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 foetuses 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. But we know that twins do not only act differently, and travel to very different places, but that they also suffer from different kinds of sickness; for which Hippocrates would give what is in my opinion the simplest reason, namely, that, through diversity of food and exercise, which arises not from the constitution of the body, but from the inclination of the mind, they may have come to be different from each other in respect of health. Moreover, Posidonius, or any other asserter of the fatal influence of the stars, will have enough to do to find anything to say to this, if he be unwilling to im pose upon the minds of the uninstructed in things of which they are ignorant. But, as to what they attempt to make out from that very small interval of time elapsing between the births of twins, on account of that point in the heavens where the mark of the natal hour is placed, and which they call the "horoscope," it is either disproportionately small to the diversity which is found in the dispositions, actions, habits, and fortunes of twins, or it is disproportionately great when compared with the estate of twins, whether low or high, which is the same for both of them, the cause for whose greatest difference they place, in every case, in the hour on which one is born; and, for this reason, if the one is born so immediately after the other that there is no change in the horoscope, I demand an entire similarity in all that respects them both, which can never be found in the case of any twins. But if the slowness of the birth of the second give time for a change in the horoscope, I demand different parents, which twins can never have.
 This fact is not recorded in any of the extant works of Hippocrates or Cicero. Vives supposes it may have found place in Cicero's book, De Fato.