From the ancient sceptics' suspension of judgment to Descartes' method and Peirce's fallibilism — doubt as poison, as tool, and as tranquillity.
Can we suspend judgment about everything — and if we doubt, is it a road to despair or the beginning of knowledge?
Doubt sits at the hinge of every theory of knowledge: is it a corrosive that dissolves all claims, or a discipline that clears the ground for firmer ones? The ancient sceptics turned suspension of judgment into a way of life aimed at tranquillity; the medieval and early-modern thinkers used doubt as a crisis to be passed through toward faith or certainty. Descartes made methodical doubt the engine of a new philosophy, only to arrest it at the thinking self; Hume showed how naturally we live in defiance of the doubts reason cannot answer. Later thinkers distinguished paper doubt from the real thing, and asked whether doubting everything is even coherent. Reading these positions together maps the difference between doubt that paralyses and doubt that inquires.
Ancient Greek scepticism
Since things are equally indeterminate and unmeasurable, we should hold no firm opinions and simply say of each thing that it 'no more' is than is not. This suspension of judgment (epochē) leads not to anxiety but to tranquillity (ataraxia), which follows suspension like a shadow.
Left no writings; his views survive through Timon and later Sextus Empiricus.
Academic scepticism
Following the New Academy, Cicero held that nothing can be known with certainty, yet the wise person may follow what is 'probable' or persuasive as a guide to life. Arguing both sides of a question exposes the weakness of dogmatism while leaving room for reasoned, provisional assent.
Academica; On the Nature of the Gods.
Pyrrhonian scepticism
Through the 'modes' or tropes, the sceptic sets appearances and judgments against one another, finding equal force on both sides and so suspending judgment. The Pyrrhonist does not assert that nothing can be known; he keeps inquiring, lives by appearances and custom, and finds that tranquillity supervenes on the very refusal to dogmatize.
Outlines of Pyrrhonism; Against the Mathematicians.
Madhyamaka Buddhism
Nāgārjuna's dialectic dismantles every fixed thesis about the nature of things, showing that all phenomena are 'empty' of inherent existence. He claims to advance no thesis of his own, using argument to loosen the grip of views rather than to install a new certainty — a therapeutic doubt aimed at liberation from conceptual grasping.
Mūlamadhyamakakārikā (Fundamental Verses on the Middle Way).
Islamic theology and Sufism
In a personal crisis Al-Ghazālī doubted the senses and then reason itself, falling into a scepticism no argument could cure. He was rescued not by proof but by a 'light' God cast into his heart, restoring trust in first principles; genuine certainty, he concluded, comes through experiential knowledge (dhawq) beyond mere demonstration.
The Deliverance from Error (al-Munqidh min al-Dalāl).
Renaissance humanism / scepticism
'What do I know?' (Que sais-je?) is Montaigne's motto: human reason is weak, custom-bound, and endlessly variable, so dogmatic certainty is presumption. His scepticism is not despairing but humane, counselling modest self-examination, tolerance and acceptance of our limits rather than the tranquillity of doctrine.
Essays (1580), especially 'Apology for Raymond Sebond'.
Rationalism
Descartes deploys hyperbolic, methodical doubt as an instrument: he sets aside everything that can possibly be doubted — the senses, mathematics, even a deceiving demon — to find one indubitable point. That point is the cogito: while I am deceived, I must exist as a thinking thing, and on this rock he rebuilds knowledge.
Meditations on First Philosophy (1641); Discourse on the Method (1637).
Empiricism / scepticism
Hume argues that reason cannot justify our beliefs about causation, the external world, or the self — these rest on custom and imagination, not proof. Yet radical scepticism is unlivable: nature compels belief, and after leaving his study the philosopher plays backgammon and rejoins common life. His is a mitigated scepticism, humbling reason while trusting habit.
A Treatise of Human Nature (1739–40); An Enquiry Concerning Human Understanding (1748).
German idealism / critical philosophy
Roused from 'dogmatic slumber' by Hume's doubt, Kant answers scepticism not by defeating it head-on but by limiting knowledge to appearances structured by the mind's forms and categories. We can have certain knowledge of the phenomenal world while acknowledging that things-in-themselves lie beyond it — a critical philosophy that both answers and disciplines doubt.
Critique of Pure Reason (1781); Prolegomena (1783).
Existentialism / Christian thought
Doubt cannot be overcome by more thinking, for reflection is potentially endless; it is halted only by an act of will and, ultimately, by faith. Against the Cartesian project, Kierkegaard holds that in the deepest matters certainty is won not by disinterested proof but by the passionate, risking commitment of the existing individual.
Johannes Climacus, or De Omnibus Dubitandum Est; Concluding Unscientific Postscript.
Pragmatism
Peirce rejects Cartesian make-believe doubt: 'Let us not pretend to doubt in philosophy what we do not doubt in our hearts.' Genuine doubt is a real, uneasy irritation that prompts inquiry until belief is restored. All our knowledge is fallible and self-correcting; certainty is an ideal limit approached by the community of inquirers, not a starting point.
'The Fixation of Belief' (1877); 'Some Consequences of Four Incapacities' (1868).
Analytic philosophy
Doubt itself requires a background of unquestioned certainties: 'if you tried to doubt everything you would not get as far as doubting anything.' Some propositions — that the world has existed for years, that this is my hand — form the 'hinges' on which inquiry turns, exempt from doubt not because they are proven but because doubting them would dissolve the practice of doubting.
On Certainty (posthumous, 1969).