§ 1
The best case for skepticism about the external world? (Stroud, âThe Problem of the External Worldâ)
Weâre all intimately familiar with what goes on in our own minds. We make plans, form opinions, experience pleasure and pain, and so on. Itâs 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, itâs 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 Descartesâs Meditations on First Philosophy (published originally in 1641), the most influential work of one of the most influential philosophers of all time.
Letâs note a couple important points before we proceed. First, people who reflect on knowledge nearly unanimously agree that knowledge requires truth, or as itâs 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 thatâs a different matter.) Now if you think that knowledge is not factive, I recommend a simple solution: everywhere we here speak of âknowledge,â understand it to mean âknowledge of the truth,â and every time we claim or ask whether someone âknows that so-and-so,â understand it to mean âknows itâs true that such-and-such.â Second, something can be possible without being real or actual. Indeed lots of things are possible that arenât actual. For instance, itâs 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, letâs 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 heâs near a fire. But do these sense experiences enable him to know heâs near a fire?
Itâs hard to imagine Descartesâs 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 âNo,â 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 heâs near a fire, and itâs this appearance that he trusts when judging that heâs 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. Itâs certainly possible for him to have all those sensations despite merely dreaming that heâs 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 heâs merely dreaming that heâs near a fire, then he certainly doesnât know that heâs near a fire. And he also recognizes it is at least possible that heâs merely dreaming. So he knows that a certain genuine possibility, the dream-possibility (as Stroud calls it), is incompatible with his knowing that heâs near a fire. So in order to know that heâs 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 Descartesâs knowledge of the fire, he doesnât 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 wouldnât know that heâs near a fire.
Could Descartes ever come to know that the dream-possibility is false? Sense experience itself wonât enable such knowledge because, as weâve already said, any sense experience is perfectly compatible with the dream-possibility. But isnât 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, heâd need to know that he wasnât merely dreaming that he was performing the test!
If youâre wondering why he couldnât then just perform a second test to determine whether heâs merely dreaming that he performed the first test, consider: he could equally well be dreaming that heâs performing the second test. The same is true for a third test he might perform to determine whether heâs merely dreaming that he performed the second test. And so on. No matter how many tests he performs, the same problem recurs. And since itâs not possible to perform an infinite series of tests, we find no relief in this direction.
Letâs 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 doesnât know that heâs near a fire, then we never know anything about the external world. (Premise)
2. Descartes doesnât know that heâs 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 heâs 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 heâs near a fire, he must know that the dream-possibility is false. (From a and b)
d. But Descartes couldnât know that the dream-possibility is false. (Premise)
e. So Descartes doesnât know that heâs 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 itâs 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 argumentâs flaw. Whatever it is, it isnât 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 âembodiedâ in our ordinary procedures for âmaking and assessing knowledge-claims.â Consider for instance a bird watcher who judges a certain bird to be a goldfinch. We ask her why she thinks itâs a goldfinch. âBecause itâs yellow,â she says. âBut for all youâve said,â we respond, âitâs possible that itâs a canary â canaries are yellow too.â We donât think she knows itâs a goldfinch, because she knows very well that canaries arenât goldfinches, and yet she doesnât know itâs not a canary. She must rule out this relevant possibility, the canary-possibility, in order to know itâs 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 thereâs a fire nearby, as the bird watcher must rule out the canary-possibility in order to know that sheâs 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 âthe problem of the external world.â Therein lies the challenge, and potential reward, of confronting philosophical skepticism.
References
Allan Hazlett, âFactive Presupposition and the Truth Condition on Knowledge,â Acta Analytica 27.4 (2012): 461â478.
John Turri, âMythology of the Factive,â Logos & Episteme 2.1 (2011): 143â152.
§ 2
Proving the external world exists (Or: Letâs all give Moore a hand!) (Moore, âProof of an External Worldâ)
Suppose we disagree about the number of books on the desk. You say there are at least two. I disagree. And itâs no mere verbal disagreement â weâre referring to the same desk, and mean the same thing by âbookâ and âat least two,â etc. How might you prove your point?
Hereâs one way. You walk over, point to one book sitting on the desk, and then point to another, all while saying, âHereâs one book on the desk, and hereâs another. So there are at least two books on the desk.â I couldnât rightly criticize the proof. Iâd have to concede the point. What else could I possibly be looking for in a proof? Your premises (âhereâs one book the desk, and hereâs anotherâ) are different from your conclusion (âthere are at least two books on the deskâ), in which case you didnât simply beg the question. Your conclusion follows straightforwardly from your premises, and you know that it does. And you obviously know the premises â after all, you arenât blind, youâre looking right at the books, and youâve 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 âa scandal to philosophyâ that the existence of real, mind-independent external objects âmust be accepted merely on faithâ rather than a âsatisfactory proof.â1 Kant thought he had rescued philosophy from scandal by giving such a proof, indeed, the only possible such proof. Setting aside the merits of Kantâs own proof, G.E. Moore denied that Kantâs was the only possible such proof. A much simpler and fully convincing proof is readily available.
Moore offers his proof by saying, âHereâs one hand, and hereâs another. So external objects exist,â as he gestures and holds his hands up before us. This proof, Moore says, is âperfectly rigorous.â 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 âconstantly take proofs of this sort as absolutely conclusive.â
Note an interesting connection with Stroudâs discussion from §1. We wondered whether Descartes was right to claim that he must know the dream-possibility is false in order to know that heâs near a fire. And Stroud worried that Descartes was indeed right about that, because such a requirement might be ânothing more than an instance of a general procedure we recognize and insist on in making and assessing knowledge-claims in everydayâ 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 Mooreâs proof to really succeed, he must also prove his premises â prove that here is one hand and that here is another. If this critic is right, then the three criteria we earlier identified arenât 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 heâs not merely dreaming that he has hands. And even though he has âconclusive reasonsâ (or âconclusive evidenceâ) that heâs 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 Stevensonâs The Strange Case of Dr. Jekyll and Mr. Hyde. A shaken and ill Jekyll tells his friend Utterson that Mr. Hyde âwill never more be heard of.â When Utterson suggests â rightly, it turns out, as the story subsequently unfolds â that Jekyllâs assertion might not be entirely warranted, Jekyll replies, âI have grounds for certainty that I cannot share with anyone.â2 What does Jekyll mean by âcannotâ here? He might mean that he cannot prudently sh...