# Nicene and Post-Nicene Fathers: Series I/Volume II/City of God/Book V/Chapter 5

Chapter 5.—In What Manner the Mathematicians are Convicted of Professing a Vain Science.
Do not those very persons whom the medical sagacity of Hippocrates led him to suspect to be twins, because their disease was observed by him to develop to its crisis and to subside again in the same time in each of them,—do not these, I say, serve as a sufficient refutation of those who wish to attribute to the influence of the stars that which was owing to a similarity of bodily constitution? For wherefore were they both sick of the same disease, and at the same time, and not the one after the other in the order of their birth? (for certainly they could not both be born at the same time.) Or, if the fact of their having been born at different times by no means necessarily implies that they must be sick at different times, why do they contend that the difference in the time of their births was the cause of their difference in other things? Why could they travel in foreign parts at different times, marry at different times, beget children at different times, and do many other things at different times, by reason of their having been born at different times, and yet could not, for the same reason, also be sick at different times? For if a difference in the moment of birth changed the horoscope, and occasioned dissimilarity in all other things, why has that simultaneousness which belonged to their conception remained in their attacks of sickness? Or, if the destinies of health are involved in the time of conception, but those of other things be said to be attached to the time of birth, they ought not to predict anything concerning health from examination of the constellations of birth, when the hour of conception is not also given, that its constellations may be inspected. But if they say that they predict attacks of sickness without examining the horoscope of conception, because these are indicated by the moments of birth, how could they inform either of these twins when he would be sick, from the horoscope of his birth, when the other also, who had not the same horoscope of birth, must of necessity fall sick at the same time? Again, I ask, if the distance of time between the births of twins is so great as to occasion a difference of their constellations on account of the difference of their horoscopes, and therefore of all the cardinal points to which so much influence is attributed, that even from such change there comes a difference of destiny, how is it possible that this should be so, since they cannot have been conceived at different times? Or, if two conceived at the same moment of time could have different destinies with respect to their births, why may not also two born at the same moment of time have different destinies for life and for death? For if the one moment in which both were conceived did not hinder that the one should be born before the other, why, if two are born at the same moment, should anything hinder them from dying at the same moment? If a simultaneous conception allows of twins being differently affected in the womb, why should not simultaneousness of birth allow of any two individuals having different fortunes in the world? and thus would all the fictions of this art, or rather delusion, be swept away. What strange circumstance is this, that two children conceived at the same time, nay, at the same moment, under the same position of the stars, have different fates which bring them to different hours of birth, whilst two children, born of two different mothers, at the same moment of time, under one and the same position of the stars, cannot have different fates which shall conduct them by necessity to diverse manners of life and of death? Are they at conception as yet without destinies, because they can only have them if they be born? What, therefore, do they mean when they say that, if the hour of the conception be found, many things can be predicted by these astrologers? from which also arose that story which is reiterated by some, that a certain sage chose an hour in which to lie with his wife, in order to secure his begetting an illustrious son. From this opinion also came that answer of Posidonius, the great astrologer and also philosopher, concerning those twins who were attacked with sickness at the same time, namely, “That this had happened to them because they were conceived at the same time, and born at the same time.”  For certainly he added “conception,” lest it should be said to him that they could not both be born at the same time, knowing that at any rate they must both have been conceived at the same time; wishing thus to show that he did not attribute the fact of their being similarly and simultaneously affected with sickness
to the similarity of their bodily constitutions as its proximate cause, but that he held that even in respect of the similarity of their health, they were bound together by a sidereal connection. If, therefore, the time of conception has so much to do with the similarity of destinies, these same destinies ought not to be changed by the circumstances of birth; or, if the destinies of twins be said to be changed because they are born at different times, why should we not rather understand that they had been already changed in order that they might be born at different times? Does not, then, the will of men living in the world change the destinies of birth, when the order of birth can change the destinies they had at conception?