Reason

Pre-Socratic

Reason is not primarily a human faculty but the Logos — the universal rational principle and measure according to which all things come to pass, even though most people live 'as though they had a private understanding.' Wisdom consists in listening to this common Logos and grasping the hidden harmony of opposites. The order of the cosmos and the order of the mind share a single rational structure.

Fragments (On Nature): 'Listening not to me but to the Logos...'

Ancient Greek (Platonism)

Reason (logistikon) is the highest of the three parts of the soul, whose proper role is to rule over spirit and appetite as a charioteer guides two horses. Only reason apprehends the eternal Forms and the Form of the Good, and the just, healthy soul is one in which reason governs with wisdom. In the well-ordered city as in the soul, the rational element must lead if there is to be harmony.

Republic (Books IV, VI–VII); Phaedrus (the charioteer).

Ancient Greek (Peripatetic)

Rationality (the possession of logos) is the distinctive function of the human being, so the good life is the activity of the soul in accordance with reason. Aristotle distinguishes theoretical reason, which contemplates necessary truths, from practical reason (phronēsis), which deliberates about action; the highest happiness lies in contemplation, the most divine activity available to us. Reason is both what we share with the gods and what sets us above the other animals.

Nicomachean Ethics, Books I, VI, X; De Anima, Book III.

Rationalism

'Good sense is the most evenly distributed thing in the world': reason is the one power that makes us human and, rightly directed by method, can reach certain knowledge. Descartes trusts reason over the deceptive senses, deriving truth from clear and distinct ideas apprehended by the 'natural light' of the mind. Reason, not tradition or authority, is the sovereign judge of what is true.

Discourse on Method (1637); Rules for the Direction of the Mind.

Rationalism

Reason grasps things through their causes and 'under a certain aspect of eternity' (sub specie aeternitatis), understanding them as following necessarily from the one substance, God-or-Nature. To live by reason is to form adequate ideas and thereby transform passive emotions into active ones, achieving freedom and blessedness. The highest knowledge is an intuitive rational understanding culminating in the 'intellectual love of God.'

Ethics (1677), Parts II, IV–V.

Rationalism

Reason rests on two great principles: non-contradiction and the principle of sufficient reason — nothing is so without a reason why it is so rather than otherwise. Leibniz dreams of a 'universal characteristic,' a formal language that would let thinkers settle disputes by calculation: 'let us calculate.' Truths of reason are necessary and hold in all possible worlds, while truths of fact depend on God's choice of the best among them.

Monadology (1714); Discourse on Metaphysics (1686).

Christian apologetics / mathematics

Pascal both honours and humbles reason: 'the heart has its reasons of which reason knows nothing.' Reason's last step is to recognize the infinity of things that surpass it, and first principles are grasped by the heart (intuition), not by reasoning. Reason is powerful yet perpetually unsettled by the passions and by our condition; in matters of faith it must yield to a knowledge it cannot itself supply.

Pensées (publ. 1670).

British empiricism

'Reason is, and ought only to be, the slave of the passions.' Reason discovers relations of ideas and matters of fact, but it cannot by itself move us to act or set our ends; only desire and sentiment do that. In morality especially, reason is inert: it can serve our passions by finding means, but it cannot supply the motivating force or the ultimate values.

A Treatise of Human Nature (1739–40), Book II, Part III.

Critical philosophy

Kant subjects reason to its own critical tribunal, mapping its legitimate use and its illusions. Theoretical reason, when it tries to reach beyond possible experience to God, freedom and the soul, falls into unavoidable antinomies; but practical reason legislates the moral law and grounds our freedom and dignity. Enlightenment is precisely reason's courage to use itself: 'Sapere aude!' — dare to think for yourself.

Critique of Pure Reason (1781); 'What Is Enlightenment?' (1784).

German idealism

'What is rational is actual, and what is actual is rational': reason is not a merely subjective faculty but the very structure of reality unfolding dialectically through history. Reason works even through passion and conflict — the 'cunning of reason' uses individual aims to realize larger rational ends. History is the progressive self-realization of Spirit becoming conscious of its own freedom and rationality.

Elements of the Philosophy of Right (1820); Lectures on the Philosophy of History.

Sociology

Weber diagnoses modernity as a process of 'rationalization': the spread of instrumental, means-end rationality (Zweckrationalität) through bureaucracy, science and capitalism, at the expense of value-rationality and enchantment. This brings efficiency and calculability but also the 'disenchantment of the world' and the threat of an 'iron cage' in which humans are trapped by the very systems they built. Instrumental reason answers 'how' but is silent about 'why.'

The Protestant Ethic and the Spirit of Capitalism (1905); Economy and Society.

Frankfurt School / critical theory

Enlightenment reason, in seeking to master nature and dispel myth, turns into its opposite: a purely instrumental, calculating rationality that dominates nature, society and the self. This 'dialectic of enlightenment' shows reason reduced to a tool of control, feeding the culture industry and even, catastrophically, modern barbarism. A reason that asks only about efficient means and abandons reflection on ends becomes a new form of unfreedom.

Dialectic of Enlightenment (1947).

Frankfurt School / critical theory

Against the reduction of reason to instrumental control, Habermas locates a 'communicative rationality' in the everyday practice of reaching understanding through argument. When people speak to coordinate action, they implicitly claim to be intelligible, true, sincere and right, and only 'the unforced force of the better argument' should decide — an 'ideal speech situation' free of domination. Reason is thus intersubjective and dialogical, not the monological calculation of an isolated subject.

The Theory of Communicative Action (1981).