St. Augustine explains why he stopped believing in horoscopes by telling the story of Firminus & The Slave. I have bolded all the main characters (except Augustine).
… I struggled against Vindicianus, that keen old man, and Nebridius, a young man admirable in mind. The first affirmed vehemently and the second said frequently, although with some hesitation, that there is no art of foreseeing the future, and that men’s conjectures are often assisted by chance: for since they say many things, some of them actually come to pass, and apart from any knowledge in the speakers, they hit upon these things by the mere fact that they do not remain silent.
You [God] provided me with a friend who was neither a foolish client of the astrologists nor one well versed in their studies, but, still, as I said, a curious consultor of them. Furthermore, he had some knowledge, which he said he had heard from his father, but he did not know how it would serve to topple over his belief in that art. This man, Firminus by name, who was possessed of a liberal education and well trained in rhetoric, consulted me, as one of his dearest friends, as to what I might think, in the light of his so-called constellations, about certain of his affairs, upon which his worldly ambitions were taking rise. I had already begun to incline towards Nebridius’s opinion in this matter, but I did not refuse to interppret them and to tell him what came into my mind, still undecided as it was. However, I submitted that I was now almost persuaded that these are empty and ridiculous fables. He then told me that his father was very much addicted to such books, and had a friend who studied them at the same time and with equal passion. By joint study and discussion they so fanned in their hearts the desire for such trifles that they even made observations on the moments when their dumb animals were born, if they were brought forth at home, and noted the position of the heavens at those times. From these things they would gather proofs for their so-called art.
He told me that he had heard from his father that, when his mother was carrying himself, Firminus, a servant of one of his father’s friends was likewise pregnant. This fact did not escape her master, who even took pains to know by very careful examination the time when his dogs littered. Thus, while the two men, one for his own wife, the other for his servant, by most painstaking observations, figured out the day, the hour, and the most minute particles of the hour, both women were delievered at the same time. As a result, they were compelled to draw up identical horoscopes, right down to the same minute, for each of the newborn infants, one man for his son, the other for his tiny slave. For when the women began to be in labor, each man indicated to the other what was happening in his home. They arranged to send messengers to each otehr, as soon as the expected birth was announced to them. Each man in his own estate easily provided for word to be sent immediately. The messengers sent by the two men met, he said, exactly at the midpoint between their houses, so that neither of them could determine a different position for the stars or different moments of time. However, Firminus, who was born to an ample estate within his own family, ran his course on life’s brighter paths, increased in weatlth, and rose to places of honor, whereas that slave served his masters with never a lightening of the yoke of his condition, as I was told by Firminus, who knew him.
After I had listened to and believed this story, for such a man had related it, all the reluctance of mine was dissolved and gave way. First, I attempted to recall Firminus from that fond study. I said to him that, after I had inspected his horoscope, if I were to make true predictions, I would surely have to see therein his parents, eminent among their fellow citizens, a family nobly placed in its city, gentle birth, good education, and liberal learning. But if that slave had consulted me about the same horoscope, for the two were identical, I ought again, so as to speak truly to him also, see therein a family most abjectly poor, a servile condition of life, and other things far different and far removed from the first. Hence it would be that from an inspection of the same horoscope I would state different things, if I were to speak the truth, but if I made identical statements, I would speak falsely. From this I gathered with absolute certainty that any true statements made after an inspection of such horoscopes would be uttered not by art but by luck, while false statements would be made not out of ignorance of the art but by the trickery of chance.
Having taken this approach to the problem, I ruminated within myself upon related things. So that none of the dotards following such a trade, whom I longed to attack right off and to refute with ridicule, might object to me that either Firminus had given me a false account or his father had given him one, I turned my attention to those who are born twins. For the most part, one issues from the womb so close upon the other that that brief difference in time, however great the power they may claim it to have in the nature of things, cannot be determined by human observation, nor can it be written down at all in those tables which the astrologer must inspect in order to make true predications. Yet they will never be true, for after inspecting the same tables, he must say the same things of both Esau and Jacob, although the same things did not befall both men. Therefore, he would make false statements; or if he made true ones, he would not be saying the same things. Yet he inspected the same tables. Not by art, therefore, but by chance would he make true statements.
This excerpt was taken from The Confession of Saint Augustine (New York, NY: Doubleday Dell Publishing Group, Inc., 1960), 163-166.