Philosophical Scepticism and What We Can Know

Suppose you see a tree outside a window. You may believe the tree is there because vision usually provides useful information. A sceptic asks how you know you are not dreaming, looking at a convincing image or being deceived in some other way.

These possibilities seem far-fetched, but they reveal assumptions behind ordinary knowledge. You trust that your senses connect you with a world, that your memory is reasonably accurate and that the future will often resemble the past. Those assumptions are practical, yet proving them without using the same tools can be difficult.

Radical scepticism argues that we may not be able to know much about an external world with complete certainty. More limited scepticism targets particular areas, such as testimony, memory or moral claims. Philosophy has therefore managed to create several carefully labelled varieties of doubt, because one apparently lacked sufficient administrative detail.

Scepticism can protect you from accepting claims too quickly. It encourages questions about evidence, source reliability and alternative explanations. In science, organised doubt supports testing, replication and revision rather than simple trust in authority.

However, demanding absolute certainty for every belief creates a problem. Almost no ordinary claim would qualify as knowledge. You could not know that breakfast happened, that other people have minds or that the floor will continue supporting you after the next step. Absolute certainty may still matter in a few philosophical arguments, but everyday reasoning usually works with degrees of justified confidence.

Many philosophers therefore use standards such as strong evidence, reliability or the ability to rule out relevant alternatives. You do not need to eliminate every imaginable possibility. You need enough support for the context and consequences involved. A casual weather claim and a criminal conviction reasonably require different levels of evidence.

A sceptical approach is most helpful when it improves judgement rather than freezing it. Instead of asking only whether a claim is certain, ask how the belief was formed and what could show it to be wrong.

Useful questions include:

• •What evidence supports the belief?
• •How reliable is the source or method?
• •Which alternative explanations are realistic?
• •What would change my mind?
• •How much confidence does the situation require?

Sceptical ideas have a long history. In ancient Greece, the Pyrrhonian sceptics, named after the philosopher Pyrrho, argued that for almost any claim, equally strong reasons could be found on both sides. Their response was to suspend judgement, and they claimed this brought a kind of calm. Much later, René Descartes, in his Meditations published in 1641, imagined a powerful deceiver feeding him false experiences. He looked for something that could survive even that doubt and settled on his own thinking: while doubting, he must exist to be doing the doubting.

Replies to scepticism have been just as varied. The twentieth-century philosopher G. E. Moore held up his hands and argued that he was more certain he had hands than he could ever be of the premises in any sceptical argument, so it was reasonable to reject the argument instead. Another approach, called contextualism, suggests that the standards for saying you know something shift with the situation. In an ordinary conversation, you know where your car is parked, while in a philosophy seminar focused on dream scenarios, the bar is raised.

For a long time, many philosophers treated knowledge as justified true belief: you know something if you believe it, it is true and you have good reason for it. In 1963 the American philosopher Edmund Gettier published a short paper with cases that seem to break this definition. A similar example appeared earlier in the work of Bertrand Russell. Imagine you glance at a clock that has always been reliable and read the time as three o'clock. Your belief is reasonable, and it is in fact three o'clock.

The catch is that the clock stopped exactly twelve hours earlier. Your belief is true and justified, yet it seems wrong to say you knew the time, because you got it right by luck. Cases like this show that sceptical worries are not only about wild scenarios such as dreaming. Even ordinary good reasons can connect to the truth in the wrong way. Philosophers have proposed fixes, such as requiring that the belief come from a reliable process or that no hidden falsehood supports it, but none has won complete agreement.

One of the most powerful sceptical arguments targets something you do constantly: expecting the future to resemble the past. The Scottish philosopher David Hume pointed out in the eighteenth century that every time you predict that bread will nourish you or the sun will rise, you rely on the assumption that nature behaves uniformly. You cannot prove that assumption by pure logic, because a world where it failed tomorrow involves no contradiction. And you cannot prove it from experience either, since arguing that it has worked before, so it will keep working, already assumes the very thing in question. This puzzle is called the problem of induction.

Bertrand Russell gave the worry a memorable image. A chicken is fed by the farmer every day and comes to expect food whenever the farmer appears, until the day the farmer wrings its neck instead. Past regularity did not guarantee the future. Philosophers have answered in different ways. Some argue that induction cannot be justified but is still the best policy available, since if any method of prediction works, induction will too. Karl Popper argued that science does not need to prove theories by induction at all, only to test them and reject those that fail. Others treat beliefs as probabilities, updated as evidence arrives.

One ancient sceptical argument needs no dreams or demons, only the question why. Suppose you believe a claim. Asked for a reason, you give a second claim, but that needs a reason too, and so on. According to an argument credited to the Pyrrhonian sceptic Agrippa and preserved by the later writer Sextus Empiricus, the chain can only end in one of three ways. It can go on forever, it can loop back on itself in a circle, or it can stop at some claim you simply accept. Each option seems to leave the whole chain resting on nothing secure.

Philosophers have answered by defending one of those ends. Foundationalists argue that the chain can rightly stop at basic beliefs that need no further support, such as how things seem to you right now. Coherentists argue that beliefs support one another like a web, so a circle need not be vicious if the system is large and fits together well. The philosopher Otto Neurath compared knowledge to sailors who must rebuild their ship plank by plank while staying afloat at sea, never able to start again on dry land. A smaller group even argues that an endless chain of reasons can be acceptable.

Philosophical scepticism tests whether beliefs are supported well enough to count as knowledge. Radical doubt reveals the assumptions beneath everyday confidence, while more practical scepticism helps you examine evidence and reliability. The goal need not be permanent uncertainty. It can be a calmer habit of matching confidence to reasons.