One of the Scholastic axioms says:

7.7 The same causes in the same circumstances produce always the same effects.

How do Thomists justify that axiom when in Quantum Mechanics we see a different result? We can have the same circumstances that lead to different outcomes.

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    There are two strategies. One is to pick an interpretation of quantum mechanics other than the Copenhagen, Bohmian mechanics, and even some versions of MWI, are compatible with causal determinism, see Why Modern Physics Does Not Refute Thomistic Philosophy, and its critique on Quantum Thomist. Another is to weaken causality to something less deterministic, which is needed anyway under the libertarian concept of free will that Aquinas himself (arguably) favored.
    – Conifold
    Jun 7, 2019 at 3:30

1 Answer 1


It seems you're asking about what Thomists think about determinism vs. indeterminism, whether causes infallibly produce their effects—whether "physical laws" admit exceptions.

A good overview of indeterminism is Thomist Charles de Koninck's "The Problem of Indeterminism" (1935) and "Reflections on the Problem of Indeterminism" (1937), pp. 357-442 of Writings of Charles de Koninck (vol. 1).

He quotes these key texts:

Summa Theologica I q. 115 a. 6 co.:

it is not true that, given any cause whatever, the effect must follow of necessity. For some causes are so ordered to their effects, as to produce them, not of necessity, but in the majority of cases (in pluribus), and in the minority to fail in producing them. But that such cases do fail in the minority of cases is due to some hindering cause
non est verum quod, posita quacumque causa, necesse sit effectum poni. Sunt enim quaedam causae quae ordinantur ad suos effectus non ex necessitate, sed ut in pluribus, quae quandoque deficiunt in minori parte. Sed quia huiusmodi causae non deficiunt in minori parte, nisi propter aliquam causam impedientem

In VI Metaphysicorum l. 2 n. 1182:

there are other beings which are neither always nor of necessity, but for the most part, i.e., in the majority of cases (in pluribus). And this, i.e., what occurs in the majority of cases, is the principle and the cause of the accidental.
quaedam vero non sunt ex necessitate, nec semper, sed sunt secundum magis, idest ut in pluribus. Et hoc, scilicet ens ut in pluribus, est causa et principium quod aliquid sit per accidens.

St. Thomas summarizes the refutation of Aristotle (Physics II, 4) of those who denied chance and fortune in Summa contra Gentiles III q. 86:

  1. […] some of the ancients […] denied chance and fortune on the basis of the view that there is a definite cause for every effect. If the cause be granted, then the effect must be granted. Thus, since everything occurs by necessity, there is nothing fortuitous or by chance.
    quorundam antiquorum […] negabant casum et fortunam, per hoc quod cuiuslibet effectus est aliqua causa determinata; posita autem causa, ponitur effectus de necessitate; et sic, cum omnia ex necessitate proveniant, non est aliquid fortuitum neque casuale.

  2. He answers this argument, in Metaphysics VI [2-3], by denying two propositions which the argument uses. One of these is: “if any cause be granted, it is necessary to grant its effect.” Indeed, this is not necessary in the case of all causes, for a certain cause, though it may be the direct, proper and sufficient cause of a given effect, may be hindered by the interference of another cause so that the effect does not result. The second proposition that he denies is: “not everything that exists in any way at all has a direct cause, but only those things that exist of themselves; on the other hand, things that exist accidentally have no cause.” For instance, there is a cause within a man for the fact that he is musical, but there is no cause for the fact that he is at once white and musical. As a matter of fact, whenever plural things occur together because of some cause they are related to each other as a result of that cause, but whenever they occur by accident they are not so related to each other. So, they do not occur as a result of a cause acting directly; their occurrence is only accidental. For instance, it is an accident to the teacher of music that he teaches a white man; indeed, it is quite apart from his intention; rather, he intends to teach someone who is capable of learning the subject.
    Hanc autem rationem ipse solvit in VI Metaphys., negando duas propositiones quibus haec ratio utitur. Quarum una est quod, posita causa quacumque, necesse sit eius effectum poni. Hoc enim non oportet in omnibus causis: quia aliqua causa, licet sit per se et propria causa et sufficiens alicuius effectus, potest tamen impediri ex concursu alterius causae, ut non sequatur effectus. Alia propositio est, quam negat, quod non omne quod est quocumque modo, habet causam per se, sed solum ea quae sunt per se; quae autem sunt per accidens, non habent aliquam causam; sicut quod sit musicum, habet aliquam causam in homine, quod autem homo sit simul albus et musicus, non habet aliquam causam. Quaecumque enim sunt simul propter aliquam causam, ordinem habent ad invicem ex illa causa: quae autem sunt per accidens, non habent ordinem ad invicem. Non igitur sunt ex aliqua causa per se agente, sed solum per accidens hoc evenit: accidit enim docenti musicam quod doceat hominem album, est enim praeter eius intentionem, sed intendit docere disciplinae susceptibilem.

cf. C. S. Peirce's trichotomy, contra the determinism/indeterminism dichotomy

  • I am sorry but I do not see how this answers my question. What Aquinas seems to say is that some causes can fail to produce the proper effect, but only if there is another cause that interferes (eg. sperm is directed to always produce male offspring, but the reason why it does not is that there is some hindering cause, and if it not for that hindering cause the male would of necessity follow.). Are you saying that there is some hindering cause which makes QM system to behave in a way it does? And that the QM system is not really in the same circumstances?
    – Thom
    Jun 8, 2019 at 17:26
  • @Thom "Are you saying that there is some hindering cause which makes QM system to behave in a way it does?" Yes, interactions with the environment are very pivotal. Schlosshauer emphasizes this point in his Decoherence: And the Quantum-to-Classical Transition. "And that the QM system is not really in the same circumstances?" Just because one can setup QM systems in identical quantum states does not mean the circumstances are the same. The "hindering causes" are in the "circumstances".
    – Geremia
    Jun 8, 2019 at 23:28
  • @Thom Also, check out my question on how one cause can impede the action of another cause. I still haven't received a satisfactory answer; perhaps the status quæstionis could be made preciser…
    – Geremia
    Jun 8, 2019 at 23:31
  • Is then axiom which was mentioned in my question falsifiable with any experiment? Because as soon as some experiment would apparently falsify it, one would say: "oh, that experiment does not falsify my axiom, it just means there is some hidden hindering cause."
    – Thom
    Jun 9, 2019 at 16:05
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