Introduction:

Although there are several voices that shine the brightest as philosophers of philosophical skepticism, it is a topic which has captured the attention of a huge number of philosophers throughout time. The so-called challenge of radical skepticism has been raised and allegedly met time and time again. One of the aforementioned notable voices in the past century was G.E. Moore, who advocated what he and others have termed a ‘common sense’ response to radical skepticism (where radical skepticism refers to the position that knowledge—or certainly knowledge of the external world—is impossible).

Formally, Moore’s response proceeds from what is now in certain contexts called a Moorean shift—changing a modus ponens argument’s second premise to create a modus tollens argument which has an opposing conclusion (explained at more length below)—to support what is now in certain contexts called a Moorean fact (a notion that is more intuitively knowable to a person than philosophical premises that contradict the notion). That naming scheme ought to tell you how influential these ideas have been. G.E. Moore was a capable and perceptive philosopher, and his work on skepticism was inspirational for Ludwig Wittgenstein (who later tried to formulate a more rigorous account of Moore’s approach in notes which were assembled into a book after Wittgenstein’s death). Now I will point out why G.E. Moore’s confident argument is insufficient for meeting the challenge of radical skepticism.

An Argument for Radical Skepticism, and G.E. Moore’s Response:

The argument which Moore is confronting is an epistemological argument, which takes roughly the following form (where the text of these premises and conclusions are extrapolated from Moore’s argument as it appears in, for instance, his essay “A Defence of Common Sense”):

(1) If I do not know with absolute certainty and correctness that my sense experience is accurately representing reality, then I do not know that external reality exists (or at least that it is like I think it is).

(2) I do not know with absolute certainty and correctness that my sense experience is accurately representing reality.

(3) Therefore, I do not know that external reality exists (or at least that it is like I think it is).

This is a modus ponens argument, meaning it is of the following deductively valid form: if A, then B; A; therefore, B. Moore restructures the argument as follows:

(1) If I do not know with absolute certainty and correctness that my sense experience is accurately representing reality, then I do not know that external reality exists (or at least that it is like I think it is).

(2′) I do know that external reality exists (and at most that it is like I think it is).

(3′) Therefore, I do know with absolute certainty and correctness that my sense experience is accurately representing reality.

This is a modus tollens argument, meaning it is of the following deductively valid form: if A, then B; not-B; therefore, not-A. The crux of G.E. Moore’s contention that this solves the challenge of radical skepticism is that he feels (2′) is more intuitively likely than (2). This, he feels, is not only demonstrably so but also appeals to common sense.

Two Weaknesses of this Moorean Shift:

The first weakness of Moore’s response that I will discuss is perhaps readily apparent to you. Moore has at times given three criteria for a good proof (e.g. in his essay “Proof of an External World”), and, without belaboring the point by going through all of them, the relevant criterion that Moore claims to pass (but which I think he clearly fails) is that the truth of the proof’s premises must be demonstrable. He takes his second premise to be demonstrable by, for instance, observation of his own limbs. But the premise states that he knows that the limbs exist, and that knowledge is not demonstrable.

It is no problem for the skeptic that they do not know whether they do not know themselves to be lacking knowledge. It is precisely that state of affairs that grounds their skepticism. But it is a problem for someone attempting a proof of knowledge that their purported proof of some possessed knowledge makes use of knowledge that is in turn unproven (perhaps unproveable). In effect, the fallibility of (2′) makes Moore’s argument unsound.

So Moore’s overall project, then, reduces to an appeal to his own intuition on the subject of one premise being more intuitively likely to him than another. An appeal to intuition, while useful for clarifying and organizing thoughts, is not a proof. And it is precisely the proveability and fallibility of the claims in question which are under study. After all, if G.E. Moore’s argument really just boils down to an assertion masking a fallibilist conception of knowledge, then he might as well say, ‘While I don’t know that external reality exists, I do casual-know that external reality exists.’

And this exposes a second weakness of Moore’s response: its basis may support his own opposition. After all, the above weakness concerning demonstrability is only going to come into play if you do have the same exact intuition as G.E. Moore, which it is unlikely that everyone does (and, certainly, which it is highly unlikely that each philosopher familiar with all relevant premises does). One person’s Moorean facts may be another person’s unintuitive premises, and Moore’s own defense of intuition would then form the basis of radical skepticism for any individual who finds one of the various academic skeptical scenarios which inform (2) to be more intuitively likely than its negation.

Conclusion:

So, if Moore, like so many others, has failed to definitively conquer and settle the challenge of radical skepticism, then how do I myself overcome it? Well, in a perhaps-unsatisfying-though-characteristic move, I do not. It would possibly be more accurate to say that I ignore it, or else view it as pragmatically irrelevant. One of the primary strengths of my allegiance to phenomenology, to whose strength I pragmatically hold and which I have praised elsewhere when discussing morality and free will, is that it is—by definition—the study of phenomena rather than the study of reality. I take statements about the external world to be equivalent to statements about perceived reality and objects of consciousness.

The real virtue of my pragmatist and phenomenological stance is its opposition of baseless claims and needless assertions, even in relation to radical skepticism. Note that it also guides a person away from impassioned and ignoble arguments over minutiae when the two opposing stances have identical effects on beliefs and practices. I deal only in what we can or do know, and never accept my own intuition as a proof of anything. So, for all that, I have no more difficulty than G.E. Moore in discussing my experience of external reality, and nor should you.