3. A first look at the skeptic’s argument

The sceptic claims that if you entered the machine, pressed the button, and found yourself waking up on Mars, you’d be unable to tell (upon waking) whether your memory of having entered the machine was genuine or false.

You wouldn’t know if you were the person who originally entered the machine, or just a freshly-minted copy with a false memory of having done so. After all, what it’s like for the original subject with the genuine memory to wake up on Mars is exactly like what it’s like for the copy with the false memory to do so. So you’d be unable to tell who you were, and thus if the machine had worked. (If you happened to be the copy, the machine would in fact have failed.)

The alleged threat here is the false memory. As you wake up on Mars, the thought that you might be the copy with the false memory is supposed to give you pause. But for this possibility, you could just go by your memory of having entered the machine and judge that the machine had worked. But—wait—you might be the copy! Then your memory would be false and the machine would in fact have failed! You’d be in danger of making the wrong judgement here if you went by your “memory.”

This line of thought appears to be rather straightforward at first sight, but it could really use a little clarification, since it should be obvious that the only person, surely, who could end up making the wrong judgment here is your copy, and not you. You yourself could never be “in danger” of judging wrongly that the machine had worked, since you were the one who entered the machine.

In other words, the line of thought above, despite surface appearances, is not comparable to the simpler sort of logic that a fellow skeptic might advance if you had a brain defect (say) that saddled you with false but realistic memories from time to time. In such a case, this fellow skeptic might quite reasonably suggest, should you seem on some occasion to remember something having happened, that you don’t really know whether it did happen. Because of your brain defect, he’d say, your memory might be false; you can’t really tell. The force of the sceptical reasoning in this comparison case is easily felt and would generally be granted without ado, but our case, despite the superficial resemblance, is not of this sort. The difference, again, is that, with the brain defect, it is your memory that would be in danger of being false, whereas, in our case, the only one who could have the false memory is someone else (your copy), and not you. Your memory of having entered the machine could never be false because you were the one who entered the machine.

This makes the skeptical reasoning in our case a little more challenging to understand. It clearly cannot be as straightforward as the reasoning in the brain defect case, but there must obviously be some logic to it all the same.

This proves indeed to be the case, although the skeptic’s logic, while simple enough, is not apparent to the casual eye. In what follows, I will lay out what I think it’s supposed to be, and point out a tacit assumption within it that the skeptic appears to need. This assumption, while entirely natural, is not necessarily true, and this, I will suggest, is where his argument falters.