What Is Skepticism? (A Philosophical Approach)

In philosophy, skepticism (from the Greek skepsis, or ‘investigation’) is the position that many commonly held beliefs are unjustified or do not constitute knowledge. Everyone should be skeptical of certain claims. If someone says this coin will appear heads on a fair toss, or that one year from today the temperature in Toronto will be exactly fifteen degrees Celsius, we should be doubtful. But other claims typically do not invite skepticism. That Boston is the capital of Massachusetts, that this tree has green leaves, or that functioning cars typically have four wheels are claims widely regarded as unremarkable knowledge, something we can take for granted. 

Skepticism is a philosophical position that denies this. It says the only justified response even to these commonsense claims is to suspend belief because knowledge of them is elusive.

Skepticism dates back to the genesis of Western philosophy. Socrates, the interlocutor in Plato’s dialogues, sets out in the Apology to determine whether anyone is wiser than he, for he believes he is not wise at all. Likewise, in the Meno, Socrates raises a puzzle about whether rational inquiry is possible: to inquire we must have an idea of what we’re looking for, otherwise we won’t recognize it; but if we already have an idea of what we’re looking for, why must we inquire? 

In dialogues like Phaedo and The Republic, finally, Socrates’ apparent skepticism extends even to ordinary perceptual claims. Imagining two examples of our own, a house appearing big might seem small to someone, and a lake feeling cold to someone who was just in a sauna might feel warm to someone just out of a freezer. So, even perceptions of the same parts of the world appear to vary from person to person. 

Two Varieties of Skepticism Are Common in Western Philosophy

We can begin defining skepticism by distinguishing three kinds of attitudes (what philosophers call ‘doxastic attitudes’, where doxa is Greek for ‘belief’) towards any claim: belief, doubt, and suspension of belief. Skepticism is the view that the only defensible doxastic attitude towards a certain class of claims is to suspend belief, that is, to neither believe nor disbelieve it. 

The ‘class of claims’ determines the scope of the skepticism in question. Skepticism about the external world, for example, might counsel suspension of belief about perceptual claims, but endorse beliefs about one’s own mental states. Skepticism about the past might counsel suspension of belief about historical claims, but endorse beliefs about the present. ‘Local’ skepticism, like these varieties, is restricted to certain classes of claims. ‘Global’ skepticism extends to all claims.

In addition to the scope of skepticism, we can distinguish its strength. Two positions are common in Western philosophy. Academic (or ‘Cartesian’) skepticism is a weaker form of skepticism because it says we should suspend belief in certain classes of claims, like those about the external world, but we are entitled to believe some higher-order claims, like that we should suspend beliefs in certain classes of claims. Cartesian skepticism is therefore weaker because it does not iterate to higher-order claims about skepticism itself.

Pyrrhonian (or ‘philosophical’) skepticism is considerably stronger because it does iterate to higher-order claims about skepticism itself. So in addition to counseling suspension of belief in all regular claims, like those made on the basis of perception, Pyrrhonian skepticism denies even that we have reason to believe claims like ‘We should suspend belief in perceptual claims.’ An obvious question for Pyrrhonists is whether their strong skepticism is self-undercutting. 

Let’s say you know the United States has more inhabitants than Germany, and that Germany has more inhabitants than Canada. Do you also know on this basis that the United States has more inhabitants than Canada? It is very plausible that you do, and the reason is that knowledge is very plausibly closed under known entailment. This is the closure principle: if I know some proposition p, and I know that p entails another proposition r, then I also know r. 

This principle is exceedingly plausible. But it also plays a central role in arguments for Cartesian skepticism. Consider this canonical argument: 

1. If I know this is my hand in front of me, then I know I am not a brain in a Matrix-like vat where I have no hands (from Closure)
2. I do not know I am not a brain in a Matrix-like vat where I have no hands
3. Therefore, I do not know this is my hand in front of me (from 1 and 2)

Premise (1) applies the closure principle to a common skeptical hypothesis, similar to Descartes’ evil demon hypothesis. (2) says that we cannot disprove this hypothesis. Why not? The argument is that if this skeptical hypothesis is true, we would be unable to know this because all the evidence available to us would appear the same: in the vat it would still appear to us that we have hands. The conclusion, (3), then endorses Cartesian skepticism about the external world: we should suspend belief about ordinary perceptual claims, like those about our own hands. The lesson is that either the closure principle is false or Cartesian skepticism is true. 

Arguments for Pyrrhonian Skepticism Use Agrippa’s Trilemma

Pyrrhonian skepticism is stronger than Cartesian skepticism because it counsels suspension of belief in any claim whatsoever. What would an argument for this radical position look like? The answer is the Agrippa’s Trilemma. 

Continuing with our example, the Agrippean Trilemma says that there are three ways to justify your belief you see your hand before you: you assert it as obvious (‘dogmatism’); you justify it by appeal to another belief, then another, without end (‘infinitism’); or your chain of belief terminates in a circle (‘coherentism’). The Pyrrhonian skeptic maintains these three options are exhaustive and devastating, because none is defensible. 

Dogmatism is indefensible, the Pyrrhonist believes, because it is no different from mere insistence something is true. Infinitism is indefensible because no finite mind can possibly entertain an infinite chain of beliefs. And coherentism is indefensible because circular chains of justification assume just what you want to prove: here, that you see your hand before you. 

The upshot is that Pyrrhonists believe the only response to skepticism is ataraxia: a mental tranquility that they think arises from suspending belief in all matters—including in skepticism itself. 

Generally, then, whether skepticism is of a Cartesian or Pyrrhonian variety, it is a philosophical position that says much, if not all, of the knowledge we take for granted is actually a fiction.