External World Skepticism

Author: Andrew Chapman
Category: Epistemology
Word Count: 1000

Introducing and Motivating External World Skepticism

Look around your environment—turn your head this way and that and really take in everything around you. Let’s use a variable for the sake of ease and say:

Things seem to you to be P.

P is just a complete description of the way things seemed to you to be when you looked around. It is certainly obvious to you that things seem to you to be P. You might say that you have a special sort of access to how things seem to you. Now, which of the following scenarios is better supported by, that is, is more justified by, its seeming to you to be P?:

  1. Things actually are P.
  2. You’re being deceived by a very powerful evil demon right now. This demon has the ability to manipulate your sensory impressions such that it will seem to you that things are some way when they are not that way at all. Accordingly, things are actually nothing like P.

For example, suppose it seems to you as though you are in a room with a table and chair in it and that you are reading from a computer screen, etc. If (1) is true, then you actually are in a room with a table and chair in it and you are reading from a computer screen, etc. If (2) is true, then you are not in a room with a table and chair in it and you are not reading from a computer screen, etc. If (2) is true, things are very different from how they seem to you to be.1

Philosophers call (2) a skeptical scenario. In skeptical scenarios, you are radically misled, deceived, or bamboozled by your evidence in such a way that how things seem to you is different from how things actually are. Perhaps the most famous propounder of skeptical scenarios in the history of philosophy is René Descartes (1596-1650) in his Meditations on First Philosophy (1641). In the Meditations, Descartes considers that he might be dreaming or that he might be being deceived by the evil demon from our scenario (2) above. Hollywood has made much of skeptical scenarios in movies like Total Recall, The Matrix, and Inception.

So back to our original question: Which of (1) or (2) is best supported or best justified by its seeming to you that P? If you’re being honest with yourself, you’ll conclude that how things seem equally well supports (1) and (2). From your internal, first-personal perspective, either of (1) or (2) could be true given how things seem to you. And if that weren’t bad enough, here comes the kicker: If both (1) and (2) are equally well supported by your evidence, how can you ever possibly know anything about the world outside your own skin? This is the problem of external world skepticism, perhaps the central problem of modern epistemology.

Some people are tempted to shrug (or laugh) off skeptical arguments when they first encounter them. I hope that you see their power. The thrust of the skeptical argument isn’t just that it’s a remote possibility that we could be wrong about the external world. It’s that we have no reason to believe that things are the way we think they are rather than some other, massively different, way. If you have no good reason for believing that things are a certain way, then aren’t you a mere dogmatist to believe that you’re in a room with a table and chair, looking at a computer, living on Earth in the 21st Century?

Two Responses to External World Skepticism2

The famous defender of common sense, Scottish Enlightenment philosopher Thomas Reid (1710-1796), argued that absent some positive reason to believe that a skeptical scenario is true, common sense tips the epistemological scales in favor of the conclusion that we are not radically deceived. Similarly, following Michael Huemer (2001), recent philosophers have defended an epistemological principle known as Phenomenal Conservatism. This principle holds that, absent defeaters (i.e., things that could override or undercut our justification), we are justified in believing that the world is the way it seems to us to be.3 This position has some essentially Reidian elements to it. We needn’t let the possibility of skeptical scenarios destroy our knowledge of the external world unless there is some good, positive reason to consider those scenarios. Notice that both Reid’s and Huemer’s doctrines rely on a sort of justificational inertia. Our default epistemic position is justified and it takes some positive reason to destroy or question this justification.

Some philosophers have been happy to accept our general ignorance of the external world. According to the Greek philosopher Pyrrho of Elis (c. 360 – 270 BCE), founder of the Skeptical school of philosophy, knowledge is impossible (Pyrrho’s writings are all lost—we know of his doctrines through the writings of Sextus Empiricus (c. 160 – 210 CE)). It is even impossible, says Pyrrho, to know that knowledge is impossible! This is the doctrine of acatalepsia, and is a form of what philosophers call global skepticism (global skepticism is the denial of knowledge tout court and is more extreme than mere external world skepticism). We can never know the true natures of things, says Pyrrho, and even if we accidentally believe something accurate about the world, we could never know that we’ve briefly stumbled upon the truth. While this doctrine might strike some as pessimistic, the Skeptics took the doctrine to be liberating. By relinquishing pretensions to knowledge, Pyrrho and his followers hoped to attain a state of ataraxia or peace of mind.


The discussion of our knowledge of the world around us, then, begins with a discussion of our justification. If we can find some way, like Huemer’s Principle of Phenomenal Conservatism, to get justification off the ground, then we might have a chance of attaining at least some knowledge. If, however, we find that knowledge is impossible (or find it impossible to know whether knowledge is possible…), then we might do well to adopt the quasi-religious attitude of the Skeptics in order to not worry so much about what we can’t possibly know.


1If the evil demon scenario is too far-fetched for you, imagine that you are dreaming or that you are hallucinating or even that you are in a laboratory and your visual cortex is being stimulated by electrodes.

There are nearly as many responses to skepticism as there are philosophers who think about skepticism. Here, I present only two due to space constraints. See DeRose & Warfield (1999) for a number of other interesting responses.

This formulation of Phenomenal Conservatism is actually too hasty, as has been discussed by Tooley and Huemer in Tucker (2013). A more careful formulation would be something like: If it seems to a person, S, that P, then, absent defeaters, S has thereby gained some degree of prima facie justification for P.


Descartes, René. Meditations on First Philosophy: In Which the Existence of God and the Distinction of the Soul from the Body Are Demonstrated. Ed. Donald A. Cress. Indianapolis: Hackett Pub., 1993.

Huemer, Michael. Skepticism and the Veil of Perception. Lanham, MD: Rowman & Littlefield, 2001.

Tucker, Chris. Seemings and Justification: New Essays on Dogmatism and Phenomenal Conservatism. Oxford: Oxford UP, 2013.

Reid, Thomas. Thomas Reid’s Inquiry and Essays. Ed. Ronald E. Beanblossom and Keith Lehrer. Indianapolis: Hackett Pub., 1983.

Sextus Empiricus. Scepticism, Man, & God; Selections from the Major Writings of Sextus Empiricus. Ed. Philip Paul Hallie. Middletown, CT: Wesleyan UP, 1964.

Vogt, Katja. “Ancient Skepticism.” Stanford Encyclopedia of Philosophy. Stanford University, 24 Feb. 2010. Web. 03 Feb. 2014. <http://plato.stanford.edu/entries/skepticism-ancient/>.

Andrew is a lecturer in philosophy at the University of Colorado, Boulder. He holds a PhD in philosophy from the University of Colorado, Boulder, an MA in philosophy from Northern Illinois University and a BA in philosophy and a BM in bassoon and sound recording technology from Ithaca College. He specializes in epistemology, metaethics, and the history of philosophy (especially Kant and the 20th Century Anglophone and Phenomenological traditions). When not philosophizing, Andrew is skiing, hiking, listening to great music, or playing the bassoon.
Website: http://andrewdchapman.org/