6 ideas
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 itself. This subjective character is the defining feature of experience, and no functional or causal description of behavior captures it, because such descriptions would fit equally well a system with no experience at all.
Bats perceive the world through echolocation, sensing distance, size, shape, and texture from reflected high-frequency shrieks. This sense works nothing like any sense humans have. Imagining ourselves hanging upside down or flying at dusk only tells us what it would be like for us to behave like a bat, not what it is like for a bat to be a bat.
Physical science becomes objective by stripping away features that depend on a particular observer's perspective, such as how things look or feel. Applied to experience, this method discards the very thing to be explained, because the subjective character of experience is essentially tied to a single point of view. So the usual path of reduction, from appearance to underlying reality, has nothing to move toward once the appearance is itself the target.
There can be facts that human beings could never represent or grasp, because our concepts are built from our own kind of experience. The facts about what bat experience is like are real even if we can never formulate them. Realism about the mental does not depend on whether minds like ours can express it.
Nagel does not argue that physicalism is false. He argues that we currently cannot understand how it could be true, much as a pre-Socratic could not have understood that matter is energy.
Nagel proposes building a new vocabulary that describes the structural features of experience without relying on empathy or imagination. The test is whether it could convey to a being who lacks a given experience something about its nature, for example explaining sight to someone blind from birth. Such a phenomenology might eventually connect subjective experience to physical theory.