek knowledge of hidden things through means vain for that purpose, and expects it from these means alone, not from God. For then a tacit pact is presumed to be present.
From this head he will be excused from divinatory superstition who, first, gives attention to this divination only for the sake of play, knowing it to be useless. No one would judge him superstitious who looked in a hen’s brain for an emerald, or on the back of an ape for a tail, knowing that he would not find it there. Second, a person is excused who, knowing that a thing is by its own nature ineffective for this purpose, nevertheless expects from God, by that thing, knowledge asked for with prayers and desires. In this way the consultatory lots in Sacred Scripture are excused: concerning Achan’s theft, the cause of the storm, and Jonathan, according to the saying of the wise man: lots are cast into the lap, but they are governed by the Lord.
So too those are excused who, in a grave cause and an arduous matter, when no other remedy is available, have asked a sign from God and desire to be taught from heaven or earth, or through some human voice or action spoken or done outside the intention of the person. The servant of Abraham and Jonathan acted in this way. And these things are truer than the too-rigid positions which Nider set down in Precept 1, chapter 9, letter D, at the end.
[margin: Sixtus V’s bull against astrology; Cajetan, Secunda Secundae, question 95, articles 5-6, on probability and divinatory inquiry; Scriptural lots concerning Achan, the storm, Jonathan, and the saying from Proverbs 16; Thomas Aquinas, Secunda Secundae, question 95, article 7, and Nider, Preceptorium, book 1, chapter 9, letter D near the end, on signs sought from God]