Meet Mary the Super-scientist. She is the world’s foremost authority on human vision, according to a famous thought experiment by Frank Jackson. We’ll deal with that in due course. For present purposes, she is also the leading expert on transporter technology and the designer of the transporter aboard the Enterprise.
Now, imagine that you are James T. Kirk, Captain of the Starship Enterprise. You have an important mission requiring you to transport down to the surface of the planet the Enterprise is orbiting. Of course, you’ve been transported many times. It’s routine and entirely safe. What goes on down on the planet will be perilous, but not stepping into the transporter. You think it will be you yourself—token identity—who comes out on the other side, down on the planet.
Today, Mary is aboard ship, studying the transporter mechanism. You strike up a conversation about the technology. She has something surprising to say:
Actually Captain, the body that comes out the other end and arrives on the planet is only type identical to you, not token identical. You—the guy standing right here—are destroyed by the process. But don’t worry. The copy of you will be just like you. He will be your functional equivalent. The crew will notice no difference at all. Of course, when the copy of you beams back up to the Enterprise, that copy will be destroyed. What appears back on the Enterprise will be a copy of a copy, ready to assume command and fully capable. That second copy will think he is the Kirk that has been around all along. The very first Kirk—the one whose childhood you remember— was destroyed a long time ago, the first time he stepped into the transporter. You— the guy I’m talking to right now— is a 133rd copy of the original, who is long gone.
Now, Captain, if you’ll just step into the transporter, we can get on with the mission. Oh, and… well… good-bye.
Are you stepping into the transporter? Surely you are going to hesitate, at least. You’ve got a job to do. Your mission requires you to step into the transporter. But if Mary is right— and she knows more about the transporter than anyone—your existence will end in a few seconds. This is the most important decision of your life and possibly your last.
A few thoughts flash through your mind. “I’m a copy of a copy, of a copy, etc. Does that matter? Right now, I am a fully functioning biological entity with my own thoughts and feelings. I am not an android or a fake. I have human rights. My ongoing existence matters to me just as much as if I’d never been in a transporter. I am Captain Kirk. There is no other. There’s something it’s like to be me. Will that something end in a few seconds?”
And you wonder how Mary can know that her transporter spits out only type identity, not token identity. The Kirk who comes out looks just like the one who went in, acts just like him, has exactly the same ability to command the Enterprise. Other than location, the new Kirk is indistinguishable from the old; that’s what makes him type identical. He will claim to be the old.
What is the test— scientific or otherwise— that determines whether he is the same token or not? For the Enterprise crew, what difference does it make whether the Kirk’s identity is token or only type? They can’t tell the difference. For anyone other than Kirk, there is no difference. Put a book through the transporter. Out comes a type identical book. Same token? Who cares? It makes no difference. Maybe it’s a meaningless question. There is no “what it’s like to be” a book from the inside.
This, then, is the paradox. For Kirk’s crew, it makes no difference whether the transporter spits out the token identical guy, or merely someone type identical. But for Kirk, it means everything. From the outside, who cares. From the inside, it’s the only thing that matters.
Update: see my comments on Am I a Type or a Token, guest post by Henry Shevlin 8/24/17 at Eric Schwitzgebel’s blog The Splintered Mind http://schwitzsplinters.blogspot.com/2017/08/am-i-type-or-token-guest-post-by-henry.html