What do we know? It’s an intriguing question, also for philosophers. Once I discussed this case: My wife and I are driving home on a Friday afternoon and stop at the bank to deposit our paychecks. However, there was a long queue in front of the counter, so I said: “I’ll do it tomorrow. I know that the bank will be open.” But my wife says: “Maybe the bank won’t be open on Saturday. Maybe it has changed its opening hours.” Should I check it? If I am in a hurry and can deposit my paychecks also on Monday, in case the bank happens to be closed tomorrow, I’ll not check it. If it is important to deposit my paychecks before the weekend, I’ll do. In other words: What I know depends on the context. (for a full explanation see my blog dated 12 December 2011: http://philosophybytheway.blogspot.com/2011/12/what-do-we-know.html)
Contextuality can affect what you think you know. Possible alternatives are another condition that can affect it, as Fred Dretske has made clear in his article “Epistemic Operators”. To illustrate this he discusses a “silly” example, as he calls it, although it is no more silly than many other philosophical examples. In short, it is this:
“You take your son to the zoo, see several zebras, and, when questioned by your son, tell him they are zebras. Do you know they are zebras? Well, most of us would have little hesitation saying that we did know this. We know what zebras look like, and, besides, this is the city zoo and the animals are in a pen clearly marked ‘Zebras.’ Yet, something’s being a zebra implies that it is not a mule and, in particular, not a mule cleverly disguised by the zoo authorities to look like a zebra. Do you know that these animals are not mules cleverly disguised by the zoo authorities to look like zebras?” (p. 39)
Probably you’ll answer this question with “Yes”, for you simply don’t find the idea that the “zebra” is a mule in disguise reasonable. If Dretske hadn’t asked this question, it wouldn’t simply have come to your mind that the zebra might be a mule in disguise. And why should the zoo authorities deceive you? And is it really possible to disguise a mule that way that you’ll not notice it? Etc. In other words, what you believe to be true in this case, depends on what you think what the plausible alternatives are. You “know” that the animal in the zoo is a zebra, for what else would it be? (or so you think). But you don’t have checked it. So even if the animal is a zebra, actually you don’t know. For, as Dretske says, “the question here is not whether [the] alternative is plausible, not whether it is more or less plausible than that there are real zebras in the pen, but whether you know that this alternative hypothesis is false.” (ibid., italics Dretske) Nevertheless, we think that we know, or as Dretske says a few lines hereafter: “[W]e simply admit that we do not know that some ... contrasting ‘skeptical alternatives’ are not the case, but refuse to admit that we do not know what we originally said we knew.” (ibid., italics Dretske)
What we think to know depends on the alternatives we judge relevant. That’s why this approach of knowledge is called the “Relevant Alternatives Theory”. But since at first sight non-relevant or left out alternatives might be true, it may always happen that we don’t know what we know, even if we belief that our knowledge is justified.
Sources- Dretske, Fred, “Epistemic Operators”, in his Perception, Knowledge and Belief. Cambridge, UK: Cambridge University Press, 2000; pp. 38-40. You can find it also here: https://people.ucsc.edu/~farkas/sclp/papers/Epistemic_Operators.pdf