Participatio
Finite perfections are possessed according to measure and point toward an unreceived source
Things are more or less true, good, noble, alive or intelligent, not because every comparison presupposes a largest specimen within the same genus, but because finite subjects participate in perfections according to limited modes. A warm object has heat received and bounded; a knower possesses truth according to the capacity of a finite intellect; a good action realizes order without exhausting goodness itself.
Participation means possession of a perfection in a partial, received or limited way. The participant is not identical with the perfection considered in its fullness. A creature is good, but it is not goodness itself; it has being, but its essence is not subsistent being. This structure connects the fourth way to the distinction between essence and existence. Limited actuality requires a principle that accounts both for the perfection and for its received mode.
The fourth way is often caricatured as “there are degrees of beauty, therefore there must be a most beautiful thing.” That is not enough. The argument turns upon transcendental and causally participated perfections, especially being, truth and goodness, whose limited presence in composites requires a source not receiving them by participation. A maximum in a genus need not cause every member; the cause sought lies beyond the genus as the source of actuality itself.
This ascent must be handled with care. Comparisons among subjective tastes do not automatically prove God. The metaphysician asks whether a perfection is pure or mixed with limitation, whether it admits more and less by participation, and whether the finite subject contains the reason for possessing it. The fourth way then converges with the other ways in a source that is not merely one good being but goodness by essence because it is being by essence.
Reader path: Fourth Way dossier · Summa Theologiae I, q. 4-6