There are a number of good reasons to want a definition for knowledge. For instance, you might be a lexicographer. Or you might be a philosopher, wondering what knowledge is.
Either way, you’re out of luck, because knowledge turns out to be a tricky beast.
Know vs. Believe
The easiest way to start is to compare know with believe. What is the difference between:
I believe it’s Friday.
I know it’s Friday.
The latter is more certain, but that’s not all. It’s possible to believe it’s Friday on a Thursday. It’s not possible to know that it’s Friday on a Thursday. So we might be tempted to define
knowledge = true belief
That’s not going to be enough, though. Suppose John just woke up from a coma. He knows he was in a coma, and he hasn’t seen a calendar. Still, his intuition tells him it’s Friday. Can he say he knows that it’s Friday?
Well, he can say it. But even if it turns out that today really is Friday, we still would be uncomfortable saying John knows that it’s Friday, unless we believe in ESP or some similar phenomenon.
Similarly, I might say that I know Barack Obama will be the next president of the United States. Even if it turns out that I am right and Obama does become the next president, it’s a little weird to say that I knew it. It seems better to say I strongly believed it.
So we might try the following definition:
knowledge = justified true belief
The idea being that it only counts as knowledge if I have sufficient evidence.
Unfortunately, that won’t work, either, though it took some fancy philosophizing to prove it. Consider the following example.
Suppose I am watching the Red Sox play the Yankees. Unbeknownst to me, there has actually been an electrical outage at Fenway, so the cameras aren’t working. NESN quickly substitutes a tape of a previous game in which the Red Sox played the Yankees, but I don’t realize it.
In this rebroadcast, the Red Sox beat the Yankees. At the same time as I am watching the taped game, the Red Sox are actually beating the Yankees. So if I then say, “Today, the Red Sox beat the Yankees,” my statement is true (the Red Sox really did beat the Yankees) and justified (I have every reason to believe what I am saying), but still it seems very strange to say that I know that the Red Sox beat the Yankees.
Where does this leave us?
You might try to save justified true belief by fiddling with justified, but most philosophical accounts I’ve seen just stop there and claim there is no definition. I am inclined to agree, and this is just one more reason to suspect that words just don’t have definitions.
As I’ve pointed out before, Greg Murphy has a pretty good explanation of why it makes sense that words don’t have definitions. The original post is here, but in short, words are used to distinguish objects, but it is always possible to come up with a new object (or idea) that is midway between two words — that is, fits both and neither, just as the baseball game example above seems to fit both knowledge and belief and neither.
I find this pretty convincing, but if he is right, it raises the following question: why do we think words have crisp definitions? Even more, why do we really want words to have crisp definitions? It seems generations of philosophers would have saved a lot of time.