Peers and twins

I just realized something that I should have known earlier. Suppose I have a doppelganger who is just like me and goes wherever I go—by magic, he can occupy a space that I occupy—and who always sees exactly what I see and who happened always to judge and decide just as I do. What I’ve just realized is that the doppelganger is not my epistemic peer, even though he is just like me.

He is not my peer because he has evidence that I do not and I have evidence that he does not. For I know what experiences I have and he knows what experiences he has. But even though my experiences are just like his, they are not numerically the same experiences. When he sees, it is through his eyes and when I see, it is through my eyes.

Suppose that on the basis of a perception of a distant object that looked like a dog I formed a credence of 0.98 that the object is a dog, and my doppelganger did the same thing. And suppose that suddenly a telepathic opportunity opens up and we each learn about the other’s existence and credences.

Then our credences that the distant object is a dog will go up slightly, because we will each have learned that someone else’s experiences matched up with ours. Given that the other person in this case is just like me, this doesn’t give me much new information. It is very likely that someone just like me looking in the same direction would see things the same way. But it is not certain. After all, my perception could still be due to a random error in my eyes. So could my doppelganger’s be. But the fact that our perceptions match up rules makes it implausible to suppose the random error hypothesis, and hence it raises the credence that the object really is a dog. Let’s say our credences will go up to 0.985.

Now suppose that instead this is a case of slight disagreement: His credence that there is a dog there is 0.978 and mine is 0.980, this being the first time we deviate in our whole lives. I think the closeness to me of the other’s judgment is still evidence of correctness. So I think my credence, and his as well, should still go up. Maybe not to 0.985, but maybe 0.983.