Arthur Lovejoy’s The Great Chain of Being is often read as a history of metaphysical exuberance: plenitude, overflow, and the slow erosion of contingency under explanatory pressure. What’s easier to miss is that Lovejoy also gives us a sharp diagnostic lens for understanding how philosophers attempt to contain that pressure.
What that lens reveals is a recurring structural move. I call it a modal firewall.
A modal firewall is a principled restriction on explanatory reach: a point at which intelligible reasons are permitted to explain structure, value, or possibility, but are forbidden from determining actuality. The firewall does not reject explanation. It commands it, then tells it where it must stop.
Seen through this lens, Thomas Aquinas becomes a revealing case—not because he invents the maneuver, but because Lovejoy’s framing makes its structure unusually clear.
The pressure Aquinas accepts
Lovejoy emphasizes that Aquinas fully accepts the rationalist pressures that make necessitarianism tempting. God is pure actuality. God is perfect goodness. God’s intellect contains the complete intelligible structure of all possible beings. Explanation is not optional; it is constitutive of intelligibility itself.
Given that package, a familiar question presses itself: if goodness is fully intelligible, why would a perfectly good being fail to actualize what is good? Why would possibility outrun actuality?
This is not yet Spinoza speaking. It is the pressure Aquinas must confront before Spinoza radicalizes it.
Crucially, Aquinas does not respond by weakening explanation. He does not appeal to mystery, opacity, or brute divine choice. God’s intellect remains exhaustive; reasons remain reasons. The pressure toward necessity is fully in place.
The firewall: divine will as modal cutoff
Aquinas’s response is not to deny the pressure but to block its extension.
Although God necessarily knows all possibles and their relative perfections, God does not necessarily will any of them. Creation is therefore not a logical consequence of divine nature but a contingent act of volition. God could have created a different world, or no world at all.
This is the firewall.
Explanatory reasons may explain what creatures are like, if they exist. They may explain why creation would be fitting or good. But they are not permitted to explain that creation occurs, or why this possible world is actual rather than another. At precisely that juncture, explanation is told where it must stop.
For a rationalist, this is the most uncomfortable possible outcome. Explanation is not refuted; it is obeyed everywhere except where obedience would eliminate contingency. The firewall does not emerge from explanation; it is imposed upon it.
Leibniz rationalizes plenitude; Spinoza completes it
This is where Lovejoy’s tripartite schema earns its keep.
To say that Leibniz rationalizes plenitude is to say that he removes Aquinas’s firewall while preserving contingency in name. God necessarily acts for sufficient reason, but contingency is relocated into the structure of reasons themselves. Different possible worlds are intelligible; God freely selects the best among them. Plenitude is no longer blocked. It is disciplined by optimization.
Spinoza, by contrast, refuses the remaining distinction. If God’s nature explains everything that exists, then nothing could have been otherwise. Possibility collapses into actuality. There is no selection among alternatives because there are no genuine alternatives to select among.
Where Leibniz preserves contingency by complicating reason, Spinoza abolishes it by allowing reason to run without remainder.
In Lovejoy’s narrative: Aquinas contains plenitude by prohibition; Leibniz rationalizes it through sufficient reason and optimality; Spinoza completes it by removing the prohibition altogether.
Lovejoy’s diagnostic contribution
It would be a mistake to claim that Aquinas invents this maneuver. Voluntarist strategies of this sort have deep roots, including in Islamic theology and medieval debates about divine freedom. What Lovejoy contributes is not a genealogy of voluntarism but a structural diagnosis: he shows how Aquinas’s appeal to divine will functions as a targeted containment strategy within an otherwise rationalist framework.
This matters, because not all appeals to will stand in the same relation to reason. An occasionalist denial of secondary causation, for example, may dismantle the explanatory machinery itself rather than merely restricting its scope. Aquinas’s strategy is different. He preserves explanation almost everywhere. That’s what makes the firewall both powerful and philosophically unstable.
Seen through Lovejoy’s lens, Aquinas is not someone who rejects necessitarian pressure, but someone who cordons it off. Contingency survives only because explanation is commanded to halt at a specific boundary.
And once that boundary is made visible, a further question becomes unavoidable: What justifies telling explanation where it must stop?
That question, Lovejoy suggests without fully pressing, will not remain contained.
Petronius Jablonski 