HistoricalChristian.Faith

Does “potuit, decuit, ergo fecit” prove the Immaculate Conception?

Related arguments: the theological inferences — see also Does Mary’s sinless life entail her immaculate conception?

Type: inference-form argument (off-timeline) · Assessment: weak as a proof, plausible as an illuminative mode — the schema “God could do it, it was fitting, therefore he did it” cannot bridge possible-and-becoming to actual without proving too much, and its own defenders — Aquinas included — deny that fittingness arguments are demonstrative. As a disciplined guide that displays the coherence and “beauty” of a doctrine held on other grounds, the same reasoning is a legitimate and ancient theological mode; as the thing that establishes the doctrine, it fails.

This page adjudicates the inference form — the completing move “decuit, ergo fecit” common to Eadmer’s si voluit, fecit, William of Ware, and the Scotist tag. For the distinct question whether the mechanism it completes — Scotus’s preservative redemption — coherently reconciles the privilege with a universal Redeemer, see Does Scotus’s “preservative redemption” dissolve the objection? (assessment: plausible). The two are deliberately kept apart: a mechanism can be coherent (that page’s finding) and the inference that says it obtained still fall short of proof (this page’s).

The claim

The Son of God was able to make Mary without original sin… It was fitting and decent that the Son of God would honor His Mother this way… Therefore, God did make his Mother without original sin.

— a modern statement of the schema by the apologist Taylor Marshall, offered as a proof to put to a skeptic: ask whether God could preserve Mary, whether it would have been fitting, and conclude that he did.¹ The historic form is older and more careful: Eadmer of Canterbury reasoned “Potuit plane et voluit; si igitur voluit, fecit” (“He clearly could and he willed it; if therefore he willed it, he did it”, c. 1125); William of Ware put it “what he could do, it was fitting that he did, and… it follows that he did so”; and the later Franciscan school compressed both into the four-word tag potuit, decuit, ergo fecit. The question weighed here is not whether the conclusion is true but whether this inference-form proves it.

Who advances it

The schema descends from Anselm’s method of convenientia (fittingness) through Eadmer and William of Ware to Scotus, who used it — on his own showing — only “by way of illustration,” resting the real weight of his case on preservative redemption rather than on the formula. In modern popular apologetics it is frequently detached from that reserve and presented as a stand-alone proof (so Marshall, above). The interpretation this page assesses is that stand-alone use: the fittingness inference as something that establishes the Immaculate Conception.

The evidence for

The steelman is real and should not be waved off. Convenientia is a legitimate, ancient mode of theological reasoning under revelation: Anselm’s Cur Deus Homo argues the whole fittingness of the Incarnation this way, and Aquinas opens his treatise on the Incarnation by asking whether it was fitting (conveniens) that God become incarnate — answering that “it belongs to the essence of the highest good to communicate itself… in the highest manner.”² Because the economy of salvation is God’s free act rather than a chain of necessities, what is fitting to that plan is a genuine — if defeasible — guide to what is actual: a doctrine that displays greater coherence, unity, and “beauty” has, other things equal, a claim on our assent, much as physicists take the loveliness of a theory as evidence of its truth. On this reading the fittingness of an unstained Mother of God is not decorative but probative in the weak sense — a reason, not a proof.³

Adversarial assessment

What holds. As an illuminative or supporting mode, the inference is sound theology and no embarrassment: it exhibits the fit between the privilege and the rest of the economy (a sinless Mother for a sinless Son, the New Eve answering the New Adam), and a fittingness that coheres with everything else believed is a legitimate, defeasible consideration in its favour. Held this way — as Scotus and Aquinas held their fittingness arguments — it strengthens a doctrine received on other grounds.

What is overstated. As a proof the schema is invalid, and this is not a hostile verdict but the common judgement of its own friends. The decisive step is decuit, ergo fecit — from “it was fitting” to “therefore he did it” — and fittingness is not necessity. Three things sink it as a demonstration. First, its authors concede the point: Aquinas ranks the Incarnation itself as “not… necessary” in the strict sense but fitting — a “better and more suitable” means God freely chose — and because such convenientia arguments bear on God’s contingent, freely-chosen acts, they cannot be demonstrated from first principles without necessitating what God left free.⁴ Second, it proves too much: if “could + fitting→did” were valid, it would license countless things God could and fittingly might do but demonstrably did not — a schema that establishes its conclusion establishes far more than anyone will accept. Third, the doctrine’s own tradition treats the tag as a weak reed: the standard reference calls the completing inference “somewhat dangerous and doubtful,” and notes that Scotus advanced it only “by way of illustration.”⁵ The general caution behind all of this is this wiki’s own: that God could and fittingly would act does not establish that he did — possibility and congruity are not actuality. So the inference can carry a doctrine already held; it cannot lift one into place.

Assessment

Weak as a proof; plausible as an illuminative mode. A fittingness inference cannot bridge “possible and becoming” to “actual” without proving too much, and its defenders — Aquinas foremost — deny it is demonstrative; deployed as the stand-alone proof modern apologetics sometimes makes it, it does not hold. But the same reasoning, kept in its proper office — displaying the coherence and beauty of a doctrine received on other grounds, a real if defeasible guide because the economy is God’s free and fitting work — is a legitimate and ancient theological mode, and on that use it stays plausible. The distinction to keep: this page rates the inference form; the preservative-redemption mechanism it completes is rated on its own page (plausible as a mechanism, coherence not being actuality), and the objection both were built to answer on a third. For the witnesses who framed the schema and the road to 1854, see the Immaculate Conception timeline.

Sources