Ā§ 1
The best case for skepticism about the external world? (Stroud, āThe Problem of the External Worldā)
Weāre all intimately familiar with what goes on in our own minds. We make plans, form opinions, experience pleasure and pain, and so on. Itās also natural to suppose that we know a lot about what goes on outside our own minds too, about the world around us, based on the information we get through our senses of sight, hearing, smell, taste, and touch. Natural as that idea may be, itās surprisingly easy to get yourself into a skeptical frame of mind about the possibility of such knowledge. Can we really know anything about the world outside our own minds?
Barry Stroud aims to understand the attraction of skepticism about the external world, why knowledge of the external world based on sense experience poses a philosophical problem. To accomplish this, he focuses intensely on the argument presented at the beginning of Rene Descartesās Meditations on First Philosophy (published originally in 1641), the most influential work of one of the most influential philosophers of all time.
Letās note a couple important points before we proceed. First, people who reflect on knowledge nearly unanimously agree that knowledge requires truth, or as itās sometimes put, that knowledge is factive.1 This means that we can know something only if it is true or a fact. We cannot know a falsehood. (We can of course believe a falsehood, but thatās a different matter.) Now if you think that knowledge is not factive, I recommend a simple solution: everywhere we here speak of āknowledge,ā understand it to mean āknowledge of the truth,ā and every time we claim or ask whether someone āknows that so-and-so,ā understand it to mean āknows itās true that such-and-such.ā Second, something can be possible without being real or actual. Indeed lots of things are possible that arenāt actual. For instance, itās possible for winged horses to exist, even though none actually do. Likewise for wizards, dragons, phlogiston, the luminiferous ether, etc. With those points in mind, letās proceed.
Imagine Descartes at work in his study on a cold night, sitting a few feet from a comforting fire. Unsure for the moment how his narrative should best proceed, he takes a break and turns his attention to the fire. He sees its colorful flames flitting and flickering; he hears it crackling and popping; he feels its heat emanating; he smells the fragrant wood burning. In light of all this, Descartes of course believes heās near a fire. But do these sense experiences enable him to know heās near a fire?
Itās hard to imagine Descartesās senses putting him in a better position to gain knowledge of the external world. He is as well-positioned as any of us could ever hope to be. So if the answer to our question at the end of the previous paragraph is āNo,ā then it seems very likely that we never know anything about the external world, at least by way of our senses.
The way things look, sound, smell, and feel make it appear to Descartes as though heās near a fire, and itās this appearance that he trusts when judging that heās near a fire. But of course things might appear exactly the same in a perfectly realistic dream. And a perfectly realistic dream is a genuine possibility. Itās certainly possible for him to have all those sensations despite merely dreaming that heās near a fire. Indeed, any sensory experience might be a mere component of a perfectly realistic dream. Thus sense experience, being equally compatible with dreaming or waking, could never enable him to know that he is awake rather than merely dreaming.
As Descartes recognizes, if heās merely dreaming that heās near a fire, then he certainly doesnāt know that heās near a fire. And he also recognizes it is at least possible that heās merely dreaming. So he knows that a certain genuine possibility, the dream-possibility (as Stroud calls it), is incompatible with his knowing that heās near a fire. So in order to know that heās near a fire, he must know that the dream-possibility is false.
Notice that, on this way of thinking, in order for the dream-possibility to potentially threaten Descartesās knowledge of the fire, he doesnāt need to know, or even so much as believe, that it is actually true. No, the dream-possibility threatens simply because Descartes recognizes that it is possibly true, and that if it were actually true, he wouldnāt know that heās near a fire.
Could Descartes ever come to know that the dream-possibility is false? Sense experience itself wonāt enable such knowledge because, as weāve already said, any sense experience is perfectly compatible with the dream-possibility. But isnāt there some test he could perform to determine whether he is merely dreaming? Unfortunately not, because in order for him to learn from the test, heād need to know that he wasnāt merely dreaming that he was performing the test!
If youāre wondering why he couldnāt then just perform a second test to determine whether heās merely dreaming that he performed the first test, consider: he could equally well be dreaming that heās performing the second test. The same is true for a third test he might perform to determine whether heās merely dreaming that he performed the second test. And so on. No matter how many tests he performs, the same problem recurs. And since itās not possible to perform an infinite series of tests, we find no relief in this direction.
Letās encapsulate the preceding line of thought in the following argument, broken up into two parts to enhance clarity. The main argument goes like this:
1. If Descartes doesnāt know that heās near a fire, then we never know anything about the external world. (Premise)
2. Descartes doesnāt know that heās near a fire. (Premise)
3. So we never know anything about the external world. (From 1 and 2)
The argument is logically valid: if its premises are true, then its conclusion must be true too. That leaves us to ask whether its premises are true. 1 is at least very plausible, and Stroud is willing to grant it. That leaves only 2 to seriously question. The following supplementary argument supports 2:
a. Descartes knows that the dream-possibility is incompatible with his knowing that heās near a fire. (Premise)
b. If Descartes knows that a possibility is incompatible with his knowing some specific claim, then in order for him to know the specific claim, he must know that the possibility in question is false. (Premise)
c. So in order for Descartes to know that heās near a fire, he must know that the dream-possibility is false. (From a and b)
d. But Descartes couldnāt know that the dream-possibility is false. (Premise)
e. So Descartes doesnāt know that heās near a fire. (From c and d)
Notice that (e) is exactly the same as 2.
Should we accept this argument? Stroud wonders whether we can seriously entertain the skeptical conclusion expressed by 3, because itās allegedly either absurd or even unintelligible. But merely rejecting it as absurd or unintelligible deprives us of the opportunity to learn something potentially important about knowledge (or at least about our concept of knowledge). Accordingly, he challenges those of us inclined to reject the conclusion to locate the argumentās flaw. Whatever it is, it isnāt obvious.
Stroud suggests that (c) is false. Yet (c) follows from (a) and (b), so rejecting (c) requires us to reject at least one of (a) and (b). (a) is obviously true, which leaves (b).
The problem is that (b) is arguably āembodiedā in our ordinary procedures for āmaking and assessing knowledge-claims.ā Consider for instance a bird watcher who judges a certain bird to be a goldfinch. We ask her why she thinks itās a goldfinch. āBecause itās yellow,ā she says. āBut for all youāve said,ā we respond, āitās possible that itās a canary ā canaries are yellow too.ā We donāt think she knows itās a goldfinch, because she knows very well that canaries arenāt goldfinches, and yet she doesnāt know itās not a canary. She must rule out this relevant possibility, the canary-possibility, in order to know itās a goldfinch.
The question then becomes whether the dream-possibility is in all relevant respects similar to the canary-possibility, so that when we insist that the bird watcher must rule out the canary-possibility, we thereby commit ourselves to insisting that Descartes must rule out the dream-possibility. Does Descartes have to rule out the dream-possibility in order to know thereās a fire nearby, as the bird watcher must rule out the canary-possibility in order to know that sheās looking at a goldfinch? If not, why not? Each subject knows the possibility in question is incompatible with his or her knowing the claim in question. So what could be the difference?
A plausible explanation of the difference, should there be any, would go a long way toward resolving āthe problem of the external world.ā Therein lies the challenge, and potential reward, of confronting philosophical skepticism.
References
Allan Hazlett, āFactive Presupposition and the Truth Condition on Knowledge,ā Acta Analytica 27.4 (2012): 461ā478.
John Turri, āMythology of the Factive,ā Logos & Episteme 2.1 (2011): 143ā152.
Ā§ 2
Proving the external world exists (Or: Letās all give Moore a hand!) (Moore, āProof of an External Worldā)
Suppose we disagree about the number of books on the desk. You say there are at least two. I disagree. And itās no mere verbal disagreement ā weāre referring to the same desk, and mean the same thing by ābookā and āat least two,ā etc. How might you prove your point?
Hereās one way. You walk over, point to one book sitting on the desk, and then point to another, all while saying, āHereās one book on the desk, and hereās another. So there are at least two books on the desk.ā I couldnāt rightly criticize the proof. Iād have to concede the point. What else could I possibly be looking for in a proof? Your premises (āhereās one book the desk, and hereās anotherā) are different from your conclusion (āthere are at least two books on the deskā), in which case you didnāt simply beg the question. Your conclusion follows straightforwardly from your premises, and you know that it does. And you obviously know the premises ā after all, you arenāt blind, youāre looking right at the books, and youāve correctly verbally identified them. Without question, your proof perfectly settles the matter in your favor.
Immanuel Kant, perhaps the most influential of all modern philosophers, once said it was āa scandal to philosophyā that the existence of real, mind-independent external objects āmust be accepted merely on faithā rather than a āsatisfactory proof.ā1 Kant thought he had rescued philosophy from scandal by giving such a proof, indeed, the only possible such proof. Setting aside the merits of Kantās own proof, G.E. Moore denied that Kantās was the only possible such proof. A much simpler and fully convincing proof is readily available.
Moore offers his proof by saying, āHereās one hand, and hereās another. So external objects exist,ā as he gestures and holds his hands up before us. This proof, Moore says, is āperfectly rigorous.ā It meets the three criteria we noted earlier when discussing your proof about the number of books on the desk. Its premises are different from its conclusion; its conclusion follows, as Moore knows, straightforwardly from its premises; and finally, Moore obviously knows the premises.
Might a satisfactory proof require more than meeting those three criteria? Not if our ordinary practice is any indication. As with your earlier proof about the books, we āconstantly take proofs of this sort as absolutely conclusive.ā
Note an interesting connection with Stroudās discussion from Ā§1. We wondered whether Descartes was right to claim that he must know the dream-possibility is false in order to know that heās near a fire. And Stroud worried that Descartes was indeed right about that, because such a requirement might be ānothing more than an instance of a general procedure we recognize and insist on in making and assessing knowledge-claims in everydayā life, which procedure helps to define out concept of knowledge. Here Moore appeals to our everyday procedures for offering and evaluating proofs.
Moore anticipates that some will say his so-called proof fails. One type of critic insists that in order for Mooreās proof to really succeed, he must also prove his premises ā prove that here is one hand and that here is another. If this critic is right, then the three criteria we earlier identified arenāt sufficient for a conclusive proof after all. At least sometimes, a conclusive proof requires more. Moore rejects this, and explicitly disavows any intention to prove his premises. He doubts it could be done, because proving them requires proving that heās not merely dreaming that he has hands. And even though he has āconclusive reasonsā (or āconclusive evidenceā) that heās not merely dreaming, he cannot articulate that evidence to us, which he of course must do in order to offer a proof.
One is reminded of a scene in Robert Louis Stevensonās The Strange Case of Dr. Jekyll and Mr. Hyde. A shaken and ill Jekyll tells his friend Utterson that Mr. Hyde āwill never more be heard of.ā When Utterson suggests ā rightly, it turns out, as the story subsequently unfolds ā that Jekyllās assertion might not be entirely warranted, Jekyll replies, āI have grounds for certainty that I cannot share with anyone.ā2 What does Jekyll mean by ācannotā here? He might mean that he cannot prudently sh...