An email friend writes to complain that Thomas Nagel’s prose in Mind & Cosmos sometimes becomes difficult as the book goes along. The correspondent points, for example, to this passage:

It would be a mistake to try to find a common denominator such as pleasure and pain to accommodate in a single realist conception the diverse values that are generated by all the actual, not to mention imaginable, forms of life. Instead, value must be seen as pluralistic: The domain of real value, if there is such a thing, is as rich and complex as the variety of forms of life, or at least of conscious life. Just as most of these lives are only dimly accessible….

What does that mean?
Nagel thinks there are such things as brute moral facts (i.e., facts that don’t reduce to, say, certain brain states giving rise to the “illusion” of personal satisfaction), that that is what moral judgment is often about, and that such judgment is at least capable of being true and false in relation to such facts.
Most people are moral realists in this sense, and so is Nagel, though he calls the view value realism instead of moral realism. Nagel is a value realist who finds value realism at least seemingly incompatible with Neo-Darwinian materialism, and so much the worse for Neo-Darwinian materialism, which is one of the arguments he makes against that view of the world in Mind & Cosmos.
Nagel’s values, his goods, do not exist objectively, however. They are dependent on subjects for existence, each bound up with a certain type of life. So for Nagel there may be as many versions of the good life as there are living, conscious creatures, and these goods do not all simply reduce to, say, undifferentiated pleasure or plain, as a Benthamite utilitarian might insist.
I’ll explain further with an example.
I don’t know what it is like to be a bat, to “see” via echolocation, for one. I also don’t know what is good for a bat to do or to experience, not really. I know what pain is, but I don’t know that pain is the same for a bat as it is for me, that it is felt by a bat as I feel it, etc.; hence what Nagel says about lives being only “dimly accessible.” But I take it that there is such a thing as what is good for a bat, for bats in general, or at least for this bat, the one before me who seems to go about satisfying desires and otherwise acting purposefully as if according to its own sense of what is good for it to do.
Anyway, I think the key to understanding Mind & Cosmos is getting a handle on other stuff Nagel has written. Throughout his body of work Nagel is consistently sympathetic to a more mysterious view of the objective world and subjective experience than typically allowed by Neo-Darwinian materialism, and for good reason.