Reasonable Skepticism about Radical Skepticism


Hendrik van der Breggen

Article ID:



Aug 25, 2022


Jan 20, 2011

This article first appeared in the Christian Research Journal, volume31, number5 (2008). For further information or to subscribe to the Christian Research Journal go to:


Radical skepticism about the external world is the idea that we cannot have accurate knowledge about the physical world outside of our minds. That idea, if true, would block the truth-seeker’s attempt to gain knowledge of God based on God’s revelation in the physical world. We can, however, examine four types of radical skepticism concerning the external world—funky/pop skepticism, sensory skepticism, Kantian skepticism, and linguistic skepticism— and show that they fail.

According to funky/pop skepticism, our knowledge of the external world is blocked because various logical possibilities can be raised—that we are in a dream or are living in a computer-generated virtual reality, for example. This type of skepticism confuses possibility with plausibility.

According to sensory skepticism, we do not know the external world because we cannot trust our senses, since they have deceived us in the past. This skepticism fails, however, because from the fact that our senses sometimes deceive us, it does not follow that they always do.

According to Kantian skepticism, we do not know the world because the mind’s structures are a distorting influence on our knowledge of what is real. This view, however, seems to require at least some accurate, i.e., undistorted, knowledge of the reality and influence of the mind’s structures. But this requirement contradicts the core of Kantian skepticism (that the mind’s structures are a distorting influence on our knowledge of what is real), rendering its broader skeptical claims dubious.

According to linguistic skepticism, we do not know the world because language refers only to other language, it is a “prison” that keeps us from the world. This view of language, however, is false, because the existence of ostensive definition (definition by pointing) makes it possible for people to get out of the dictionary and to the world.

Other reasons also render the radical skepticisms seriously problematic. One significant reason is that the burden of proof rests on the shoulders of those who would deny the obvious; when radical skeptics fail to provide such proof, the obvious—the idea that we can know the external world—remains.

Radical skepticism concerning the external world is the philosophical view that we cannot have accurate knowledge about the physical reality that exists outside our minds. Sadly, if a person believes that the external world cannot be known, then it will be difficult for that person to know that (as Psalm 19:1–2 and Romans 1:20 state) the physical world—its glorious heavens included—declares the existence of its Creator. l Following the apostle Paul’s mandate to “demolish arguments and every pretension that sets itself up against the knowledge of God” (2 Cor. 10:5), in this article I will look at four types of radical skepticism—funky/pop, sensory, Kantian, and linguistic—and will show that they fail. Those who deny the obvious, as do radical skeptics, must shoulder the burden of proof;the radical skeptics’ failure to disprove the obvious means that the obvious remains: we can know the external world.


Funky/pop skepticism is my label for a radical skepticism about the external world that probably is best explained by considering some examples. (The examples are a bit weird, hence funky. The notion is common due to the influence of media on contemporary popular culture, hence pop.)

Consider the film The Matrix. Suppose we are characters in this story: what we perceive to be real is merely a computer-generated illusion, but in actuality, each of us is floating in an amniotic-sac-like pod with our nervous systems and brains wired into a common virtual reality. Whatever we sense—that is, whatever we think we sense—is merely what a supercomputer programs for us to sense. Nothing we see, hear, smell, taste, or touch is real.

Prior to The Matrix series of films, philosophers entertained a similar possibility. Some suggested that we are merely brains in vats, kept alive by a mad scientist who feeds us patterns of electrical impulses that mimic our sensory organs.

Consider also the possibility that you are at this very moment dreaming. (This example originates with René Descartes [1596–1650].)1 Whatever you see, hear, smell, taste, touch—and read—is simply part of your dream.

How do you know that, right now, you are not in something like The Matrix? Or that you are not a brain in a vat? Or that you are not dreaming? I might answer that I believe I am not in The Matrix because I have not yet met agent Smith. (Smith, according to the film series, is a representative of the supercomputer.) The skeptic would respond that the supercomputer wants to keep me in the dark. I might argue that I am not a brain in a vat because I can feel my skull with my hands. The skeptic answers that the mad scientist has wired me to perceive that I am touching my skull when in fact I’m not really touching anything. Let’s say that I argue that I’m pretty sure that I’m not dreaming because I heard my alarm go off this morning. The skeptic answers that it’s not at all unusual for one to hear one’s alarm go off in one’s dream. Alarmingly (sorry), any evidence that I present against the skeptic can be subsumed under the Matrix hypothesis, the brain-in-a-vat hypothesis, or the dream hypothesis. Should I thus give up my knowledge of the external world?

A Rational Reply to Funky/Pop Skepticism

There is a reasonable way to answer the funky/pop skeptic. In fact, there are five ways, which together constitute a formidable cumulative case argument.2

First, following the philosopher Ludwig Wittgenstein (1889–1951), we can point out that to imagine a doubt is not really to have that doubt.3 We can imagine, say, that the Statue of Liberty is a robot—but that’s not really to believe it actually might be a robot. That is, we can imagine the doubt that the statue isn’t really a statue, but that’s not actually to doubt it’s a statue. So, yes, I can imagine that I am in a computer-generated world, but that doesn’t mean I truly believe I might in fact be in such a world. Simply put: imagining isn’t doubting. To think otherwise is to conflate two distinct cognitive categories.

Second, we can point out that if one were to be convinced of any of the above skeptical hypotheses, then one would be confusing logical possibility with plausibility/probability. Yes, it is logically possible that the moon is made of green cheese (i.e., there is no logical contradiction in this claim), but from this it does not follow logically that the moon actually is made of green cheese. In other words, the mere logical possibility of X is not the same as an adequate justification for X; therefore, the mere possibility of doubt does not constitute sufficient grounds for doubt.

Third, we can point out that there is no compelling reason to accept any of the funky/pop hypotheses. After all, all we have is the skeptic’s mere assertion (of a mere logical possibility).

Fourth, we can point out that belief in any of the funky/pop hypotheses requires a denial of many of our prior beliefs that are logically incompatible with those hypotheses. Furthermore, these prior beliefs are not without epistemic weight—that is, they also count as contenders for knowledge.

Fifth, we can point out that if, for the sake of argument, we accept mere assertions of bare logical possibilities as sufficient grounds for the truth of those assertions, then, to be consistent, we should believe all mere assertions of logical possibilities as truths. This, however, would mean that all logical possibilities are true, which is plainly absurd. We would have to believe that The Matrix is true, and that the brain-in-a-vat hypothesis is true, and that the dream hypothesis is true, and that The Lord of the Rings is true, and that Batman is true, and that the moon is made of cheese, and that the moon isn’t made of cheese, and…you get the picture.

In other words, rational persons can weigh the pros and cons—in this case, one pro that consists of a mere assertion of funky/pop skepticism versus five cons or counter-considerations—to conclude that it is reasonable not to believe funky/pop skepticism.4


Sensory skepticism tells us that we cannot know the external world because our senses deceive us. (This skepticism, like the previously mentioned dream hypothesis, also is inspired by Descartes.) Consider the following examples. When rowing my boat, I put an oar into the water, but then the oar appears bent. When walking in the countryside, I see a flat wall on a distant farm building, but as I get closer the wall turns out to be curved. When strolling along the railroad tracks, I see that the metal rails look straight and parallel, but on the horizon they appear to meet. When driving my car on a hot summer day, I see water on the road ahead, but as I continue to drive I observe that the road is dry. While volunteering as a subject for psychology research, I see a red six of hearts as the researcher flashes a playing card, but later discover that it was a red six of spades.5 Clearly, my senses do deceive me. They therefore should not be trusted.

A Rational Reply to Sensory Skepticism

There are three reasonable criticisms that we can set out against radical sensory skepticism. I describe each of them as follows.

First, always does not follow logically from sometimes. The fact that we are sometimes deceived by our senses does not mean that we are always deceived by them.

Second, to know that our senses sometimes deceive us requires that they sometimes or often do not. Indeed, for us to discern that I mistakenly think that the oar is bent, that the wall is flat, that the tracks do not remain parallel, that the road is wet, or that the card is a red six of hearts requires that we have clear and accurate sensory knowledge. It presupposes that we know—accurately—that the oar is in fact straight, that the wall is in fact curved, that the tracks are in fact parallel, that the road is in fact dry, and that the card is in fact a red six of spades. The argument of the sensory skeptic, then, requires as legitimate and true what it purports to show is not legitimate and true. It self-refutes.

Third, because the argument for sensory skepticism very apparently fails, our senses’ prima facie veridicality—that is, their very apparent truthfulness—remains. As mentioned, the burden of proof belongs to those who deny the obvious, so the senses are innocent until proven guilty. It is reasonable, then, to go with what our senses tell us about the world, as long as we have no overriding reason to doubt them, and as long as we’re careful.


Immanuel Kant (1724–1804) set out a theory of knowledge that inspired yet another form of radical skepticism.6 According to Kant, all our knowledge begins with sensory experience, but the human mind—via its conditioners of sense-experience and its categories of thought—makes a significant contribution to this knowledge. These conditioners and categories determine what we can experience, and even shape our experience. Using Kant’s terminology, all we can perceive is phenomena (what our mind has conditioned and categorized), not noumena (the things themselves).

In effect, our mental conditioners and categories are like rose-tinted glasses that project pink onto all we see. Our mental conditioners project space and time, and our mental categories project category-specific thought onto all we experience.

Via our mind’s category of, say, causality, we project causation onto events we experience. We project cause onto, say, a pool cue hitting a pool ball, and thereby we “know” that the cue “causes” the ball’s movement. Via the mind’s category of substance, we project the notion of material stuff onto what appears to be, say, a brick. We do the same with various other mental categories, such as existence. (According to Kant, there are twelve such categories.)

Our mental conditioners and categories are even more like a meat-grinder/sausage-making machine than tinted glasses. What we can perceive and know—the phenomena, which have been shaped by the conditioners and categories—would be the sausages. What is real—the noumena, besides the meat that is shaped into sausages (which we only know as sausages)—would be the stuff that does not fit into the grinder (e.g., the knives, the butcher, bicycles, and suspension bridges). No matter how much we grind, we won’t really know the external world.

A Rational Reply to Kantian Skepticism

We need not accept Kantian skepticism, for four reasons.

First, if Kantian skepticism is true, then science’s search for causal connections/laws ultimately is a search for connections/laws that are not really in the world, but in our heads. That this is a search for what is not really in the world is plainly false; therefore, Kantian skepticism is false.

Second, as philosopher Jim Leffel astutely observes, “The success of scientific technology is a strong argument that our perceptions of the world are relatively accurate. Countless achievements attest to the reliability of human knowledge. We can engineer enormously sophisticated rockets to propel men to the moon, and provide health care that has more than doubled human life expectancy. We couldn’t do these things without an essentially reliable correspondence between our ideas of reality and reality itself.”7

Third, for Kantian skeptics to perceive that the mind cannot perceive things as they are requires that the mind can. The Kantian skeptical position assumes that the skeptic can stand outside the meat-grinder/sausage-making machine and see the meat, the grinder, the table, and so on. If the skeptic is correct that the mind cannot see things as they are, however, then he or she should only see sausages, and nothing else. If the skeptic can have this “outside” view, then, surely, so can everyone.

The previous point can be argued more carefully as follows. The Kant-inspired skeptic holds to the thesis that humans misperceive the world through their colored and distorted concepts of it (hereafter, this thesis will be referred to as the Kantian thesis). In other words, the Kantian thesis has to do with a particular aspect of the world, that is, that humans in fact misperceive the world via their concepts. To gain traction, Kantian skepticism must involve an admission that we can know, via our concepts, that the Kantian thesis is true. This, however, means that the skeptic must presuppose an alternative non-Kantian thesis, a thesis that holds that humans, via their concepts, actually do know the world in a noncolored, nondistorting way. Now, because this alternative thesis is not self-contradictory (and thus not knocked out of the explanatory competition right at the start); because the Kantian thesis requires that the alternative thesis is true (albeit with respect to a limited domain); and because there seems to be no overriding reason to limit the domain of the alternative thesis in the way the Kantian thesis does, we can conclude that the doubt the Kantian thesis casts on observation is seriously weakened. This means that it is quite reasonable to accept as accurate the evidence that our observations of the everyday/scientific sort very apparently and very often are accurate.8

Fourth, Kant’s theory of knowledge faces other deep problems. The categories of understanding are supposed to apply to phenomena, not noumena. The category of causation, however, is applied to noumena (as the cause of the phenomena). The category of existence is applied to noumena too (noumena is said to exist), and so on. In other words, Kant’s view is contradictory in some of its crucial tenets; thus, it is reasonable not to succumb to Kantian skepticism.


According to linguistic skepticism (which lurks behind some postmodernist philosophizing), we cannot know truth about the world in an objective way because of the distorting effect of language. On this view, we think only in language, and language refers only to other language, so language is a “prison” (of signifiers) that keeps us from knowing anything outside language. There is no reference to an extra-linguistic world, and words continually refer to each other; because of this there is a never-ending deferral of meaning. Also, the semantics (word meanings) and syntax (grammatical structure) of languages are not fixed; they are, rather, social constructions (cultural creations), so the way people understand reality is dependent on culture, which varies. Consequently:

  • There is no objective truth; each community has its own mere “story” or ”narrative.”
  • There is no objective rationality; we reason in language, which is culture-dependent.
  • There are no objective ethics; values are relative to culture, too.
  • Therefore, power rules; the dominating cultural group ultimately controls the language (wittingly or unwittingly), so it determines “truth,” rationality, and ethics.

We thus should be radically suspicious of alleged knowledge of the external world.

A Rational Reply to Linguistic Skepticism

We should not be persuaded by linguistic skepticism. The following reasons demonstrate that language does not always shield us from truth, as linguistic skepticism suggests.

First, and most important, the linguistic skeptic’s view of language is false. To be sure, language often refers to other language (check any dictionary). It is simply not the case, however, that language is completely defined by other language. There is such a thing as ostensive definition—that is, the fact that we define our words by physically pointing at the extralinguistic thing(s) to which we intend our words to refer.9 Language, then, is not a “prison” that keeps us from reality; there is no endless deferral of meaning. We do use words to communicate information about the extralinguistic world, and we—linguistic skeptics included—do this quite well. Linguistic skeptics refer us to their writings, but those writings are extralinguistic markings (called print) found on extralinguistic objects (called pages), found in other extralinguistic objects (called books), located on yet other extralinguistic objects (called shelves), and so on. Linguistic skeptics even be­come upset if we misrepresent their written work.

Second, although the semantics and syntax of languages are not absolutely fixed (they are contingent social constructions), it does not follow that our understanding of reality depends wholly on language and so is wholly socially constructed. Yes, labels and how they are used are in fact dependent on the language system in use and are in a sense arbitrary. The word “dog” is actually an arbitrary collection of letters (in France, people use “chien,” in the Netherlands, “hond”). The semantics and grammar we use with the word are conventional (culturally dependent, not absolutely fixed). Such labels, nonetheless, can refer successfully to extralinguistic entities. Think of the dog down the street. The dog itself clearly is not a mere social construction, as the torn pant leg will attest. In other words, relativity of term selection and use does not mean that language cannot refer to external reality, nor that external reality has no say (or bite).

Third, we legitimately can ask, is there really no objective truth? To answer this question, it may be helpful to look to Friedrich Nietzsche (1844–1900), who, famously, wrote the following:

What then is truth? A movable host of metaphors, metonymies, and anthropomorphisms: in short, a sum of human relations which have been poetically and rhetorically intensified, transferred and embellished, and which, after long usage, seem to a people to be fixed, canonical and binding. Truths are illusions we have forgotten are illusions; they are metaphors that have become worn out and have been drained of sensuous force, coins which have lost their embossing and are now considered as metal and no longer as coins.

If truth is mere metaphor or illusion, then Nietzsche’s claim, which purports to be true, is mere metaphor or illusion. If truth is not mere metaphor or illusion, then Nietzsche’s claim is false. Either way, why bother with Nietzsche’s claim?10

The fact is that there is something called simple truth. Simple truth is the (correspondence) notion of truth of which we are all aware and which we all use in science and in everyday life. For example, it is true that water freezes at 32° Fahrenheit, it is true that my desk is made primarily of wood, and so forth.

Fourth, the claim that standards of rationality are wholly relative to the community or tribe is false. The principle of noncontradiction, for example, is a fundamental principle of logic that is applicable to all. The principle of noncontradiction states that something cannot both be and not be, at the same time and in the same respect. (We can see this as necessarily true: can anyone be taller and not taller than a particular neighbor of his or hers, at the same time and in the same respect? Moreover, argument forms exist that are deductively valid,11 and thus applicable to all as well.12

Fifth, that moral relativism is true can be seriously challenged. Poking pins into a baby’s eyes for fun surely is wrong for everyone, everywhere, always.13

Sixth, the fact that language and power are often intertwined is grounds for caution, not radical skepticism. People who have power (e.g., politicians) might use words that carry persuasive emotional appeals rather than truth (e.g., “only rednecks vote for [the politician you like the least]”). The claim, however, that language is wholly a power play and thus not capable of communicating knowledge is false. The truth or falsity of sentences depends on the facts of the world, not on power agendas. That’s why we are able to check up on power-mongering politicians to hold them accountable. Moreover, if (contrary to fact) language were wholly a power play, then we would have no good grounds to believe the linguistic skeptic’s arguments, because he or she merely would be using language to exert power over us.

In sum, we should be careful with language, because sometimes our language is unclear, ambiguous, emotionally loaded, or false, for example. Language, nevertheless, need not and does not always blind us to truth about the extratextual world; thus, linguistic skepticism fails.14


We have examined funky/pop skepticism’s alleged blinding by its assertion of mere logical possibilities, sensory skepticism’s alleged blinding due to our senses getting it wrong from time to time, Kantian skepticism’s alleged blinding by the influences of the mind’s structures/filters, and linguistic skepticism’s alleged blinding by language—and we have found some good reasons to be skeptical about each of these radical skepticisms. Of course, we often make mistakes—but sometimes we don’t. Of course, we don’t know the external world exhaustively or absolutely—but sometimes we do know some of it reasonably and fallibly. Of course, we don’t have X-ray vision—but we are not blind either.

This is good news. It turns out that because we can know at least some of the external world (in a limited way), we can find reasonable evidence for the existence of God. Scientifically based evidence and good reasoning lead us to believe that the universe had a beginning; that it was caused; that that cause transcends matter, energy, space, and time; that the arrangement of the universe was fine-tuned for life; and that life itself—the cell’s molecular machines and DNA’s code/language—is exquisitely fine-tuned. All of this points to an intelligent and powerful supernatural cause. Historical investigation of the external world gives us further reason to believe the New Testament’s witness concerning Jesus’ life, death, and resurrection. In other words, the external world points us to the Christian worldview, the gospel, and a reasonable faith in Jesus Christ.15

Hendrik van der Breggen, Ph.D. (University of Waterloo), is assistant professor of philosophy at Providence College and Seminary, Otterburne, Manitoba.


1 René Descartes, “Meditation One,” in Discourse on Method and Meditations on First Philosophy, trans. Donald A. Cress (Indianapolis: Hackett, 1986), 58.

2 A cumulative case argument consists of a collection of arguments that, individually, may not provide strong or decisive support for a conclusion, but jointly do—just as one strand of string may not be strong enough to lift a heavy load but several interwoven strands are.

3 Ludwig Wittgenstein, Philosophical Investigations, trans. G. E. M. Anscombe (Oxford: Basil Blackwell, 1967), no. 84, p. 39e.

4 The idea of weighing the above considerations (especially the third and fourth) against the skeptical position comes from Tom Morris, Philosophy for Dummies (Foster City, CA: IDG Books, 1999), 72–74.

5 See Thomas Kuhn, The Structure of Scientific Revolutions (Chicago: University of Chicago Press, 1970), 62–64, 112–13. The card example (as well as the other examples) can also fit under Kant-inspired skepticism, i.e., skepticism arising from conceptual categories, schemes, or even paradigms, as well as under linguistic or postmodern skepticism, to be discussed.

6 Immanuel Kant, Critique of Pure Reason, trans. Norman Kemp-Smith (London: Macmillan, 1929; reprint 1986).

7 Jim Leffel, “Postmodernism and ‘The Myth of Progress’: Two Visions,” in The Death of Truth, ed. Dennis McCallum (Minneapolis: Bethany, 1996), 52.

8 My argument is heavily influenced by Thomas Nagel, The Last Word (New York and Oxford: Oxford University Press, 1997), 92–96. “The second-order theories [e.g., Kant’s theory, i.e., the view that our concepts/perceptions do not get us to the external mind-independent world] cannot avoid competition with the content of what they are trying to reduce or debunk [i.e., that our concepts really do get us to the external mind-independent world]” (Nagel, 96).

9 Wesley C. Salmon, Logic, 3rd ed. (Englewood Cliffs, NJ: Prentice Hall, 1984), 145. This point is from Douglas Groothuis’s excellent book Truth Decay: Defending Christianity against the Challenges of Postmodernism (Downers Grove, IL: InterVarsity Press, 2000), 95.Friedrich Nietzsche, “Truth and Lies in a Nonmoral Sense,” in The Nietzsche Reader, ed. Keith Ansell Pearson and Duncan Large (Oxford: Blackwell, 2006), 117.

10 If, contrary to what his words seem to mean, Nietzsche is talking only about some or many (but not all) alleged “truths” not being genuinely true, then we should have no quarrel with him. Such a scenario would warrant a careful case-by-case investigation, however difficult, but not radical skepticism. If in his rejection of truth, however, Nietzsche means that truth is nothing but interpretation that precludes accurate knowledge of the external world, then Nietzsche seems unduly influenced by Kantian skepticism and unaware of the significance of ostensive definition apart from Kant’s influence, which provides us with what philosophers call knowledge by acquaintance. For more on knowledge by acquaintance, see J. P. Moreland, Kingdom Triangle (Grand Rapids: Zondervan, 2007), 120–130. For a helpful and generally sympathetic look at Nietzsche, see Robert Wicks, Nietzsche (Oxford: Oneworld, 2002).

For more on the concept of simple truth, see: Michael Luntley, Reason, Truth, and Self: The Postmodern Reconditioned (London and New York: Routledge, 1995), 103–23; and J. P. Moreland, “Truth, Contemporary Philosophy, and the Postmodern Turn,” in Andreas Köstenberger, ed., Whatever Happened to Truth? (Wheaton, IL: Crossway, 2005), 75–92.

11 Deductive validity means that whenever an argument’s premises are true, then the argument’s conclusion is true too; that is, it’s not possible for the premises to be true and the conclusion false.

12 The valid argument form modus ponens is stated thus (where P and Q respresent declarative sentences): If P then Q; P; therefore Q. Consider this form as exemplified by the following argument: If Fido is a dog, then Fido is mortal; Fido is a dog; therefore Fido is mortal. The argument is deductively valid everywhere and always. See also Salmon, Logic, chap. 2 , and Trudy Govier, A Practical Study of Argument, 6th ed. (Belmont, CA: Thomson/Wadsworth, 2005), chaps. 7–8.

13 For further explanation see Paul Chamberlain, Can We Be Good Without God? (Downers Grove, IL: InterVarsity Press, 1996); Russ Shafer-Landau, Whatever Happened to Good and Evil? (New York and Oxford: Oxford University Press, 2004).

14 It seems to me that the text has priority in interpretation: we seek out the context (historical and linguistic) to which the text directs us, then we seek out specific meaning in that context, to which the text also directs us. In other words, the text directs through its context to the meaning that the text was intended to point to by its author, and this meaning is picked up by the reader. For more, see the appendices of Grant R. Osborne, The Hermeneutical Spiral, 2nd ed. (Downers Grove, IL: InterVarsity Press, 2006).

15 On the external world’s evidence for the Christian worldview, see Chad V. Meister, Building Belief: Constructing Faith from the Ground Up (Grand Rapids: Baker, 2006); and William Lane Craig, Reasonable Faith: Christian Truth and Apologetics, 3rd ed. (Wheaton, IL: Crossway, 2008).

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