Pain is an experience and brain activity is a material event. Those are two different things. Brain activity correlates with pain in that they happen at the same time, but it’s a category mistake to say they are identical to one another. You can’t objectively measure pain. What you can measure is brain activity and then make an assumption that similar brain indicators reflect similar pain experiences. But ultimately that’s an assumption we can’t test because we can’t share each other’s experiences directly. We can only infer by comparing others’ pain to our own. You don’t need a neural scanner to do this. Often we can see people’s pain on their faces. And in an everyday sense we can say “I see your pain”. Looking at electrical impulses is a more technical method of doing the same thing. But of course we know that when we say we can see someone’s pain on their face, we don’t literally mean that there face is identical to the experience of pain. Electrical impulses in the brain are a more detailed way of looking at pain than looking at someone’s face but essentially the same kind of inference is being drawn between a physical representation and an experience. So it’s a category mistake to say that a brain scan allows us to “see” pain just as it would be a mistake to say that pain and a facial expression are the same thing.
Why is this important? Because experience is different from a physical event it has properties of its own. Most importantly we can’t share each other’s experience. For example I can tell you I broke my leg and, how big the break was, how long it will take to heal and so on. All those things can be measured and shared. What I can’t share directly is the actual experience of having broken my leg. If you’ve broken your leg too, we can compare descriptions of the experience and infer a lot in common. But again, what we can’t do is share each other’s experience or know how the other person experienced the pain itself.
David Chalmers has a good example that demonstrates the different between physical properties that can be measured and experiences that can’t be quantified. He gives the example of a scientist who studies colour and knows everything there is to know about colour. They know the wavelengths, how light refracts, how the eye works, how the brain processes colour. They know absolutely everything there is to know about colour. Yet if they can’t see colour there is still one thing about colour that they don’t know: namely the experience of seeing colour. Because no matter how exactly you can measure a thing, measurement is not identical to the experience of the thing.