Reason

← Encyclopedia

Reason is the mind’s capacity to move, by inference, from one thing known to another — to weigh, conclude, and give an account of why a thing is so. It is the faculty the West has most often named when asked what sets the human being apart, and also the one it has argued hardest over: how far it reaches, where it stops, and whether anything lies above it. The two questions are not the same. One asks what reason does — links premises to conclusions, tests a claim against its grounds, builds an order of thought that can be retraced and checked. The other asks what reason is worth — whether the order it builds touches the order of things, or only the order of the mind that builds it.

The Greek term-cluster

The vocabulary that still shapes the discussion is Greek, and the central word carries more at once than any single English term. Logos, in its account-giving sense, meant word and speech, but also ratio, proportion, the reckoning a thing answers to, and the ordering principle held to run through the world — so that to have logos was to be the kind of creature who could give reasons, render an account, say why. The same root yields logizesthai, to calculate or reason out, and logikos, the rational. Beside it stood other words for the mind’s activity: dianoia, thought that moves through steps; and nous, the higher seeing that takes its object whole. The career of reason in the West is in large part the career of these distinctions — preserved, blurred, narrowed, and recovered across two and a half thousand years.

A boundary holds here. Logos also came to name a divine utterance, the Word through which the world was made — a sense that belongs to its own line of thought and is treated under logos proper. What concerns reason as a faculty is the narrower, account-giving sense: the capacity to give grounds.

Plato and the rule of the rational part

Plato set the architecture that later thought worked within. In the Republic the soul has three parts — the reasoning part, the spirited part, and the appetites — and justice in the soul is the rule of the first over the other two, as justice in the city is the rule of those fit to reason over those who labor and those who fight. Reason here is not merely a tool for getting what the appetites want; it is the part that knows what is genuinely worth wanting, and philosophy is the discipline by which it comes to rule. The famous image of the divided line ranks the soul’s ways of knowing: at the lowest reach, conjecture about images; then belief about visible things; above that dianoia, the reasoning of the geometers, which argues from assumptions it does not examine; and at the summit noēsis, the unhypothetical grasp of the Forms themselves. Already the distinction is in place that will run through the whole tradition — step-by-step reasoning set below an immediate seeing it can approach but not equal.

Aristotle, Plato’s pupil, gave reason a different center of gravity. He defined the human being as the animal that has logos, made the study of valid inference into a discipline — the syllogism, the analytics of demonstration — and so handed the West its first formal theory of reasoning as such. Where Plato had reason climb toward a vision, Aristotle had it descend into the structure of argument: what follows from what, and on what grounds a conclusion is secured. Both moves are inherited together, and the tension between them never fully resolves.

The Stoic identification

The Stoics pressed the Platonic intuition into cosmology. On the Stoic account the universe is a single living body ordered throughout by an immanent reason — the logos that is also fire, breath, fate, and god — and the reason in a human being is a fragment of that cosmic reason, a portion of the divine running through the whole. This made the human capacity for inference continuous with the structure of the world: to reason well was to think along the grain of things. And it gave ethics its shape. To live well, on the Stoic teaching, was to bring one’s own judgment into agreement with the order that governs everything — to want what happens because one has understood why it must. The Discourses of Epictetus press this again and again: the one thing fully a person’s own is the use made of impressions, the assent the mind gives or withholds, and the whole of the discipline is the schooling of that assent. Of the Stoic divisions of philosophy — logic, physics, ethics — it is logic, the study of logos, that underwrites the rest, because the rational order it traces in argument is the same order the physics finds in the cosmos and the ethics asks the self to join.

From the Latin ratio — itself a reckoning-word, the term a Roman would use for a calculation or an account — the concept descended into the European languages: French raison, Italian ragione, English reason, each carrying the double charge of the mental act and the justifying ground. The descent was not neutral. Ratio translated logos but lost some of its reach, dropping the cosmic and the utterance-bearing senses and keeping the calculating one, so that the Latin West inherited a word already tilted toward inference and account.

Dianoia and nous: the Platonic distinction

Within the Platonic tradition a distinction grew up that later thought rarely keeps clean. Reason proper — dianoia, the discursive movement of thought from premise to conclusion — was set below a higher faculty, nous or intellect, which grasps its objects whole and at once, without the labor of inference. The discursive mind must travel; intellect simply sees. In Neoplatonism the ranking becomes a metaphysics. Plotinus held the reasoning soul to be a lower image of Intellect, which is itself a lower image of the One; the soul reasons because it cannot, at its own level, hold its objects together in a single act, and the philosopher’s ascent runs from reasoning toward an intuition that reasoning can prepare but not itself reach. The discursive is the unfolding, across time and steps, of what intellect possesses all at once — reason is the soul thinking in succession what nous thinks in simultaneity. The later school carried the structure forward and elaborated it. Iamblichus and Proclus built vast graded hierarchies in which the human reasoning soul occupies a fixed rung, below the divine and demonic intellects and above the merely living, and in which the soul’s task is the recovery of a higher knowing it has descended from. For these thinkers reason is genuine and necessary — the indispensable ladder — but it is a ladder, valued for where it leads.

The medieval inheritors preserved the pair as ratio and intellectus: the faculty that argues and the faculty that beholds, the one proper to a creature who must reach its truths by steps, the other a faint participation in the way angels and God are said to know — completely, at once, without discourse. Much of later mysticism turns on the same gap. The recurring claim is that the deepest knowing arrives where argument leaves off — that the mind, having reasoned as far as reasoning goes, is brought to a stop and given what it could not have inferred. What the mystic calls union and the Neoplatonist calls the return to the One name the far side of a boundary that reason itself marks out by failing to cross it.

The modern narrowing

The modern career of the word narrowed it, and in narrowing made it sovereign. In the rationalism of the seventeenth century — the systems built by Descartes, Spinoza, and Leibniz on the model of geometry — reason became the tribunal before which every claim was to be tried, the court that grants or withholds certainty by its own light. Descartes resolved to doubt whatever could be doubted and to admit only what the mind could see clearly and distinctly to be true, taking the order of reasons, not the testimony of the senses or the authority of the schools, as the foundation of knowledge. With the Enlightenment the word came to stand for the authority of the human mind as such, set against inherited belief — the claim that custom, revelation, and established power must answer to it rather than the reverse. Reason was no longer one faculty among several with a place in a graded order; it was the measure to which the rest were summoned.

Against this stood the empiricist counter-argument: that the senses, not reason working alone, are where knowledge begins, and that the mind contributes no substantial truth about the world that experience has not first supplied. Locke’s blank sheet, Berkeley’s reduction of bodies to what is perceived, and Hume’s doubt of the very inferences reason most relies on were the pressure applied from this side — the dispute between rationalism and empiricism that organized European philosophy for two centuries, and that turned, at bottom, on exactly the question the older tradition had asked in other terms: how far the mind reaches when it works on its own.

A third current ran beneath both. From late antiquity onward the skeptical tradition — Pyrrho’s heirs, codified in the modes of suspension and preserved in the works of Sextus Empiricus — had pressed reason against itself, arguing that for every argument an opposing argument of equal force can be found, that every proof rests on a premise that itself demands proof, and that the honest end of inquiry is the suspension of judgment. The recovery of these texts in the Renaissance gave the early modern doubt much of its edge. Skepticism is not the denial of reason but its turning inward: reason used to measure its own reach, and finding, on the skeptic’s account, that the reach does not close.

Sources and study

The ancient material survives unevenly, and the public-domain record reflects the difference. For the Stoic identification of human reason with the cosmic logos, the most direct surviving voice is Epictetus, whose Discourses and Manual — transcribed by his pupil Arrian — are available in P. E. Matheson’s 1916 Oxford translation, hosted in full at sacred-texts.com; George Long’s 1877 rendering circulates alongside it. The early Stoa survives only in fragments and testimonia, gathered in Hans von Arnim’s Stoicorum Veterum Fragmenta (Teubner, 1903–1924), and reported at length in Diogenes Laertius’ Lives of Eminent Philosophers, Book VII, read in English through C. D. Yonge’s Bohn translation (1853) or R. D. Hicks’s 1925 Loeb. The standard modern anthology of the Hellenistic schools’ epistemology is A. A. Long and D. N. Sedley, The Hellenistic Philosophers (Cambridge, 1987).

For the Platonic ranking of dianoia below nous, the loci are the Republic (the divided line at 509d–511e) and Plotinus on the soul’s discursive thinking, read in Stephen MacKenna’s Enneads (Medici Society, 1917–1930); the fifth Ennead, on Intellect and the soul, is where the distinction is drawn most sharply. The late Platonist elaboration in Iamblichus and Proclus reaches modern readers chiefly through Thomas Taylor’s nineteenth-century renderings and the in-copyright Ancient Commentators series.

For the skeptical turning of reason against itself, the surviving systematic corpus is that of Sextus Empiricus; the only complete public-domain English of a single book is Mary Mills Patrick’s translation of the first book of the Outlines of Pyrrhonism in Sextus Empiricus and Greek Scepticism (Deighton Bell, 1899), available at Project Gutenberg; Richard Bett, ed., The Cambridge Companion to Ancient Scepticism (Cambridge, 2010), is the current scholarly survey. The early modern defense of reason as a real but non-final instrument is documented in the writings of the Cambridge Platonists, whose recurring emblem cast reason as a candle of the Lord — a light kindled in the mind but not its own source — and is collected in the Cambridge Platonism Sourcebook, alongside Ralph Cudworth’s True Intellectual System of the Universe (1678).

Reason and what it serves

Esoteric currents have generally treated reason with respect and reservation together — honoring it as a real instrument while declining to give it the last word. That last word is reserved for gnosis or direct illumination, a knowing held to lie past the reach of argument, or for the revelation that comes from outside the mind rather than from its own working. The figures who pressed reason hardest from within these currents did not despise it: Giordano Bruno built his cosmology by argument and reasoned the universe open to infinity before he reasoned past argument toward a vision he held argument could only point at. The relation is rarely simple hostility. It is a placement.

And placement is the structure to hold onto. To rank reason below something else is not to demote a rival but to assign an office. The Neoplatonist who sets dianoia under nous, the mystic who sets argument under union, the esotericist who sets reason under gnosis — each grants the faculty its work in full and then names the thing that work is for. Reason is the ladder, and a ladder is honored precisely by being climbed and left standing. It gives the account; it tests the grounds; it carries the mind as far as steps can carry it. What it does not do is supply its own summit — and a faculty that knows the edge of its own reach, and keeps working up to that edge, is not diminished by being told where the edge is. It is given its measure.

→ In the library: Plotinus — The Enneads (MacKenna) · Epictetus — The Discourses and Manual (Matheson, 1916)

→ Related: Nous · Logos · Empiricism · Neoplatonism · Gnosis · Plato · Stoicism · Aristotle · Soul · Knowledge · Mysticism · Revelation · Iamblichus · Proclus · Giordano Bruno