Theme
Knowledge and scepticism
40 philosophers, 5 traditions
What can we know, and how? Thinkers in several traditions have asked which sources of knowledge can be trusted, and whether the sceptic's doubts can ever be answered.
Philosophers in many traditions have asked what we can know, how we come to know it, and whether doubt, once raised, can be answered. In India the theory of knowledge (pramāṇa-śāstra) became a genre of its own, spanning almost twenty centuries, in which schools debated which sources of knowledge, such as perception, inference and testimony, are genuine. Nāgārjuna asked what certifies those sources themselves; Nyāya philosophers replied that a doubter’s own actions contradict the doubt.
In China, Zhuangzi questioned whether any standpoint could be shown to be the right one, and later Confucians argued over whether moral knowledge comes from studying things or is already present in the mind. In the Islamic world, Suhrawardi set aside knowledge through mental representations in favour of the direct presence of what is known.
In Europe, Plato contrasted knowledge of unchanging forms with the error-filled world of the senses. Descartes made doubt a method for finding certain foundations, Hume argued that our expectations about the future rest on habit rather than reason, and Kant tried to show how some knowledge of the world can be certain after all.
Through time
6th century BCE
Xenophanes
No one has seen the clear truth about the gods or the nature of things, and even a lucky guess would not be knowledge; mortals have only opinion. Yet by searching over time, people find out what is better.
5th century BCE
Protagoras
Each person is the measure of things: as they appear to me, so they are for me. Critics from Democritus to Aristotle objected that the claim refutes itself, but it may be read as saying that truth is always relative to someone.
4th century BCE
Plato
Plato held that the senses show a defective world full of error, so real knowledge is of the unchanging forms, which the soul grasps best when free of bodily distraction. In the Meno, learning is recollecting what the soul knew of the forms before birth.
Argued against Protagoras
Zhuangzi
Zhuangzi doubted that any standpoint can be shown to be the right one: winning a dispute doesn’t make you correct, and no judge can settle it. His scepticism is modest rather than total, denying final certainty while allowing degrees of knowing, and it invites openness to other perspectives.
3rd century BCE
Chrysippus
Against the sceptics, some impressions carry their own guarantee: they come from a real object, match it exactly and are clear. These “grasping” impressions are the standard of truth.
1st century BCE
Cicero
Certainty is not available, so no view should be treated as settled. We can still judge some positions more plausible, more like the truth, than others, and act on them while remaining open to revision.
Drew on Plato
1st century CE
Wang Chong
Received teachings, even those of the sages, must be tested by questioning and challenging them, and accepted only if they prove true and useful. Wang rejected common beliefs in ghosts and in a Heaven that rewards and punishes, because the evidence did not support them.
2nd century CE
Sextus Empiricus
For every argument there is an equally persuasive counter-argument, so the sceptic suspends judgement on philosophical and scientific questions while continuing to inquire. Scholars still debate whether this leaves the sceptic with any beliefs at all.
Nāgārjuna
Nāgārjuna asked what certifies the sources of knowledge themselves: each needs a further warrant or must simply be declared self-validating. Sources and their objects establish each other, so neither has intrinsic nature, and he claimed to have no thesis of his own to defend.
About 400 years later
7th century CE
Dharmakīrti
Dharmakīrti recognised only two sources of knowledge: perception, which grasps real, momentary particulars without concepts, and inference, which works through concepts. A cognition counts as knowledge if it is reliable enough to act on, as when the vase we saw really does hold water.
Kumārila Bhaṭṭa
Every cognition carries its own claim to truth and should be trusted until a specific contrary cognition overturns it. Requiring each cognition to be proved reliable first would start a regress that no knowledge could survive.
10th century CE
Saadia Gaon
There are four sources of certain knowledge: the senses, reason, inference and reliable tradition. His book sets out to cure the errors and doubts that keep people from the truth and so from living rightly.
12th century
Judah Halevi
Sceptical arguments undermine the philosophers’ claims about God and the world: they disagree with one another and their proofs meet equal counter-proofs. Certainty comes instead from what witnesses saw with their senses and passed on as tradition.
Śrīharṣa
Every attempt to define knowledge fails: true awareness can be true by luck, as when a gambler guesses right. More generally, any philosophical argument can be met by an equally persuasive counter-argument, so reason cannot settle philosophical questions.
Fakhr al-Din al-Razi
We cannot acquire knowledge by defining essences, and our concepts and beliefs are not up to us; what we know is given to us directly. Later readers suspected him of scepticism, but he held that knowledge is attainable, even if certain demonstration is not possible on many questions.
Suhrawardi
Suhrawardi denied that we know things by acquiring forms or definitions of them in the mind, as the Peripatetic philosophers held. The most basic knowledge is the direct presence of a thing to the self, starting with the self’s awareness of itself, and only direct experience guarantees true knowledge.
Drew on Plato
14th century
Ibn Taymiyya
Knowledge comes from outer and inner sense perception, from reliable reports, from reason and from our natural constitution. Aristotelian definitions add nothing: our concepts arise from experience, and definitions merely name and distinguish things.
Argued against Fakhr al-Din al-Razi
John Duns Scotus
Using only its natural powers, without any special help from God, the human intellect can know the truth with certainty. Scotus argued against both scepticism and the view that certainty requires divine illumination.
Gaṅgeśa
Gaṅgeśa, defending the Nyāya school, held that perception, inference, analogy and testimony, working properly, yield only true awareness. Doubt is answered by the doubter’s own conduct: no one can regularly turn to fire to get smoke while doubting that fire causes smoke.
15th century
Nicholas of Cusa
Knowing is measuring, and no finite thing can be measured exactly, so all our knowledge is approximate, a matter of conjecture. The wise know the limits of their knowledge: their ignorance is learned.
16th century
Wang Yangming
Wang Yangming held that moral knowledge isn’t found by investigating external things, as Zhu Xi taught, but is the “pure knowing” already in every mind. Real knowing shows itself in action: those who know but do not act “simply do not yet know”.
Michel de Montaigne
Reason is easily deceived and our opinions are largely the product of custom, so no one can claim to have found unshakeable truths. Montaigne’s scepticism counterbalances each opinion with others, keeping judgement free rather than suspending it altogether.
Drew on Sextus Empiricus
17th century
Francis Bacon
Knowledge must be built up from the senses, but the senses and the mind are distorted by “idols”, so they need correcting by experiment, instruments and a careful method of induction. Rightly pursued, knowledge becomes a source of power over nature.
Drew on Michel de Montaigne
René Descartes
Descartes set out to doubt everything that could be doubted, supposing he might be dreaming or misled by an evil deceiver. The certainty that he exists while thinking survived, and he argued that a God who is no deceiver guarantees whatever the intellect perceives clearly and distinctly.
Drew on Michel de Montaigne
18th century
George Berkeley
Materialism leads to scepticism, since if we perceive only ideas we could never know whether matter resembles them. By identifying objects with the ideas we perceive, Berkeley claimed to defend common sense against the sceptic.
Argued against René Descartes
David Hume
Hume argued that reason cannot justify our expectation that the future will resemble the past; such beliefs come from custom or habit. Yet total, Pyrrhonian doubt can’t be sustained, so he recommended a “mitigated” scepticism that lowers our confidence and limits the scope of our inquiries.
Drew on George Berkeley and Cicero
Immanuel Kant
Woken from his “dogmatic slumber” by Hume, Kant argued that some things can be known a priori, for example that everything that happens has a cause, because the mind brings these forms to experience. Such knowledge is certain but reaches only as far as possible experience, never to things in themselves.
Drew on and argued against David Hume; drew on George Berkeley
19th century
Charles Sanders Peirce
Of the ways of settling doubt and fixing belief, the method of science is the best. Its conclusions are statistical rather than exact, and a concept is clear only when we can say what observations it leads us to expect.
Drew on and argued against Immanuel Kant; drew on John Duns Scotus
Gottlob Frege
Arithmetic is not known through experience or intuition but can be derived from logic alone, with each step of a proof justified by an axiom or a rule of inference. Frege’s own attempt failed when Russell found a contradiction, but it transformed the study of logic and mathematics.
Edmund Husserl
Knowledge needs a secure foundation, which phenomenology seeks by describing how things are given in experience. Bracketing our natural belief in the world is a method for studying how any object comes to appear to us at all.
Drew on and argued against René Descartes
Henri Bergson
Against Kant, absolute knowledge is possible. Analysis, which serves our practical needs, gives only relative knowledge; intuition rises above the quarrels of rationalists and empiricists.
Argued against Immanuel Kant
John Dewey
Knowing is not an escape from illusion to an ultimate reality, but one of the ways organisms cope with problems. Inquiry begins in doubt and proceeds by hypothesis and test; ideas are judged by how they work in practice.
Drew on Charles Sanders Peirce
20th century
G. E. Moore
We know for certain many truisms of common sense, such as that the earth existed long before we were born. Sceptics who deny this knowledge contradict what they themselves know; what remains undecided is how such truths are to be analysed.
Argued against Immanuel Kant
Rudolf Carnap
Knowledge is built from logic and experience, and scientific theories can be confirmed to a degree but never finally verified. Many traditional problems of philosophy are artefacts of imprecise language, and metaphysical claims convey nothing at all.
Studied with and drew on Gottlob Frege; drew on and argued against Immanuel Kant
Karl Popper
Induction cannot justify scientific laws, but that does not lead to scepticism. Knowledge grows by bold conjectures exposed to attempted refutation; a theory earns our trust by surviving tests it could have failed.
Drew on David Hume
Theodor W. Adorno
No object can be fully known by thinking that forces it into identity with its concepts, and none is simply “given” either. Thought must respect the nonidentical, and think in contradictions to expose the antagonisms that society covers up.
Drew on and argued against Immanuel Kant; argued against Edmund Husserl
Maurice Merleau-Ponty
Knowledge rests on “perceptual faith”, the pre-reflective conviction that perception gives us the world as it is. Empiricism and idealism both go wrong by treating the world as ready-made; phenomenology describes how it is actually experienced.
Drew on and argued against Henri Bergson and Edmund Husserl
W. V. O. Quine
There is no standpoint outside science from which to justify it. Science extends common sense, and our beliefs face experience as a whole, like a web, so almost any belief could in principle be revised, even logic and mathematics.
Drew on and argued against Rudolf Carnap
Thomas Kuhn
Scientific knowledge does not grow by steady accumulation or by bold refutations. Normal science works within a paradigm it does not question, and revolutions replace one paradigm with another that cannot be measured against it on common terms.
Argued against Karl Popper
Michel Foucault
What passes for timeless scientific truth about human beings is often the product of contingent historical forces. Knowledge in the human sciences is bound up with power, and critique means showing that what seems necessary might be otherwise.
Drew on Immanuel Kant; studied with and drew on Maurice Merleau-Ponty
Sources
- Peter Klein, Skepticism, Stanford Encyclopedia of Philosophy
- Anand Vaidya, Epistemology in Classical Indian Philosophy, Stanford Encyclopedia of Philosophy