This is the the third post in my Theories of Knowledge and Reality series. Follow the link for more on the series and for links to other entries as they appear.
One bad objection to the arguments in the previous post is that the cases it involves are fairly extreme and seem unlikely. It's a bad objection because the point doesn't involve any assumption that the cases are likely. The point is very simple and straightforward. You can't rule these cases out. They're within the realm of possibility as far as our evidence is concerned. Our evidence is just as consistent with the Matrix or an evil demon's deception as it is with what most of us believe to be true. All that matters is that it's possible. If it's possible that something is true, even something you think incredibly likely, then you can't rule it out. If it were true, it would mean your beliefs are false. That means you can't rule out the possibility that your beliefs are false, and you don't know those things that you believe. How likely the skeptical scenarios are plays no role in the argument. If the argument is bad, it's not because the skeptical scenarios are far-fetched. Some other error in reasoning must have taken place.
Nonetheless, the skeptical argument can easily be framed in very mundane circumstances that aren't all that far-fetched at all. In fact, they're things that happen all the time. For instance, I might walk to campus and then wonder if I know where my car is. I know where I parked it. Do I know it's still there? I haven't moved it. Maybe it's moved from where I put it, however. How would it do that? Well, it's possible that someone broke into it and stole it. What's more likely is that my wife got into it and drove it somewhere when I wasn't expecting her to do so. Can I rule that out? So I don't really know if my car is where I left it. It may not be all that probable that she'd go somewhere on a day when I don't know she's going somewhere, but that doesn't mean she didn't. Even if she didn't, I don't know that she didn't, so I don't know that the car is where I left it.
Similarly, I might say that I'll see my brothers in a few weeks when we plan to get together in NH. Do I know that I'm going to see them? Well, something could come up. They might not make it. I might not make it. Who knows what will happen? Since I don't know that something won't come up, I don't know that they'll be there. Therefore, I don't know that I'll see them. Interestingly, this argument comes up in the biblical epistle of James:
Come now, you who say, "Today or tomorrow we will go into such and such a town and spend a year there and trade and make a profit" -- yet you do not know what tomorrow will bring. What is your life? For you are a mist that appears for a little time and then vanishes. Instead you ought to say, "If the Lord wills, we will live and do this or that." As it is, you boast in your arrogance. All such boasting is evil. (James 4:13-16, ESV)
So this kind of skepticism even has the backing of the Bible! James was particularly concerned with boasting about one's plans, but in the process he endorses the view that you don't know what you will do even if you have clear plans and a very small likelihood that things will go wrong.
It's not hard to see how this kind of argument really will undermine most things that we might think we know. If there's any possibility at all that you might be mistaken, if there's even a very slight bit of uncertainty in your belief, it's simply not knowledge. The Cartesian skeptic doesn't need to go to the more extreme skeptical hypotheses to make the point that we don't know much at all. All that's needed is the thesis that knowledge involves the ability to rule out alternatives. Almost nothing we believe could even have a chance of having the kind of absolute certainty that Descartes thinks we need if we're to know something.
The next post will consider the first of a series of responses to skepticism.