8. But would he know that the machine had worked?

Employing the “resolve and follow through” strategy under the circumstances in question—i.e., whenever the copy doesn’t care—the original subject would be bound to judge correctly that the machine had worked, should he awaken on Mars. But would he thereby know that the machine had worked? Notoriously, there is a gap between judging correctly and knowing, and it’s a valid question whether the gap is bridged in our case. I believe that the answer to this question is yes, and that there is more than one way to see this.

Above all, as mentioned briefly in section 4, no luck would have been involved in the original subject’s arriving at his correct judgement. It wasn’t as if he was lucky to have judged correctly that the machine had worked, but could easily have gotten it wrong if things had been different. That confuses our case with the brain defect case mentioned in section 3. In our case, if things had been different, it would have been the copy who would have ended up judging wrongly that the machine had worked, and not the original subject. (The original subject himself, following the strategy in question, could never judge wrongly that the machine had worked.)

Alternatively, the accusation of luck confuses the original subject qua original subject with the original subject qua person waking up on Mars, as explained in section 6. Only the latter may be accused of making a lucky judgement, but only the former is our concern. (The original subject, as opposed to the person waking up on Mars, would never get it wrong.) Moreover, as we are supposing, the original subject employs the strategy in question precisely because he understands all this. Since a rough and ready definition of knowledge is correct judgement not arrived at by luck, it seems to me that this is not merely a case of judging correctly that the machine had worked, but one of knowing that the machine had worked. Employing the strategy in question, under the circumstances in question, the original subject would know, and thus succeed in verifying, that the machine had worked, should he awaken on Mars—which was the result advertised at the start:

I will argue that, if teleportation really works, then, under certain conditions to be specified, you can verify as much by trying a teleporter out for yourself. You’d find yourself waking up on Mars and you’d know that you were you and that the machine had worked.

A second and slightly more abstract way to see this is to notice that, under the circumstances in question, this whole hullabaloo of the potential existence of the copy with the false memory would just fall away: it would be as if there was no such complication. For, whenever the copy doesn’t care, the original subject will readily trust his memory upon waking on Mars, and there is nothing wrong with his memory, ex hypothesi. In a case like this, the potential existence of the copy is a red herring and may simply be ignored: the trumpeted hazard of the false memory would now be the copy’s problem alone.

In contrast, the original subject’s problem, all along, was only ever that of bringing himself to trust his memory upon waking up on Mars. After all, he knows that, whatever happens, he could never wake up in the body of his copy. In particular, should the machine fail, his copy would wake up on Mars, but he himself wouldn’t wake up in his copy’s body. So resolving at the outset to trust his memory should he so much as seem to find himself waking up on Mars was all along the strategy to go for, the only reason to dismiss it being the skeptic’s contention that the persona of the copy would “show up” and veto any inclination the original subject might have to follow through on his resolve. Once we see that the persona of the copy need not always show up, it should be clear that there are cases where the original subject would readily (and rightly) trust his memory should he awaken on Mars. These are the cases where he’d know that he was he and that the machine had worked.

A third way to see it is to suppose, over and above our current supposition that the copy does not care, that there is known to be an afterlife, clearly recognizable as such, as described in section 2. The skeptic should have no objection to this supposition, since his argument nowhere turns on the claim that there is no afterlife, or that it cannot be known. Imagine now that you are the original subject. Then, stepping into the machine, it should be clear to you that, if the machine failed, you’d know this at once, since you’d find yourself awakening in the afterlife. As such, if you found yourself awakening instead on Mars, you’d know at once that the machine had worked. Q.E.D. Of course, should your copy awaken on Mars, he’d be taken in by this very same reasoning, but, as before, that would be his problem, not yours. You yourself could never be led astray by the reasoning. Bringing in the afterlife makes this especially clear—a point one might otherwise easily lose sight of—since, as your copy awakens on Mars, taken in by the reasoning, there you are in the afterlife, realizing that the machine had failed.

Finally, we should consider the natural protest that, notwithstanding all that has been said, it would continue to remain indistinguishable to the original subject from the “inside” whether he was original or copy, as he awakens on Mars. So even if he manages to follow through on his resolve to “judge” that he was the original subject, as in the circumstances being imagined, one feels like saying that—come on—he wouldn’t really know that he was the original subject. He’d have no way of discerning this from the “inside” and would be bound to have recurring doubts as to his true identity. “He” might, after all, just be the copy, going through the motions of “judging” that he was the original subject, as per the assumption that the copy does not care. And so his confident surface declaration that he was the original subject, as though he had no doubt about the matter, would ultimately just be pluck, one feels like saying. He’d aver that he was the original subject but he wouldn’t really know this.

I believe that the correct way with this protest is simply to point out that the presence of such doubt does not in itself impugn the possession of the corresponding knowledge, since knowledge is not generally thought to require the absence of doubt. The traditional example is that of the nervous schoolboy who knows the answer to the test, having studied all semester, but cannot help doubting himself even as he writes the correct answer down. Or the chess prodigy who instinctively knows which pawn to push, even though, being too young to recognize her talent, doubts if she has pushed the right pawn. Likewise, the fact that the original subject may harbour doubts about his identity, despite his confident surface assertion that he is the original subject, does not itself rule out that he knows, after all, who he is. It depends on the details of the case, so let’s remind ourselves of what they are.

I’ve already laid out the positive case for saying that the original subject, should he awaken on Mars and follow through on his resolve to judge that he is the original subject, would know that he is the original subject. What remains, given my suggestion that this is an instance of knowledge in the face of doubt, is to “account” for the doubt in question. The schoolboy’s doubt may be attributed to his nerves; the chess prodigy’s to her inexperience. But why would our original subject doubt that he is the original subject if he knows that he is? Whence the doubt and why should he have it?

I believe that the answer, in this case, is that the doubt is just vestigial, an epistemological leftover from larger things afoot. The doubt, I submit, properly belongs to the person waking up on Mars, rather than to the original subject per se. Or, put another way, it properly belongs to the original subject qua person waking up on Mars, rather than to the original subject qua original subject. The person waking up on Mars, as we have seen, would not in general know whether he was original or copy, and this ignorance would generally be reflected in a mental conflict, or doubt, over his identity. (See section 7.) He would not know and would be quite right to doubt who he was.

The same cannot be said, however, of the original subject per se. The doubt, in particular, would serve no purpose on his part, since there is no possibility of him being the copy. That was indeed the point behind his resolve to judge that he was the original subject should he seem to find himself waking up on Mars. The point was to ignore any doubt over his identity that he might have upon waking, since there would be no possibility of him being the copy. But why should he have the doubt if it was inconsequential in this way? This was the question. Well, simply because the person waking up on Mars would rightly have it, and he happens, in this case, to be the person waking up on Mars. So he cannot escape having the doubt. More precisely, as explained in section 7, the doubt in question is a factual property of the person waking up on Mars, as opposed to the ignorance, which is a modal property. As such, the original subject per se will automatically inherit the doubt, whereas, depending on the case, he may or may not inherit the ignorance. In a case where he does not inherit the ignorance, as I am contending of the present one, the doubt will merely be vestigial, in the sense just explained, and this is what I take to be happening here. The doubt in the present case is just a residual echo of no significance, a gratuitous epistemological appendix.

More generally, indeed, someone who takes the doubt in question to show that the original subject does not really know who he is, would appear to be thinking of the original subject as the person waking up on Mars. The thinking is that, well, he’s right to say that he is the original subject, but, as his own doubt testifies, he could just as easily be the copy; so he doesn’t really know that he is the original subject. This way of thinking shows that “he” is being conceived of as the person waking up on Mars, rather than as the original subject per se. In particular, should “he” turn out somehow to be the copy, a triumphant, “Told you so,” would ensue. This attitude would not make sense if “he” was being conceived of as the original subject per se, since it would then be plain that he could never be the copy. The point, as before, is that it’s only the original subject qua person waking up on Mars who does not know, and who rightly doubts, who he is. The original subject qua original subject, in contrast, only vestigially doubts who he is. Apart from this vestigial doubt, he seems to me to pass every other test for knowing that he is the original subject. So I am persuaded that this is a case of knowing in the face of doubt.

Admittedly, this may not be quite the image that one associates with someone who has succeeded in verifying the reality of teleportation. One imagines, rather, the original subject waking up on Mars with something like Archimedean certainty: “Good Lord, the thing worked!”

Instead, we have something more of a guarded confidence that the machine had worked laced with lingering doubt. This takes little away from the result though. You don’t even have to call this ‘knowledge’ if you don’t want to. The important thing is that the difference from the skeptic’s position remains stark. According to the skeptic, someone (anyone) who tries out the machine at risk to his own life, and survives, would be in no better position to judge whether the contraption works than if he had not tried it out at all. So trying it out to find out if it works is a waste of time: all risk and no possibility of reward. This view seems to me to be incorrect. It seems to me that, whenever the copy doesn’t care, the original subject, should he try out the machine and survive, and should he have his wits about him, would be rightfully emboldened thenceforth to keep on using the machine—to his personal advantage, we may presume, e.g., to repeatedly and safely get to Mars and back for work. In a case like this, if someone was willing to risk his life to see if the machine did indeed take him to Mars, the experiment would have my blessing. The skeptic, in contrast, would advise him against doing anything so foolish, consigning him forever to use the old method, a space-ship journey taking several weeks.