A dozen centuries ahead of his time, he rejects astrology in favour of environment and heredity as explaining human traits. From The City of God:
'Cicero says that the famous physician Hippocrates has left in writing that he had suspected that a certain pair of brothers were twins, from the fact that they both took ill at once, and their disease advanced to its crisis and subsided in the same time in each of them. 3 Posidonius the Stoic, who was much given to astrology, used to explain the fact by supposing that they had been born and conceived under the same constellation. In this question the conjecture of the physician is by far more worthy to be accepted, and approaches much nearer to credibility, since, according as the parents were affected in body at the time of copulation, so might the first elements of the fætuses have been affected, so that all that was necessary for their growth and development up till birth having been supplied from the body of the same mother, they might be born with like constitutions. Thereafter, nourished in the same house, on the same kinds of food, where they would have also the same kinds of air, the same locality, the same quality of water -- which, according to the testimony of medical science, have a very great influence, good or bad, on the condition of bodily health -- and where they would also be accustomed to the same kinds of exercise, they would have bodily constitutions so similar that they would be similarly affected with sickness at the same time and by the same causes. But, to wish to adduce that particular position of the stars which existed at the time when they were born or conceived as the cause of their being simultaneously affected with sickness, manifests the greatest arrogance, when so many beings of most diverse kinds, in the most diverse conditions, and subject to the most diverse events, may have been conceived and born at the same time, and in the same district, lying under the same sky.'