Two people can, at the same time, have qualitatively identical experiences. For example, I experience a pain at a given time, and you experience a pain that feels exactly like my pain at the same time. The natural view is that in these cases, although our experiences are qualitatively identical, they are numerically distinct. So in the above case, there are two pains between us, not one.

In this post I’ll argue that if dualism or idealism is true, then this natural view is false.¹ If dualism or idealism is true, then there is only one pain between us in the above case. More generally, if dualism or idealism is true, then whenever two people, at the same time, have qualitatively identical experiences, then their experiences are numerically identical. I call this view ‘the identity of indiscernible experiences’ (IIE). Since I think IIE is false, I take this to be a reductio on dualism and idealism.

My argument relies on a weak version of the identity of indiscernibles. To get a sense of why the version I’ll rely on is plausibly modest, it will help to start with an implausibly strong version of the principle. This implausibly strong version says that no two things are qualitatively identical. There are well-known counterexamples to this principle. For example, there could be two qualitatively identical billiard balls that are, say, a meter apart from each other.

OK, but how is it possible for the balls to differ numerically despite being qualitatively identical? The obvious answer is that they differ in spatial location. If they were in the same spatial location, then it seems right to conclude that there is just one ball, not two. This suggests a weakened version of the identity of indiscernibles which avoids this sort of counterexample, namely: no two things are qualitatively identical at the same time, unless they differ in spatial location.

Now consider experiences. Suppose that two are, at the same time, qualitatively identical. Can we say the same thing about them that we said about the qualitatively identical billiard balls—that they differ numerically in virtue of having different spatial locations? Not if we’re dualists or idealists, who don’t think that experiences have spatial locations.² So, given dualism or idealism, even the weak version of the identity of indiscernibles is enough to entail that the experiences are numerically identical.

We can formalize this argument as follows:

1. Non-locality of Experiences: No experience is located in space.
2. Weak Identity of Indiscernibles: For all x and y, if x and y are qualitatively identical at the same time and do not differ in spatial location, then x = y.
3. Therefore, IIE: For all experiences x and y, if x and y are qualitatively identical at the same time, then x = y.

The argument is valid. Dualists and idealists are committed to (1), and (2) seems plausibly modest. Again, I’m not endorsing this argument. I’m just saying that if dualism or idealism is true, then it is sound.

Objections and replies

Objection: You claim that dualists and idealists are committed to (1), but I’ve never heard a dualist or idealist endorse it. I’m pretty sure that even they would agree that there can be a pain in, say, your foot. And that’s inconsistent with (1).

Reply: Not really. When we say that there is a pain in your foot, we mean that you are experiencing a pain that represents a location in your foot. But that doesn’t mean that the pain itself is located in your foot, or anywhere else. Similarly, a map can represent Toronto without being located in Toronto. In fact, even physicalists should say this.³ If your foot pain is a brain state, then it is located in your head even though it represents a location in your foot.

Objection: Premise (2) is false. Two qualitatively identical experiences can differ in virtue of having different causal properties or histories, even if they don’t differ in spatial location. For example, my pain was caused by me stubbing my toe, while your pain was caused by you stubbing your toe. And my pain causes me to be cranky, while your pain causes your to be cranky.

Reply: Two things can’t differ simply in having different causal properties or histories. For example, suppose that one ball moved because you pushed it, while another ball moved because I pushed it. Or suppose that one ball broke your window, while another ball broke my window. The different causal histories of the balls are possible only because the balls weren’t always in the same place at the same time.

Objection: Premise (2) is false. Two qualitatively identical experiences can differ in virtue of having different owners, even if they don’t differ in spatial location. My pain is mine, your pain is yours.

Reply 1: Imagine someone saying the same thing about what are supposedly two qualitatively identical billiard balls that don’t differ in spatial location: “Despite appearances, there are two balls here. The difference between them is that one ball is mine and the other ball is yours.” That would be silly. Two things can’t differ simply in having different owners.

Reply 2: If you insist that two experiences can differ simply in having different owners, that this is a brute fact about them, then you are assuming non-reductionism about persons. This admittedly a popular view among dualists and idealists. So I need to qualify my position. It’s not that dualism or idealism alone entails IIE. It’s dualism or idealism together with reductionism about persons. So what I really have a reductio on is (dualism & reductionism) and (idealism & reductionism).

Objection: What’s so bad about IIE, anyway?

Reply 1: Compare the following worlds:

• World 1: A million people are experiencing qualitatively identical extreme pains. It’s the worst pain imaginable.
• World 2: One unlucky person is experiencing the extreme pain that the people in world 1 are experiencing. The other 999,999 people are experiencing barely-noticeable minor pains.

Which is worse? Obviously world 1. But if IIE is true, then world 2 is worse than world 1, since there’s more pain in it. After all, if IIE is true, then all of the qualitatively identical pains in world 1 are numerically identical. So it would be pointless to completely relieve the pain of anyone in world 1 unless you completely relieve the pain of everyone, since otherwise the total amount of pain will remain the same. And it could even be bad to partially relieve the pain of anyone, since you might end up increasing the total amount of pain in the world.

Reply 2: Suppose that you and I are, at the same time, experiencing qualitatively identical pains: I stubbed my toe, while you stubbed your toe. If IIE is true, then there is just one pain between us. So if IIE is true, then my pain has the same cause as yours, since everything has the same cause as itself. But my pain wasn’t caused by you stubbing your toe.

Objection: One lesson you could take from IIE is that something really weird happens when two people have qualitatively identical experiences at the same time. But a different lesson you could take from IIE is that two people never have qualitatively identical experiences at the same time. And that doesn’t seem so strange. After all, even if my pain feels extremely similar to your pain, there might always be some subtle difference between them.

Reply 1: Leibniz had an a priori argument that no two leaves are exactly the same shape. Most people think that there must be something wrong with that argument, because that’s not the sort of thing you can know a priori. I think the same goes for experiences as goes for leaves. Whether two people ever have qualitatively identical experiences at the same time is not the sort of thing you can know a priori. So at the very least, those who accept IIE are hostage to the possibility that some people do have qualitatively identical experiences at the same time.

Reply 2: Even if, as it turns out, no two people ever actually have qualitatively identical experiences at the same time, it is at least physically possible for this to happen. And this is enough to show that IIE is false. For example, it is physically possible for there to be a complete physical duplicate of me on the other side of the universe who is always experiencing what I am experiencing. If IIE is true, then my experiences have the same causes his experiences, since our experiences are numerically identical. But it is physically impossible for my experiences to be caused by something that happens to someone on the other side of the universe.⁴