Argument from Degrees

Among Thomas Aquinas's Five Ways presented in the Summa Theologiae, the Fourth Way is often regarded as both the most metaphysically ambitious and the most contested. Aquinas begins from an observation about ordinary experience: among existing things, we find some more and some less good, true, noble, and so forth. We routinely rank actions as better or worse, arguments as more or less true, and characters as more or less noble.

From this starting observation, Aquinas reasons that such comparative language implies an underlying scale, and that scales of this kind are measured with reference to a maximum. Just as we call something hotter the closer it approaches the maximum of heat, we call things better or truer the closer they approach a maximum of goodness or truth. This maximum, Aquinas concludes, must not only exist but must actually cause these qualities in the things that possess them in lesser degree.

Aquinas illustrates his reasoning with an analogy borrowed from Aristotle's Metaphysics: fire, being the maximum of heat, is the cause of heat in everything else that is hot. The analogy is meant to show a general pattern — that a maximal instance of a quality within a genus functions as the causal source of that quality in lesser instances, not merely as a comparative benchmark.

This reasoning depends heavily on the Aristotelian metaphysical framework Aquinas inherited, particularly the doctrine of the transcendentals: being, unity, goodness, and truth, understood as properties that convertibly apply to everything that exists, admitting of degrees culminating in an unqualified maximum. Without this background metaphysics, the inference from 'more or less X' to 'a maximally X cause' does not obviously follow, which is precisely where much contemporary criticism is directed.

Central to the Fourth Way is the classical doctrine of participation, inherited partly from Neoplatonism through thinkers like Plotinus. On this view, finite things possess perfections like goodness or being only in a derivative, dependent way, 'participating' in a fuller source of that perfection rather than possessing it as their own underived nature.

This metaphysical picture treats reality as hierarchically structured, with lesser beings deriving their limited share of perfection from a supreme, unlimited source. Historians of philosophy like Etienne Gilson and Fran O'Rourke have traced how Aquinas adapted this broadly Neoplatonic framework within an Aristotelian metaphysics of act and potency, producing a distinctively Christian synthesis used throughout the Summa Theologiae.

The most philosophically contested step in the Fourth Way is the move from 'there exists a maximal instance of a quality' to 'that maximal instance causes the quality in everything else.' Simply being the highest example of a scale does not, on the face of it, entail causal responsibility for lesser instances — a tallest person in a room does not thereby cause the height of everyone else present.

Aquinas's defenders argue this objection misses the metaphysical, not merely comparative, sense in which he intends 'maximum.' Within Aristotelian metaphysics, perfections are transmitted causally from actuality to potentiality; a purely comparative reading of 'maximum' would indeed fail to generate the causal conclusion, but Aquinas's framework is explicitly causal-metaphysical rather than merely comparative.

The philosopher Anthony Kenny offered one of the most detailed contemporary critiques of the Fourth Way, arguing that it relies on outdated Platonic assumptions no longer philosophically defensible — namely, that comparative predication ('more F than') requires an actual maximally F thing serving as a standard, and that this maximal thing must be a cause.

Kenny pointed out that we can meaningfully compare things as more or less F (for instance, more or less similar to a paradigm) without any of the items in question being maximally F, or without positing that the paradigm causes the property in the others. Thomist philosophers such as Brian Davies have responded that Kenny's critique, while highlighting genuine logical gaps, does not fully engage with the specific metaphysics of transcendental perfections Aquinas has in mind.

The Fourth Way's first premise — that goodness admits of real, objective degrees — connects directly to contemporary debates in metaethics about moral realism. If moral value judgments are merely expressions of preference or cultural convention, as anti-realists argue, the very starting point of the Fourth Way collapses, since there would be no objective scale of goodness to explain.

Contemporary moral realists, drawing on considerations of moral progress, cross-cultural convergence on core moral judgments, and the seeming objectivity of statements like 'torturing innocents for fun is wrong,' argue that some degree of objective value is defensible independent of theism, though establishing an intrinsic maximum of goodness requires further metaphysical argument beyond moral realism alone.

The argument from degrees remains a minority position even among philosophers sympathetic to natural theology, largely because it depends on a specific and contested metaphysical framework involving transcendentals and participation that many philosophers, both theist and atheist, no longer accept without qualification.

Nonetheless, contemporary Thomists such as Edward Feser have worked to restate the argument in updated terms, defending the underlying Aristotelian metaphysics on independent grounds. Whether or not one finds the argument persuasive, it illustrates how deeply interconnected classical natural theology is with broader questions in metaphysics and value theory, rather than functioning as an isolated proof.