First, let’s ask what sorts of things are “known” in the sense we are interested in. In a claim of the form “Sluggo knows X,” what sort of entity goes into the “X” spot? We could fill in the blank with phrases like “how to ride a bike,” “how to speak Esperanto,” or “how to dance like a maniac.” These are abilities Sluggo may or may not have, and we might call this sort of knowledge know-how. Philosophers typically are not very interested in know-how since we have not yet come across many interesting questions or problems to raise about it. Instead, we could put into the “X” spot phrases like “that Bangladesh and India share a border,” or “that Jupiter is larger than Mars,” or “that Francis Bacon is credited with having said that knowledge is power.” For obvious reasons, this is called knowledge-that, and what is known, or what goes in the “X” spot, are claims that may be true or false which philosophers call “propositions.” Philosophers are very interested in propositional knowledge.
So, let’s continue to focus on propositional knowledge. We will want to know what features a proposition has to have in order for us to rightly say that someone knows it. We have already seen one back when we were observing that people sometimes fight over knowledge or truth. A claim to know X is, among other things, a claim that X is true. No one would ever claim to know something they believe is false. (Well, they might if they were lying. But if they were not trying to lie, and they were well aware of what they are saying, and they were not under the influence of some strange drug or head injury, they would never say, “I know X and I think X is false.”) Furthermore, we would not claim that someone else knew something if we thought that that something were false. I would not say that Sluggo knows the world is flat since the world isn’t flat. Sluggo might believe it is, but he does not know it, because it isn’t.
This is a tricky point, so we should spend a little more time on it. Some time ago, people thought the sun revolved around the earth. They had good reason for believing this since it certainly looks like the sun revolves around the earth at the rate of once per day, and we do not feel the earth to be in motion. But would we say these people knew it? They believed it, yes, and they had their reasons, yes, and they thought they knew it, yes. But did they know it? We might say they “knew” it, but it would be important to keep those scare-quotes attached because they did not really know it. They did not know it because the claim is false, and they were wrong.
(Or so it seems to me. Perhaps my use of the word “know” has been corrupted by reading too much philosophy. But at the very least there is one widespread meaning of the word “know” which implies that what is known is in fact true, and in what follows, that is the sense of “know” we will be using.)
This discussion also suggests a further feature of propositions we claim to know: we must also believe them, or in other words, believe they are true. There may be some cases of people really knowing something deep down in their bones but not admitting it to themselves. Perhaps these are cases when someone knows something and, in a certain sense, does not believe it. But let’s mark these cases as special exceptions to the more general rule that we believe the things we know.
From what we have seen so far, knowledge is believing true propositions. But there is a further feature we need to add. Knowledge usually involves some amount of evidence or reasoning or reliable report. It is not a lucky guess, like correctly predicting a coin toss. We can call this general feature “justification,” meaning that if we really do know some proposition, we could provide some reasons or evidence for the truth of that proposition.
And so we arrive at a time-honored definition of knowledge: To know a proposition is to have a justified belief in that proposition and for that proposition to be true. If I believe it and have some good reason for believing it, and if it is true, then I know it. For obvious reasons, this is called the “justified true belief” definition of knowledge, and in fact it is so common that it is sometimes just referred to by its initials: JTB.
But philosophers love to test definitions against their own imaginative powers, and so challenges have been posed to JTB. Here is a case where the letter of the definition is met, but intuitively, it does not seem to be a case of knowledge. Suppose that Molly walks into her bedroom one night and flips on the light switch, but the light does not come on. She tries the switch a few more times, but still the light does not come on. She considers that the light bulb has been in use for a long time and that it is about time for it to have burned out. Just to be sure, she checks to make sure the bulb is screwed securely into its socket. It is. “Ah,” she thinks. “The light bulb must have burned out.” Let us also suppose that, in fact, she is right: The light bulb has burned out. But what she doesn’t know is that Doug is in the basement, and he has turned off the circuit breaker for her bedroom. So the light bulb would not have come on even if it had not been burned out. Does Molly’s belief that her lightbulb has burned out count as knowledge?
She believes the lightbulb has burned out, and she is right about that. She also has at least some justification for her belief since she has tried the switch a few times. She remembers how old the bulb is and correctly infers that it would be likely for the bulb to have burned out at this time. She confirms that the bulb is screwed in securely. So she meets the JTB conditions. But in a very important sense, Molly has simply lucked out this time. She accidentally got to the right conclusion, and her belief does not really count as knowledge.
To try to repair the JTB account, we might further require that the truth of the proposition is not accidental to the person’s justification for their belief. In Molly’s case, as we said, it was lucky for her that the bulb was in fact burned out; its being burned out did not actively contribute to the evidence she was gathering for her belief since all of that evidence would have been the same anyway even if the bulb was in good working order due to Doug’s interference. Another way to make this point is to say that, in a case of knowledge, the truth of the thing being known helps to explain why the person believes what they do.
With this in mind, we might offer the JTB+ definition of knowledge: S knows P if and only if S believes P, S’s belief has justification, P is true, and the truth of P helps to explain why S comes to believe P.
Of course, much of this is still unclear. How much justification does one need? Will any old justification be sufficient, or must the justification be of a certain kind? What does “helps to explain” mean? Is it okay to help a little, or must it be a lot? And even if we manage to make these matters clear, can we be sure that there aren’t some further clever challenges to JTB+ arising from the fertile imaginations of philosophers?
But we will leave these worries and unclarities to the side, at least for now, and be content to have given at least a set of features central to cases of knowledge. However, we should explore this “truth” business a little further.