A year or so ago, after I gave a talk on God and science, a gentleman expressed his doubt concerning the principle of non-contradiction.
The principle of non-contradiction is a self-evident first principle of reason upon which all knowledge is built. It states something cannot be and not-be in the same respect at the same place and time. Aristotle identified this principle as “the most certain of all principles” (Metaphysics IV.3).
The gentleman’s doubt exemplifies a radical form of skepticism that tends to doubt everything, even the first principles of knowledge—it’s called universal skepticism. Is there a way to refute such radical skepticism? I suggest there is.
Self-referential incoherence
First, the claim “I doubt everything” is self-referentially incoherent: it refutes itself. If a skeptic doubts everything, then he must doubt the claim “I doubt everything,” which of course is the same as saying, “I doubt that I doubt everything.” This is absurd. If a skeptic doubts his own assertion “I doubt everything,” then why is he even putting it forward?
If a skeptic retorts he is certain he doubts everything, then there would be one thing he doesn’t doubt—namely, the claim “I doubt everything.” Consequently, it wouldn’t be true that he doubts everything.
Perhaps a skeptic resorts to his original position and refuses to definitively declare his universal skepticism. Besides having to deal with the conundrum of being skeptical about his skepticism, he cannot avoid being certain about something—namely, I ought not to affirm my universal skepticism.
No argumentation allowed
Second, if a skeptic attempts to justify his claim via argumentation, then all the facts and principles that constitute his proof would thereby be declared invalid, since they imply human certitudes. As the nineteenth-century English Jesuit priest and philosopher John Rickaby writes:
[F]or this conclusion, when drawn, at once turns round on the premises and says, “Out upon you, you vile incapables, you are yourselves suspects, and can lead only to suspicious conclusions.” The premises retort, “That reproach does not come well from you” (First Principles of Knowledge ch. 8).
No matter how a skeptic approaches his claim, whether he doubts it or affirms it, he ends in a self-contradiction.
Absurdity of denying first principles of knowledge
A third way to refute radical skepticism is to show the absurdity in the denial of first principles of knowledge. If a skeptic doubts everything, then he necessarily doubts self-evident first principles of knowledge, like the principle of non-contradiction. In doing so, a skeptic denies knowledge as having any foundation. But this doesn’t work.
Consider trying to obtain knowledge without a principle that doesn’t require further demonstration, i.e., a self-evident principle. Any conclusion put forth would require an infinite series of reasons why that conclusion is true. For example, the skeptic’s claim “There are no self-evident first principles of knowledge” would be true only if A is true. But A would be true only if B is true and B only if C is true, ad infinitum.
Notice the search for a true premise upon which the conclusion can rest would never come to an end. No matter where one stops in the series of reasons, one would always have a reason that can’t be proven true because it relies on an infinite number of other reasons we can’t know to be true.
But if no reason the conclusion depends upon can ever be known to be true, then the conclusion “There are no self-evident first principles for knowledge” can’t be known to be true. This is not something the skeptic wants to conclude, since it would undermine his skepticism about first principles. Therefore, a skeptic can’t deny the necessity of knowledge having a foundation in first principles without undermining his own skepticism.
Defending the principle of non-contradiction
Fourth, we can show the absurdity of denying the principle of non-contradiction itself. A skeptic cannot deny the principle of non-contradiction without his speech already betraying him. He can only speak against the principle if his words have the intended meaning and not the opposite. For example, if a skeptic says, “The principle of non-contradiction is false,” he must intend the statement to mean what it expresses and not the opposite, namely, “The principle of non-contradiction is true.”
If a skeptic affirmed the opposite—“The principle of non-contradiction is true”— he would be affirming what he set out to deny. But if a skeptic intends to mean what his initial statement expresses, then he presupposes the principle and thus, once again, undermines his initial attempt to deny the principle.
So, a skeptic’s denial of the principle of non-contradiction ultimately ends in self-defeat.
Perhaps a skeptic could remain silent. Would that save a skeptic from the above dilemma? The answer is no, for even understanding what is meant by the principle presupposes its truth. The cognitional content must have the intended meaning and not the opposite.
Doubt presupposes certitudes
A final way we can refute radical skepticism comes from a Jesuit priest, T.V. Fleming. In his book Foundations of Philosophy, he argues there are certain presupposed certitudes embedded in the statement “I doubt everything.” Consider that, in order to make the statement, a skeptic must know what doubt is. Furthermore, his statement implies he knows doubt differs from knowledge, which in turn necessarily implies he knows what knowledge is. Related to what was mentioned above concerning the principle of non-contradiction, a skeptic also must know the meaning of the proposition he wishes to doubt and the reason for his doubt.
Moreover, if a skeptic suspends judgment concerning a proposition, he must recognize the reasons given for it as insufficient to justify his assent. This in turn means he knows the reasons given.
The presupposed certitudes do not stop there. When a skeptic continues to hold to skepticism, he does so in order not to fall into error. But that presupposes knowledge of what error is. It also presupposes a desire for truth, which in turn suggests at least a tacit certitude that truth exists.
So a skeptic simply cannot doubt everything. It’s impossible. The statement itself implies presupposed certitude, and the denial of self-evident first principles, such as the principle of non-contradiction, is not possible in denying it the principle is affirmed. Universal skepticism, therefore, is bankrupt. We have rational justification in being skeptical about skepticism.