What Makes You You
6. Why You Have to Rule Out the Dreaming Hypothesis DR1 says that you must be able to rule out the dreaming
hypothesis in order to know that you’re sitting down reading.
Why is that? Why can’t you claim to know that you’re sitting down reading, while at the same time admitting that you have no way to rule out crazy ideas like TDH? I’ll give two reasons.
The first reason for accepting DR1 involves thinking about everyday ways of challenging someone’s claim to know something. If we see a large bird in the sky and you say that it’s a hawk, I might ask how you know it’s not an eagle or a falcon.
Perhaps you are able to rule out these competing hypotheses. For instance, perhaps you can tell it’s not an eagle by its tailfeathers or by its beak. If, however, you aren’t able to rule out these competing hypotheses, then you can’t truly claim to know that it’s a hawk.
This suggests the following argument for DR1:
The Competing Hypotheses Argument
(CH1) One knows a certain thing only if one has some way of knowing that all competing hypotheses are false (CH2) TDH is a hypothesis that competes with your belief
that you’re sitting down reading
(DR1) So, if you have no way of knowing that TDH is false, then you don’t know that you’re sitting down reading
CH1 reflects a general lesson that can be extracted from the hawk example: in order to truly know what’s going on in a given situation, you have to be able to rule out competing hypotheses about what’s going on in that situation. That’s why the observation that you can’t rule out the hypothesis that the bird we saw is an eagle constitutes a genuine challenge to your claim to know that the bird is a hawk.
CH2 is straightforward: when you have all of these experiences as of sitting down and reading, and you assume that you indeed are sitting down reading, a competing explanation of what’s going on is that you’re in bed having an incredibly vivid dream in which you’re sitting down reading. I’m not saying this is an especially plausible hypothesis, just that it’s a competing hypothesis.
Now for the second reason to accept DR1. Suppose you really did know that you were sitting down reading right now. In that case, you would have a way of definitively ruling out TDH. After all, if you genuinely knew that you were sitting, then you’d be able to infer that you aren’t lying down—since you can’t simultaneously be sitting and lying down—and thus that you aren’t lying down dreaming. Knowing you’re sitting down would therefore give you a way of knowing that TDH is false, so if you truly have no way of knowing that TDH is false then you must not
know that you’re sitting down reading. Which is exactly what DR1 says.
We can develop this idea more explicitly using the notion of a deduction. A deduction is a certain type of reasoning, where the conclusion of the reasoning is logically guaranteed by the premises. In other words, you would be contradicting yourself if you accepted all the premises and yet denied the conclusion. As an illustration, if you reason from the coin either landed heads or tails and it did not land heads to the conclusion it landed tails, that’s a deduction. You deduced that it landed tails from those other two beliefs. Using this notion of deduction, we can run the following argument:
The Argument from Deduction
(DE1) If you know you’re sitting down reading, then you can deduce that TDH is false from things you know (DE2) If you can deduce something from things you know,
then you have a way of knowing that thing
(DE3) So, if you know you’re sitting down reading, then you have a way of knowing that TDH is false
DE1 says that there’s a certain kind of deduction you’d be able to perform if you really did know that you were sitting down reading. Specifically, you’d be able to perform the following deduction:
(i) I’m sitting down reading
(ii) If I’m sitting down reading, then I’m sitting (iii) If I’m sitting, then I’m not lying down
(iv) If I’m not lying down, then I’m not lying down dreaming (v) If I’m not lying down dreaming, then TDH is false (vi) So, TDH is false
This is a way of deducing that TDH is false. Steps (ii), (iii), (iv), and (v) of the reasoning are easily known conceptual truths. For instance, you know (iii) just by observing that it follows from the definition of sitting that if you’re sitting you’re not lying down.
So, if you know the first step as well—that you’re sitting down
reading—then what we have here is a way of deducing that TDH is false from things you know. That’s what DE1 says.
The idea behind DE2 is straightforward. Suppose I tell you that I flipped a normal coin and that it didn’t come up heads. You tell me that it came up tails. How did you know?? Answer: by deducing it from things you know: that it was either heads or tails, and that it wasn’t heads. Of course, if you ran through that same deduction, but you didn’t actually know that it wasn’t heads—
you were merely guessing it wasn’t heads, let’s say—we wouldn’t say that you knew it was tails. But when you deduce something from things you actually do know, then you know the thing you deduced as well. That’s what DE2 is saying.
DE1 and DE2 are both true, and they together entail DE3. But notice that DE3 says exactly the same thing as DR1:
(DE3) If you know you’re sitting down reading, then you have a way of knowing that TDH is false
(DR1) If you have no way of knowing that TDH is false, then you don’t know that you’re sitting down reading To see that these say the same thing, notice that “if A is true then B is true” is just another way of saying “if B isn’t true, then A isn’t true.” These are simply two different ways of saying that you don’t get A without B. (An example: “if Farid is from Paris then he is from France” is exactly equivalent to saying “if Farid isn’t from France then he isn’t from Paris.”) And since DR1 and DE3 say exactly the same thing, the Argument from Deduction serves as an argument for DR1.