Nagel introduces talk about subjective experience as talk about what it is like for something to have a certain mental state. In his words,
fundamentally an organism has conscious mental states if and only if there is something it is like to be that organism—something it is like for the organism.
Nagel thinks that we won’t be in a position to assess the reduction of mental things to physical things until we class="enumerate-enumitem">To say that facts about conscious experience are essentially subjective is to say that a particular perspective figures in the fact as a part.
Nagel himself claims that (a) is not the right way to understand his claim (see footnote 8). It will be interesting to see why, in light of his general argument. As long as we have a clear grasp of what it takes for conscious experience to be reducible to entities or properties to be posited by science, it could be that, as a matter of fact, the reduction can be effected, even though we may never come to known this. But this is not Nagel’s point.
According to Nagel, scientific reduction is concerned with obtaining greater objectivity. He says:
We appear to be faced with a general difficulty about psychophysical reduction. In other areas the process of reduction is a move in the direction of greater objectivity, toward a more accurate view of the real nature of things... The less it depends on a specifically human viewpoint, the more objective is our description [...] Experience itself, however, does not seem to fit the pattern... Certainly it appears unlikely that we will get closer to the real nature of human experience by leaving behind the particularity of our human point of view... (p. 223)
The point is that there is something like a conceptual gap between the kind of concepts that we use when we engage in scientific explanations or descriptions of facts, and the kind of concepts that we can fruitfully use to characterize conscious experience.
In particular, concepts of the second kind can be grasped only by someone who has the relevant kind of experiences. Imagine the predicament you would find yourself in when trying to explain a blind person what a red apple looks like. Perhaps you’ll have no trouble when describing features of the apple such as its shape, but once you get to talk about color, it’s hard to see what one could say.
This is the kind of predicament that a bat may find himself in if he was trying to explain to us what it is like to know the location of things by means of a sonar. It would not do if he said “it’s just, you know, sonar-y”, or if he invited us to go into a submarine and use its sonar to determine our position. In order to even be able to grasp the concepts required to understand the facts related to the bat’s experience, we would need to be able to have bat-experiences ourselves.
But again, the point is not just that it’s impossible for us to share the bat’s perspective. Rather, it is that grasping facts about conscious experiences necessarily requires a particular perspective, and perspectives play no role in scientific explanations. Moreover, it is part of the aims of science to eliminate perspectives in favor of greater objectivity.
The idea is that the concepts of science are by design incapable of allowing us to grasp facts from a subjective perspective, as they would have to allow if they were to afford us an understanding of conscious experience.
Notice that, if this is an accurate reconstruction of Nagel’s argument, it doesn’t show that conscious experience is metaphysically irreducible to physical things. Instead, it tries to press the point that we don’t have an accurate understanding of what such a reduction would even imply or require. Because the concepts that are useful for describing conscious experience are essentially subjective, and the concepts we use in science are essentially objective, it’s just hard to see what a reduction would amount to.
Nagel doesn’t endorse reading (c), but examining is useful to see what his argument doesn’t show. If Nagel endorsed (c), his argument could conclude that the mental is irreducible to the physical, as long as we take perspectives themselves to be irreducible to physical things. But Nagel doesn’t even talk about this.