St Anselm Versus All Possible Worlds

The question at issue in St. Anselm’s Cur Deus Homo is fundamentally about the mode of necessity (modus necessitatis) operative in divine action—that is, the manner in which God acts within the order of creation. The central problem is not whether the Incarnation is fitting, nor whether it accomplishes satisfaction for sin, but whether it was required by an intrinsic demand of divine justice such that God could not have redeemed humanity otherwise. To address this question adequately, one must distinguish carefully among the various modes of necessity, clarify the nature of the divine essence, and examine the metaphysical structure of satisfaction.

The Doctor Magnificus, Anselm of Canterbury, grounds his argument in the structure of divine justice. Sin is a failure to render to God the honor that is owed to Him (Ps 51:4). “Non est conveniens ut Deus sic dimittat peccatum impunitum.”God cannot remit sin without satisfaction, for He does not deny His own rectitude (1 Jn 1:9). Satisfaction must be proportionate to the offense, which is measured according to the dignity of the one offended (Hab 1:13). Because the offense is against the infinite majesty of God, it bears a certain infinity (Isa 6:3). Fallen humanity cannot render infinite satisfaction (Rom 3:23), yet satisfaction must be made by man, since man sinned (Rom 5:19). Therefore, the satisfaction required by divine justice can be rendered only by one who is both God and man (1 Tim 2:5).

From this structure Anselm’s argument can be rendered schematically:

∀x (Human(x) → ¬AbleToRenderInfiniteSatisfaction(x))

∀x (¬God(x) → ¬AbleToRenderInfiniteSatisfaction(x))

∴ ∃x (God(x) ∧ Man(x)) who renders satisfaction.

Thus, if God wills to save humanity consistently with justice, the Incarnation appears necessary. Anselm’s language at times suggests more than hypothetical necessity; it can be read as implying that divine justice itself requires the God-man.

When we turn to Thomas Aquinas in Summa Theologiae III, q. 1, a. 2, we find a decisive clarification. Aquinas states: “Non fuit necessarium Deum incarnari necessitate absoluta, sed ex suppositione.” The Incarnation was not necessary with absolute necessity, but only on the supposition of a divine decree. Absolute necessity (□P) holds independently of any condition. Hypothetical necessity (□(P | Q)) holds only given a prior supposition. Aquinas explicitly denies □(Incarnation). Instead, he affirms □(Incarnation | God wills to restore humanity in this mode).

This distinction follows from the metaphysics of divine simplicity and freedom. In God, justice is not an external norm imposed upon the divine will. Rather, Justice(God) = Essence(God). Since the divine essence is pure act (actus purus) and absolutely free in its operations ad extra, no created effect can be metaphysically necessitated. If the Incarnation were absolutely necessary, a reductio would follow: either (1) God would be subject to an external standard of justice, or (2) there would be unrealized potency in God actualized by human sin. Both are incompatible with classical theism and with Aquinas’s account of divine simplicity.

Nevertheless, Anselm’s central insight remains sound. The Passion constitutes sufficient and superabundant satisfaction for the sins of the human race (cf. ST III, q. 48, a. 2). The metaphysical reason lies in the doctrine of suppositum. Actions are attributed to the person, not merely to the nature. Because Christ’s person is divine, His human acts possess infinite dignity. The infinity of satisfaction must therefore be grounded not merely in the gravity of sin but in the ontological dignity of the acting subject—the eternal Word.

Formally stated:

If Dignity(x) = ∞ → Merit(x) = ∞.

Christ is a divine person acting through a human nature.

∴ Merit(Christ’s acts) = ∞.

The satisfaction is infinite because the agent is the eternal Word.

Yet Aquinas further teaches that every work of divine justice presupposes mercy (ST I, q. 21, a. 4). Mercy is not opposed to justice; rather, it is its root in execution. The entire order of redemption—including the Incarnation—is initiated by gratuitous love, not compelled by retributive equivalence. Divine justice operates within a freely established order whose very existence is the effect of mercy.

This point is sharpened by Thomas Cajetan in his commentary on ST III, q. 1, a. 2. Cajetan insists that the necessity in question is solely “ex suppositione divinae ordinationis.” Once God freely ordains that humanity be redeemed through satisfaction proportionate to sin, the Incarnation follows necessarily. But the divine ordination itself is free; there is no necessity antecedent to the decree.

In logical terms:

Let R = God wills redemption through satisfaction.

Let I = Incarnation.

Anselm’s rhetoric can appear to suggest □(R → I), with R itself treated as morally unavoidable. Aquinas and Cajetan clarify instead:

¬□I

□(I | R)

◇(Redemption ∧ ¬I)

Thus, while the Incarnation follows necessarily given the divine decree R, it remains possible that God could have willed another mode of redemption. The Incarnation is therefore not absolutely necessary, but hypothetically necessary within the order God freely chose.