According to the doctrine of St. Augustine and St. Thomas, the foundation of all divination is an implicit or explicit pact with a demon. A pact is judged implicit when someone seeks knowledge reserved to God alone from some source other than God, and by means not due to that end, that is, not at all instituted for this purpose by divine providence. For by the very fact that someone has regard to these vanities and false insanities, he plainly wishes to be taught by a demon, who is the author of curiosities of this kind and mixes himself very willingly into their functions.
An explicit pact or invocation is twofold. It is either made with express and conceived words, as enchanters do when they conjure by illicit formulas instituted by a demon; or it is made by the fact and action itself, when someone does that which he knows the demon has established as a sign or token of his help and efficacy.
The theologians, like Grillandus, reduce all explicit forms to necromancy, and implicit forms to sortilege and augury, using these words too loosely and improperly, since their meaning is narrower and does not adequately comprehend all the species, which are almost countless. The matter and order now call me to treat these species one by one. I shall divide all the species which I find into two, as it were, heads. In the former I shall include those in which an express pact is clearly recognized; in the latter, the remaining ones.
[margin: Augustine, On Christian Doctrine, book 2, chapters 22-23; Thomas Aquinas, Secunda Secundae, question 95, article 2, on divination; Cajetan on question 95, article 3; Navarro, Manual, chapter 11, number 21; Grillandus, question 6, on explicit and implicit pact]