1. What is knowing?

What can we know? Can we know the future? Can we know that things exist that we can't perceive? Can we know that there are aliens somewhere in the universe? Can we know that God created the universe or guides its history? Can we know that there is life after death? Can we know that animals have minds? Can we know that killing is wrong?

All these are questions in epistemology, the theory of knowledge. The questions can become quite complicated, as we shall see. But first it's rather important to know what knowledge is ― what it means to know something. Our first question, accordingly, will be "What is knowledge?"

We can use the word "knowledge," like any other word, to mean anything we choose. But, as the word is already used, what is it used to mean? What conditions have to be fulfilled before we are prepared to say that a person knows something? It isn't enough to believe it; it isn't enough just to guess it. But what is enough?

We don't always mean the same thing by the phrase "to know something." Here are the main ones:

1. Knowing how. Do you know how to ride a horse? to swim? to use a welding torch? to scuba dive? Such questions have to do with an ability to engage in a certain activity. Usually it is a learned ability, like knowing how to walk, but sometimes it is not, like knowing how to cry.

   Animals know how to do many things that people don't, and often ― more often than with human beings ― they know how to do something without having to learn it. Almost all creatures know how to swim (and they haven't been given lessons). Chickens know how to hide and conceal themselves from passing hawks only a day after being hatched. Animals know how to select foods that they can digest: dogs will eat meat but not (usually) leaves, and rabbits will eat leaves but not meat. They are sometimes said to "know by instinct" or (more usually) to have been "genetically programmed" to engage in these activities.

2. Acquaintance. "Do you know that man?" "Yes, I recognize him." This is not the same as, "Do you know any facts about him?" "Do you know that old country lane near the red house a mile south of town?" "Yes, I've been there ― I'm acquainted with it." You know Yosemite Falls (in this sense of "know") if you have been there and seen it, if you have confronted it through your senses in some way. This is not the same as knowing facts about it. A person who has never been there but has read all about it in an encyclopedia may know many more facts about the place than the person who has seen it but has never learned any facts about iuaintance, not knowledge. If you have seen Yosemite Falls you can hardly escape knowing a few elementary facts about it, such as its geographical location, but just "drinking it in," or staring at it, doesn't provide knowledge. Knowledge comes, they would say, only when you have before your mind some statement that is either true or false. You couldn't have knowledge (knowing that) without acquaintance with something, perhaps just the letters on a page, but still, acquaintance isn't knowledge; it only provides the material for knowledge. You don't have knowledge until you are in a position to claim something as true or false. In short, knowledge is propositional.

   We distinguish propositions from sentences. A sentence may contain the word "the" twice; it may contain eight words; it may be translated into another sentence in another language. Here are two sentences: "An elephant is larger than a mouse," and "A mouse is smaller than an elephant." The first sentence begins with the words "an elephant," the second does not. They are two different sentences. But they both have the same meaning, that is, they state (express) the same proposition. A proposition has to do with the meaning of a sentence: same meaning, same proposition. Sentences are linguistic vehicles through which people express propositions.

   This difference is important when we need to emphasize the difference between the meaning and the vehicle that is used to express that meaning. Philosophers are most interested in meanings, and they tend to speak of propositions. But the word "proposition" is not customarily used in daily discourse (except in a different sense, as in "I have a proposition to put to you"). The more common word "statement" is generally used instead, although "statement" is ambiguous: it can mean either the proposition that is expressed or the sentence that expresses it.

3. Knowing-that. Most of the time, when we use the word "know," we are referring to knowing-that: "I know that I am now sitting down," "I know that the earth has existed for many years," "I know that I care about you," and so on. Knowing-that is knowing that some situation or state of affairs actually occurs or exists.

   But what is knowing-that? What conditions must be fulfilled in order for us to be justified in saying that we know something?

Belief

Take p to stand for any proposition. In order to know p, it's necessary for you to believe p. Believing it isn't enough, since people can believe many propositions that aren't true. But if you didn't at least believe it, you wouldn't be said to know it.

People can say they believe something even if they don't. They may lie, of course, or they may delude themselves into thinking they are sincere even when they're not. It's often difficult to tell: does she really believe she's the great-granddaughter of a distinguished nobleman? After all, she's said it so many times, she may not even ask herself any more whether she really believes it. She would say she believed it if you asked her, but does she?

Suppose you've just won the million-dollar jackpot. "I know it's true," you say, "but I still can't believe it." But you do believe it, don't you? Intellectually you may believe it, but you've been disappointed so many times in the past that your emotions haven't yet caught up with your intellectual realization. You haven't yet "digested" the information. Or, a woman knows that her husband is dead ― she has seen his body lying in state ― but she has not yet come to terms with it emotionally, and she says, "I know it but I still don't believe it."

"If you really believe something," it has been said, "you must be willing to act on it." But what if it's not the kind of statement one could act on because it has no consequences for us? You believe that Saturn has twelve moons and would say so if asked, but there's not much you can do about it to prove your sincerity, other than to say "yes" if asked whether you believe it.

Belief can also be a matter of degree: you can believe something but not very strongly. You have beliefs you would stake your life on, and others on which you would stake very little. You may believe something because a friend told you, but you aren't quite sure whether your friend can always be trusted to give you correct information. Still, if you didn't believe it at all ― if you positively disbelieved it ― could you claim to know it?

Belief is sometimes said to be the subjective condition of knowing. We turn now to the objective condition: if you know it, it must be true.

Truth

To know it is to know it to be true. If I know I am sitting on a chair, I know that the statement "I am sitting on a chair" is true. I may believe what isn't true, but I can't know what isn't true. If what I believe turns out not to be true, then I didn't know it; I only thought I knew it. If you say, "I knew she was in the room next to me all the time," and it turns out that she was a thousand miles away, then you didn't really know she was in the next room, since she wasn't.

Knowing isn't the same as being certain. You were certain that she was nearby, but she wasn't. People feel absolutely certain about lots of things that aren't so. They were certain for thousands of years that the earth is flat. They were certain that certain people were witches. A woman on a recent television talk show was certain that she had visited both Venus and Mars during the past few months. She claimed to know it, as surely as she knew she was sitting there. During religious wars people on one side claim to know with certainty ("with certainty" is redundant ― if they know, don't they know with certainty?) that their side has the truth, and those on the other side also claim to know that they have the truth, even when the two beliefs contradict one another. People can have a very intense feeling of certainty ("We will win the war," "She's going to recover") about many things that are far from certain and in fact may not be true at all.

In daily life we don't have much trouble with the concept of truth. We may disagree on how we discover that a statement is true; this will be our subject in the second half of this chapter. But even a small child knows the difference between lying and telling the truth. "Johnny, tell the truth! Did you really call Jane's mother a bitch?" Johnny may or may not answer the question truly, but unless he has a sudden lapse of memory he knows whether he is giving a truthful answer. It is only when people have been tainted by a little philosophy that they ask questions such as the following:

1. "I don't believe there's any such thing as truth." "Not even the statement you just made?" one might ask. Does the person mean that all statements are false? That not only "Snow is black" is false but also "Snow is white"? Does the person really mean to say that no statements that are made are ever true ― not even that he was born, or had a childhood, or had parents, or is now alive?

2. "I mean that there is no absolute truth." What exactly does the word "absolute" add? What is the difference between "It's true that snow is white" and "It's absolutely true that snow is white"? The word "absolute" gives the utterance a certain emphasis, but what does it add to the meaning of the statement? And when someone protests, "What you say is absolutely false," how is this different (except in emphasis) from saying, "What you say is false"? Don't both sentences label the same situation?

3. "All truth is relative." "Absolute" is the opposite of "relative." If someone says, "All truths are relative," what does that mean? Relative to what? "I am now sitting in a chair." How is this statement relative? And what does it mean to say so?

   A person might say, "If I am sitting on one side of the desk and you on the other, the vase that to you is on the right side of the desk is to me on the left side. It's relative to one's position." "Right" and "left" are indeed relative terms, but is the truth they express relative? "From where Mary sits the vase is on the right," and "From where Jerry sits (on the opposite side of the desk) the vase is on the left" ― aren't both of these propositions just true, period?

   Or, one might say, "If I'm facing north, east is to my right; but if I'm facing south, east is to my left." Is that statement relative to anything? Isn't it just true (without qualifications)? If someone objects that "east is to the right" is a relative truth, one can reply that the statement itself is incomplete; you can't know whether it's true till you know what direction the speaker is facing. If the speaker is facing north, the statement is true ― that is, the statement "East is to your right if you're facing north."

4. But doesn't the truth of a statement depend on who it is that's speaking? If one person says, "Calculus is interesting," and another person says, "Speak for yourself ― it may be interesting to you, but it's boring to me," what is it that's true? Is calculus boring or not? Clearly what interests one person bores another. "X interests you," and "X bores me," can both be true, and often are. There are many statements of this kind: "I find her fascinating." "Well, I don't." They are not contradicting one another; they're each stating how they feel, and they feel oppositely.

   A statement isn't true-for-you and false-for-me even when it's about you or about me.

   It may be a truth about you that you like to handle snakes, and it's a truth about me that I do not. It's not just true-for-you that you like to handle snakes but true, period.

5. One can respond in the same way to the charge that truth is relative to time. Isn't the statement "New York City has over 6 million inhabitants" relative to the time about which the statement is made? Isn't it true at one time and not at another time? But again the relativity disappears when we include the information that's needed to complete the statement. "New York City had more than 6 million inhabitants in 1896" is false, and "New York City had more than 6 million inhabitants in 1996" is true. Neither is relative to the time; the time is included in the statement, and must be if we are to give an accurate report about New York City's population.

6. Suppose someone says, "As far as I'm concerned, it's true." What exactly does the phrase "as far as I'm concerned" add? Is the statement true? If a member of the jury says, "As far as I'm concerned, the man is guilty;" doesn't that translate into "I think he's guilty"? Isn't "it's true as far as I'm concerned" a confused way of saying "I believe it's true"? And of course what one person believes, another may not.

   Two people are arguing, and one of them says, "As far as I'm concerned, there are aliens from other worlds on the earth now," and the other says, "As far as I'm concerned, that's not so." Well, are there or aren't there? Isn't that the question? Isn't the "as far as I'm concerned" just a way of getting off the hook in case the proposition turns out not to be true?

   All this may seem too plain and obvious to mention. Still, people say this sort of thing constantly and it may be important to point out when their assertions are inaccurate. They do say things like, "As far as I'm concerned, what he says is true" ― but that leaves open the question as to whether what he says is true. If a person is asked in court, "Is it true that you killed that woman?," what is the judge or jury supposed to conclude if she says, "Well, as far as I'm concerned, I didn't"?

   If one person says of a proposition p, "It's true," and the other says, "It's not true," and it's the very same proposition in both cases, then surely one of them is mistaken. But if one says, "I believe it's true," and the other says, "I believe it's not true," both statements may be true: one believes it and the other does not. And if one of them says, "To me it's true," and the other says, "To me it's not true," one must ask them what the "to me" adds to the statement, whether the phrase can be dropped without loss of meaning, or whether it means only that the speaker believes it's true.

7. "But what if it's partly true and partly false?" If you are confronted by a mass of statements, in which you can discern a few that are true, you may say, "There's some truth in what you say." Perhaps with a few corrections a few true statements can be extracted from the mess.

   Can one and the same statement be partly true and partly false? Yes, if it's a compound statement: "Snow is white and grass is white" is false because one of its component statements is false ― as every student is supposed to be aware when taking a true-false test.

   But what of a simple statement such as, "She is a scheming, selfish brat"? Such a statement is extremely vague, and it isn't clear what the limits of application are for words like "selfish," "brat," and so on. But even apart from this, what is said may be true of this person, but other things of a different kind may also be true of her: she may on occasions be outgoing, helpful, and generous. Both can be true as long as neither statement claims to be the whole truth about her. "She is nothing but X" may not be true, but "She is sometimes X and sometimes Y' may be true. Or, "She is always X" may be false, although "Part of the time she is X, and part of the time she is Y," may be, as we say, "All too true."

8. 8. What if someone says, "It may be true, or it may be false, or it may be neither: its truth may be unknown." This is mixing truth with our knowledge of truth. A statement may be true but not known to be true and false but not known to be false. If unknown, it's still true or false but we don't know which. It's either true or false that there is an intruder in the house next-door, but since the family is gone for the weekend, the truth or falsity of the statement is unknown.

   The truth of a statement should not be confused with someone's knowledge of its truth. More truths are being discovered all the time, and many are still unknown, but are no less true for the fact that we do not yet know them.

Criteria of truth. "What is truth?" asked Pontius Pilate, who did not stay for an answer.

But what is the question? If it's true propositions we are talking about ― not some other sense of "true" such as, "She's a true friend," or "This is a true plumb," or "This is a true diamond" (that is, it is a diamond and not something else), the question asks when a proposition is true. Well, a proposition is true when it tells us what is. Aristotle devoted one sentence to answering the question: "To say of what is that it is not, or of what is not that it is, is false; while to say of what is that it is, and of what is not that it is not, is true."

Or one could put it even more simply: The statement that snow is white is true, if and only if snow is white. "The earth is round" is true if and only if the earth is round. Or, we could say that a statement is true if it reports the way some aspect of reality is, and false if it misreports reality.

1. Correspondence. What has just been described is sometimes called the "correspondence theory of truth." A statement is true if it corresponds to reality (or as is sometimes said, to a fact). Someone says there are five trees in the courtyard, and I look, count them, and say, "That's true" ― that is, the statement corresponds with the fact.

   Philosophers have given themselves headaches speculating on what the word "correspond" means in the sentence. (Do we need the word at all?) If I match color charts to cans of paint, the colors on the chart resemble the colors of the paint in the cans; they correspond in being alike. There is no such resemblance between words and facts. But correspondence doesn't require any such similarity.

   We speak of a one-to-one correspondence between two things, for example, between the books in a library and the cards in the card catalog. Is there such a one-to-one correspondence between a sentence and a fact? Surely not, for the sentence can be translated into other languages and still express the same fact. It's the meaning that counts, not the sentence per se.

   The correspondence theory applies easily to empirical statements. Are there five books on this desk? Just check the statement against the alleged fact. But suppose someone says, "If Cleopatra's nose had been half an inch longer, Egypt would not have become part of the Roman Empire" (because Marc Antony wouldn't have been attracted to tier). Historians may dispute whether this statement is true. But where is the situation to which it corresponds? There is no such situation, since it never occurred. "If X had happened (which it did not), Y would not have happened (although it did), is an example of a contrary-to-fact hypothetical (conditional) statement. Where is the reality to correspond with the statement?

   There are other types of statements as well, which we have not yet considered: statements in mathematics ("2 + 2 = 4") and logic ("If A is larger than B, B is smaller than A"). These are surely true, but what kind of truths are they? Do they correspond to any fact of reality?

   Anyway, a statement by itself corresponds to a fact only if the speaker intends it for that purpose. An instructor may say, "The moon is in syzygy," not to state a fact but to give an example of how the word "syzygy" is used. And so on. A seemingly simple issue has suddenly been riddled with complexities, of which only small hints have been given here. Meanwhile, we turn to a second conception of truth.

2. Coherence. Not correspondence but coherence, it has been suggested, is what decides whether a statement is true. Coherence with what? With other statements. Its truth consists in its coherence with a body, or system, of other statements.

   What is meant by "coherence"? One meaning is, "A proposition is coherent with a body of other propositions if it is logically consistent with them; that is, if it doesn't contradict any of them." A proposition, p, is not coherent with another proposition or set of propositions if anywhere within the set there is a not-p, the negation (denial) of p. But this is a very loose kind of requirement. "There is a vase on this desk" is consistent with "Saturn has twelve satellites." In that sense they are coherent with each other, but what has this to do with their being true? If Saturn had thirteen moons instead of twelve, would that be any less coherent with there being a vase on this desk?

   One could make the coherence requirement stronger by saying that the propositions must entail (logically imply) one another: "This is a square" entails "This has four sides." But "This is a square" entails nothing about Saturn's moons or my vacation last year or just about any other proposition we believe to be true.

   Rather, coherence among propositions has to do with a body, or system, of propositions that are not only consistent with each other but that mutually support one another; they don't logically entail one another but they provide evidence for each other. Suppose we suspect that Mr. Jones killed a certain person. There were no witnesses, but blood with the victim's DNA was found on Jones's jacket, Jones was seen leaving the victim's house a few minutes after the established time of death, Jones abruptly left his home a mile away an hour before the killing took place, and so on. The statement that Jones committed the murder is coherent with all these other statements; the other statements provide evidential support for it.

   If I can't find my pen, I say, "It must be somewhere," even though I search every place where I think I might have lost it. Its disappearance remains a puzzle to this day, yet it never occurs to me to say, "Perhaps it was annihilated ― one moment it was a collection of molecules, and the next moment, pfft! no molecules, nothing." Yet we dismiss this possibility because there is such a huge array of beliefs, garnered through long effort over many years, that is not coherent with the belief in annihilation, although my continued inability to find it is coherent with belief in its annihilation. (Maybe it could happen in a black hole, but, we say, not on earth.) "The pen suffered total annihilation" doesn't fit with ― is not coherent with ― a great mass of other statements that we already consider to be true.

   Suppose I am alone in the house, and I keep hearing what seems to be a toilet flushing upstairs. I go and find no one. Perhaps I am not alone after all, but I check every room and closet, and there's no one. I hear the sound again and go to the bathroom: the tank fills with water and then the sound stops. I don't know how it could have happened, but after a while it happens again. Perhaps it was a genie or some incorporeal spirit; I didn't consider that possibility before, but now I am not so sure. So I situation myself in the upstairs bathroom again and wait. Sure enough, it happens again; I see the lever move and the water flow out of the tank, but nothing and nobody can be seen pressing it. Well, perhaps the event depends on some other factors I haven't considered, some strange condition inside the tank that a plumber might be able to explain. Meanwhile I take a video picture of the whole thing, to prove to others that I really saw all this.

   I do call in a plumber, and he observes what I did, but he can't explain it either. Neither of us rests very easy with the idea that some immaterial spirit has pressed the lever. What if something "from the spirit world," wherever that is, is doing this? If such things started to happen frequently, we might give up our belief that only a physical force can cause a physical effect and conclude that the science of mechanics is not up to explaining what it claims to explain ― events as simple as the pressing of a lever.

   Still, we would accept the spirit hypothesis only with great reluctance ― not because we are stubborn, not because we don't want to admit any reality in the universe except the physical, but because what we see and hear on these occasions is not coherent with the whole body of knowledge we already have. The complex and interconnected network of principles of applied mechanics, which every would-be plumber spends many months learning, seems to be put in question. It's as if a monkey wrench had been suddenly thrown into the machinery of our thought ― as if we had achieved a coherent system of mechanical laws and now something turns up that threatens the structure of principles we have so laboriously built. We tend to discard the "spirit world" explanation because it is not coherent with the vast mass of other things we know, or believe we know.

   Isn't it the lack of coherence that leads us to reject it? If only this new discovery were coherent with what we already believe, it would be so much easier to accept it. But there are (as we shall see) many examples in science of discoveries, such as x-rays and electricity, that were not coherent with a body of propositions already accepted, that opened up new and hitherto unsuspected areas of investigation, and that exhibited a wider coherence (coherence with more propositions already accepted) than anything that had been thought of up to that time.

   Coherence is of great importance in the sciences, where there is a large body of propositions that are mutually dependent on one another. In scientific theory, the only means available for choosing among competing theories is their degree of coherence with other propositions already accepted (see Chapter 4).

   But what of the statements on which the theory is based? The spirit hypothesis about the flushing toilet is not coherent with propositions already accepted about plumbing, but what of the statements that led to the hypothesis in the first place, such as that I kept hearing the sound upstairs and that it had no assignable cause for it? Was the statement "I heard what sounded like a toilet flushing upstairs" itself accepted because of its coherence with other statements? (Could coherence be defined without already presupposing ― assuming ― the concept of truth?)

3. The pragmatic theory. "The truth is what works." The first question to be asked is, "What does it mean?" We know well enough what it is for a mechanical object to "work." Don't you know when your car works? You turn on the ignition, and the car fails to start; you check the battery, the carburetor, the wiring, and so on, and everything looks all right, but still the car won't start. A friend who is a mechanic makes a few changes under the hood, the car starts, and you drive it to your destination. There is no doubt that the car "works." The functioning of mechanical objects is the "home base" for our use of the word "work."

   But what does it mean for a belief to "work"? You believe you are now reading a book. Let's say you are, but does this belief "work"? What does it mean to say that it does or that it doesn't? Does your belief that the car works, itself work? Can propositions be tested by whether they "work"?

   A program or a procedure may work, that is, it may have its intended effects. A diet program may work in this sense; in fact it may work for one person and not for another. An exercise program may "work" in the sense of making you feel more vigorous and healthy. But what if someone says, "Christianity works for me"? Does this mean that it makes her feel better? That she is now happier or more hopeful than before? In this sense one might say that Christianity "works," at least for her (but perhaps not for others).

   But what has its "working" in a specific case to do with its truth? If it "works" for you but not for me, is it then true for you but not for me? What exactly is the relation between its being true and its "working"? Surely they aren't the same thing?

   If a king who wanted to put an end to religious strife within his kingdom passed an edict saying that everyone was from that moment on required on pain of death to believe in Isis and Osiris, he might succeed, if he had a large squad of enforcers. Doubters would be put to death, and everyone else would either believe or keep quiet about it. Parents would teach their children about Isis and Osiris, and no other religious belief would gain a foothold, until finally there would be no competition, and no more strife or even argument about religion. Did the belief "work"? It did in the sense that it put a stop to religious wars. But what has that to do with the belief being true?

   The contemporary philosopher Karl Popper has written, in defense of truth as correspondence, that

   there is no doubt that correspondence to the facts is what we usually call "truth" ― that in ordinary language it is correspondence that we call "truth," rather than coherence or pragmatic usefulness. A judge who admonishes a witness to speak the truth and nothing but the truth does not admonish the witness to speak what he thinks is useful either for himself or for anybody else. The judge admonishes a witness to speak the truth and nothing but the truth, but he does not say, "All we require of you is that you do not get involved in contradictions," which he would say were he a believer in the coherence theory. But this is not what he demands of the witness.[1]

Evidence

Believing isn't enough for knowing ― a person may believe what isn't true, and truth isn't enough ― a proposition may be true although you may not know that it is. If you are ignorant of it, you can hardly be said to know it. Neither can you be said to know it if you only guessit.

A player predicts that the next throw of the dice will be double-sixes. And as it turns out, that is what it is. "I knew it! " he exclaims in triumph. We feel mildly irritated, because we are convinced that he didn't really know it ― he only guessed it, and the guess turned out in his favor. "He didn't really know," we say, "he only guessed." People often say that they knew when it was only a lucky guess. What is lacking?

What is lacking is evidence. To know it, he had to have good reason to believe it. His statement can't be just a "shot in the dark." You say you know that the earth is round because you've seen the curvature from airplanes, you've flown around the earth yourself, and before that you read this fact in countless books and magazines. All this gives you good reason to believe it.

Why do you say you know that this is your sister's child? The child looks like her, she says it's her child (and you've never known her to lie), and you went with her to the hospital when she was eight-and-a-half months pregnant. But does all this entitle you to know? Couldn't there still have been some slip-up? How strong must the evidence be? considerable? incontrovertible? conclusive? And what do these three words mean ― aren't they all somewhat vague?

Let's consider a few examples of how we actually use the word "know."

While driving I hear a familiar thumping sound and say to myself, "I've got a flat tire." I get out of the car and look, and behold, the left rear tire is flat. Before I got out of the car I had strong reason to believe the statement, but now that I see it for myself, surely I know. This is a standard kind of knowing that we use every day.

2. I say, "I know that this man next to me is Ryan Carter. I've known him most of my life, and I recognize him clearly." But what if, unknown to me, he has an identical twin who just came into town? Then to my surprise I was mistaken in saying this man is Ryan. I have every reason to identify him as Ryan; my evidence is extremely strong. Ordinarily I'd say I was absolutely sure. Yet my identification of him as Ryan turned out to be mistaken ― I didn't know that this man was Ryan, since he wasn't.

3. "Do you know that your desk and books are still in your office?" "Of course I know," I say, "I left the office only five minutes ago, and they were all there." "But how can you be sure that in those five minutes some movers didn't come in and take all the things out?" "I was right next-door, and I would surely have heard some sounds of scraping and so on. And I didn't see any movers around; the place was quiet. Anyway, how could they have done it in five minutes?" "Do you mean that if someone were offered a million dollars to see to it that the stuff was moved out quietly in five minutes, they couldn't have done it? Would you stake your life on it?" "Well, perhaps not my life ― but still, I know." Just to be sure I go back to the office, and all the things are still there. I was right ― so I knew, didn't I?

   Should I say that the evidence was conclusive? No, not quite ― at the time I was questioned about, I wasn't in the room, so there was a possibility that I didn't have some evidence. Should I say that I didn't know, or didn't quite know, because of that improbable but possible contingency? Or should I say that I knew, although I did know that I might have been mistaken (but if I had been mistaken, then of course I didn't know)?

4. I open a certain book to page 100, and I see that the first word on the page is "upon." To make sure, I look at the page and at the same printed word again ― it's still the same. I ask other people, and they all say the word is "upon." I take a Polaroid picture of the page and develop it: the first word is still "upon." Surely, I would say, I don't just believe this ― I know it.

   But I run into a "perpetual doubter" ― someone who on leaving her house goes back and forth to the front door to make sure she's locked it. "You have to make absolutely sure," she says. So I look again and yet again; the result is still the same. Isn't that the end of the matter? If someone asked me what I was doing, and I said, "I'm making sure that the first word on this page is `upon,"' wouldn't my statement be a bit ridiculous? I can continue to stare at the page, but what more could I do that would be called "making sure what the first word on the page is"?

   It could be that if I stare at the page for a long time I would see, or seem to see, a different word. But then, someone might object, I would be seeing this at time t-2; but it would still be true that at time t-1, I did certainly see the word "upon" there. Or would what I saw at time t-2 cast doubt on what I saw, or was sure I saw, at time t-1?

Strong and weak senses of "know." One might say, "As long as there was any evidence I might have had that I didn't actually have, I didn't really know, because the missing bits of evidence might have turned out negatively. During those few moments I left my office, if I had been there I might have found the bookshelves empty! As long as that's a possibility, however remote, I can't say that I know that the books were there."

This is too rigorous a requirement for the needs of daily life. "In the ordinary way of speaking," I would surely say that I knew that the books were there on the shelf even when nobody saw them, and that the word "upon" was the first word on page 100. We would say that although the evidence was not conclusive, it was sufficient for knowing.

Philosophers have sometimes distinguished what they call the strong sense of "know" from the weak sense. In the strong sense, I don't know until I have conclusive evidence. The evidence is conclusive when there is nothing more that I or anyone could discover that would cast the slightest doubt on the statement. All the evidence must be in; there can be nothing lacking. In the weak sense, the "daily life" sense, we know when we have a preponderance of evidence in favor of it and (even though we have tried to find it) no evidence against it. In this sense, it might have turned out that books weren't on the shelf during those intervening minutes, and if this had turned out to be so, then I didn't know because I can't know what is false. But it turned out as expected, so I did know. I knew in the weak sense that the books were still on the shelf during my absence from the room. I know those true propositions that I believe with good reason. (Did I know in the strong sense that the first word on page 100 was "upon"? It would surely seem so, but let's wait; philosophers are ingenious at devising previously unthought-of situations in which one might still have doubt. We shall consider some of these in Chapter 3.)

1. When do you know that the accused man is guilty? Not guilty beyond a reasonable doubt, which is all that the law requires, but guilty beyond any doubt. He may have confessed, but that doesn't prove his guilt: there are false confessions all the time. If someone testifies that she saw him doing the killing, you still can't be sure: many witnesses perjure themselves ― they may not be lying, but they may make an innocent mistake in identification. They are certain that suspect number 3 in the police line-up is the man, but they don't recollect very accurately; some time has elapsed, and their memory image of the man doesn't resemble the man all that much any more. In all these cases you have evidence but not knowledge. Suppose further that someone recorded the whole murder on tape and that the tape shows this man wielding the knife. Can't we now say that we know that the suspect was guilty?

   Still, isn't it possible that what the camera shows is a look-alike, someone with so strong a resemblance to the suspect that even his mother wouldn't be able to tell the difference? Couldn't it have been the look-alike who did it? But then there's the DNA evidence, which indicts the original culprit by a million-to-one margin. We've sent men to their deaths on less evidence than this, but in this case don't we know he's guilty? Not in the strong sense ― not if we demand that nothing that might yet be discovered would make any difference; not if we demand that all the evidence be "in" so that there is no chance that any yet-to-be-discovered shred of evidence, however slight, would turn up to cast the slightest doubt on our claim to know. Of course we don't ordinarily use the word "know" in the strong sense; but if we do, even this case won't meet the requirement. Some evidence could still turn up that goes the other way. We are still entitled to doubt, to say, "We don't know ― not quite!"

Let's try one more: I can understand how you might doubt that I have a hand if you look in a bad light or in a dense fog or if there's something wrong with your eyes. But in this case we are sitting a couple of feet apart in this well-lighted room. I raise my hand and say, "Here is my hand." And you say to me, "I doubt that that's a hand." And I say, "What more do you want? Here it is, you can see it; come closer, touch it. You still aren't satisfied? Take a picture of it; call other people in as witnesses if you like. After all that, what more do you want? What more evidence is there that I can give you?" I could understand your doubt as long as some of the conditions of knowing were unfulfilled; but I've fulfilled them all, and you still doubt. What is it that you're doubting? What test is there that, if performed, would resolve your doubt? You want to touch it again? Go ahead; here it is. You keep on saying, "I doubt," but every time I give you the means of resolving your doubt, you perform the test and you still say, "I doubt." What test is there whose negative outcome you fear?

You may say, "I doubt that if I were to reach for your hand a minute from now there would still be anything there." So we wait a minute, and you still touch the hand. Do you want another minute? Do you really think that if I don't remove it, it may not still be there? Doubting is a game with you. You say, "I doubt," but these are now empty words. You're not really doubting anymore.

We have distinguished the daily-life sense of "know" (the weak sense) from the philosophically stringent sense (the strong sense), which admits no possibility of uncertainty. But we have still not reached the end of the issue. A skeptic about knowledge could still say that in stating the examples we have already assumed too much. In giving the example of the flat tire we assumed that we really did see the flat tire and didn't just imagine it. In the hand example, we assumed that there really was a hand there, that you weren't dreaming it. The skeptic would doubt everything, even that there is a physical world. The skeptic's doubt is more all-encompassing than any we have yet presented. Thus far all we have done is to "get the show on the road." We shall do more with it in the next chapter.

[1] Karl Popper, Objective Knowledge (London: Oxford University Press, 1979), p. 317.