Do twins refute astrology? — the potter's wheel, Firminius, and Jacob and Esau

Type: supporting argument (off-timeline) · Assessment: sound as a refutation of the strict natal determinism the Fathers targeted — two people born under one horoscope routinely meet opposite fates, and the astrologers' escape (that a sub-observable interval divides them) is self-defeating · pressed by Augustine, Basil, and Gregory of Nyssa, after Carneades and Cicero.

The claim

— Augustine, On Christian Doctrine II.22, newadvent.org. The assertion: if the natal chart determined the life, people with the same chart would have the same life — but they demonstrably do not, so the chart determines nothing.

Who advances it

The argument is pre-Christian. The Academic sceptic Carneades (2nd c. BC) framed it, and Cicero preserved it in On Divination II.90 — that people born at the same moment differ wildly, and people born at different moments across the world die together in one battle or one shipwreck. The Christian writers took it over wholesale and gave it a scriptural spearhead. Basil (c. 378) supplied the practical edge — the decisive interval can never be measured; Augustine (c. 397–417) assembled the fullest dossier, in three forms:

Gregory the Great (c. 591) repeats the Jacob-and-Esau form; Gregory of Nyssa (c. 380) argues the converse — whole cities of people born at different hours perishing "in a single instant" in an earthquake or flood, a common fate no shared star can explain.¹

The evidence for

The argument is strong exactly against the target the Fathers had in view: genethlialogy, the claim that the configuration of the heavens at birth fixes the course of a life. On that claim, identical charts should yield identical destinies, and they plainly do not. The astrologers' one available reply was Nigidius Figulus's potter's-wheel demonstration — that the celestial sphere turns so fast that even a heartbeat's difference between two births spans a great arc of sky, so twins are not really "under the same stars" after all. Augustine grants the physics and springs the trap: if that interval is too small for anyone to observe, then the astrologer, who likewise cannot observe it, has forfeited the very art he is defending. The reply, he says, "is more fragile than the vessels which are fashioned by the rotation of that wheel."² Modern empirical work has, if anything, strengthened the Fathers' conclusion: large "time-twins" studies — people born within minutes of one another — find no convergence in the life-outcomes natal astrology predicts.³

Adversarial assessment

What holds. Against hard astral determinism the argument is decisive, and it survives every historical counter-move. The potter's-wheel escape is self-refuting (the astrologer cannot read an interval he stipulates is unobservable); the boy-and-girl-twins case (City of God V.6) closes the last gap, since here one conception under one constellation yields even different sexes; and the empirical record of time-twins matches the ancient verdict. The scriptural instance (Esau and Jacob) is a genuine, not merely rhetorical, datum: the heel-grip guarantees a shared horoscope by the astrologers' own rules.

What is overstated or imprecise. Two qualifications keep the assessment honest. First, the argument refutes determinism, not every conceivable stellar influence: a softened astrology that claims only tendencies, probabilities, or "inclinations the wise man overrules" is not touched by the twins case, and some ancient astrologers held just such a probabilistic view (the maxim astra inclinant, non necessitant, "the stars incline, they do not compel"). The Fathers largely aimed at the deterministic version because that was the version that threatened free will and providence. Second, Cicero's companion point — that people born at different times die together — proves only that the stars do not fix the hour of death, not the whole of the natal thesis; it is the same-chart-different-life form that carries the weight.⁴

Assessment

Sound — as a refutation of genethlialogical determinism, which is what its patristic users were attacking. Identical horoscopes yielding opposite lives (up to opposite sexes) is a real disproof of the claim that the birth chart determines the life; the only historical rejoinder, Nigidius's unobservable interval, defeats the astrologer along with the objection; and modern time-twin data point the same way. The one limit to note — and it is a limit on scope, not a crack in the argument — is that it bites against deterministic astrology, and leaves a purely probabilistic or "inclining" astrology to be met on other grounds.

Sources

Cicero, De Divinatione II.43–47 (§§89–99), the Carneadean argument from twins and from common deaths; Gutenberg (Yonge trans.). Augustine, On Christian Doctrine II.22, newadvent.org; City of God V.1–6, newadvent.org; Confessions VII.6, newadvent.org. Basil, Hexaemeron VI.5, newadvent.org. Gregory of Nyssa, Against Fate, argues the converse from mass simultaneous deaths (McDonough, GNO III.2). ↩
Augustine, City of God V.3–4 (Nigidius Figulus and the potter's wheel), newadvent.org: "This argument is more fragile than the vessels which are fashioned by the rotation of that wheel." See the detail page, Augustine of Hippo. ↩
Geoffrey Dean and Ivan W. Kelly, "Is Astrology Relevant to Consciousness and Psi?", Journal of Consciousness Studies 10.6–7 (2003): 175–198 — a long-term study of "time twins" (people born close together in time) finding no astrologically predicted similarities. Journal of Consciousness Studies. ↩
On the "the stars incline, they do not compel" tradition and the deterministic-vs-probabilistic distinction, see Tim Hegedus, Early Christianity and Ancient Astrology, Patristic Studies 6 (New York: Peter Lang, 2007), esp. chs. on the twins topos. open-access PDF (OAPEN). The Latin maxim astra inclinant, sed non obligant / non necessitant is a scholastic crystallization (cf. Aquinas, Summa Theologiae I.115.4, that heavenly bodies act on the body and the passions but not on the will). ↩