Philosophy 133 - 7.4.2 Responses to Global Skepticism

The philosopher who wishes to overcome philosophical skepticism must find reasonable grounds for rejecting the skeptic’s argument. The different skeptical arguments reveal a specific conception of the level of justification required for knowledge. Skeptical arguments rely on the existence of doubt. Doubt exists when we cannot rule out a possibility. If we have doubt, we are not certain. We cannot be certain that we are not, say, a brain in a vat. And if we cannot be certain, then we cannot know anything that implies we are not a brain in a vat. Certainty is a very strict measure of justification. One clear possible response is to simply deny that one needs certainty in order to be considered justified. This section looks at some of the classical responses to the skeptic’s argument that we cannot know anything.


British philosopher G. E. Moore (1873–1958) presented an argument against skepticism that relies on common sense. In his famous paper “Proof of an External World,” Moore begins by raising his right hand and claiming, “Here is one hand,” then raising his left hand and claiming, “Here is another hand” (Moore 1939). Therefore, he concludes that skepticism is false. At first glance, this argument may seem flippant. It is not. Moore means to replace the second premise in the skeptical argument with his own premise: I know I have hands. The skeptical argument starts with the premise that if you cannot rule out a skeptical hypothesis, then you do not have knowledge of some proposition pertaining to the external world. Moore uses “I have two hands” as his proposition about the external world. In effect, he accepts the skeptic’s first premise, then uses his commonsense belief in the truth of “I have two hands” to defeat the skeptical hypothesis. Here is the argument’s structure:

  1. If I cannot rule out the possibility of SH, then I cannot be justified in believing that P.
  2. I am justified in believing that P.
  3. Therefore, I can rule out the possibility of SH.

In claiming that he has two hands, Moore claims that he is justified in believing propositions about the external world. And if he is justified, then he can rule out the skeptical hypothesis. The skeptic’s argument takes the form of what is called modus ponens, meaning a valid inference where the antecedent of a conditional is affirmed. Moore’s argument takes the form of what is known as modus tollens, meaning a valid inference where the consequence of a conditional is denied.

But notice that the two arguments contradict each other. If we accept the first premise, then either Moore’s or the skeptic’s second premise must be false. So why did Moore think his second premise is better? The choice is between thinking you are justified in believing that you have two hands and thinking you are justified in believing the skeptical hypothesis might be true. Moore thinks he has better reason to believe that he has two hands than he does for believing the skeptical hypothesis is true. For Moore, it is just common sense. You have reason to believe that you have two hands—you can see them and feel them—while you have no reason to believe the skeptical hypothesis is true.

Many philosophers remain unconvinced by Moore’s argument. Any person who accepts the possibility of the skeptical hypothesis will disagree with his premise 2. The possibility of the skeptical hypothesis effectively undermines justification in the belief that you have two hands.


As we just saw, some theorists reject the notion that you must be certain of a belief—that is, rule out all possible defeaters—in order to have knowledge. Moore thinks he has more justification to believe he has two hands than he does that there’s an evil demon tricking him. And in determining whether I am justified in believing in the bird outside my office window, I rarely consider the possibility that I could be a brain in a vat. I’m more likely to focus on my poor vision as a defeater. In the context of bird identification, wild skeptical hypotheses seem out of place. Indeed, we often adjust how much justification we think is needed for a belief to the task at hand. Contextualism is the view that the truth of knowledge attributions depends on the context. Contextualism is a theory about knowledge and justification. When we attribute knowledge to a subject S, the truth of the knowledge claim depends on the context that S is in. The context of S determines the level of justification needed for a true belief to count as knowledge. Contextualism comes from the observation that the level of confidence needed for justification changes depending on what the belief is as well as its the purpose and its importance, among other things. We expect a high degree of justification from physicians when they diagnose disease but less justification from friends recalling the title of a movie because there’s much more at stake in medical diagnoses.

Contextualism deals with skepticism in a unique way. Rarely are we in situations where we must rule out skeptical hypotheses to consider ourselves justified. Indeed, it is generally only when a skeptical hypothesis has been explicitly raised that we think we need to rule it out to be justified. And in our daily lives, the skeptical hypothesis just does not seem relevant. Yes, the possibility that we are brains in a vat technically still exists; we just do not think of it.

The content of this course has been taken from the free Philosophy textbook by Openstax