KNOWING SOMETHING WHEN YOU FEEL IT

All Nagel meant was that we all know what feeling is and what it means, and that we all know it when it is happening. Moreover, the only thing we feel is our own feelings, so those are the only feelings about which we have cartesian certainty. About the feelings of other creatures we can only guess. I'd have to be the other creature -- say, Nagel's bat -- in order to know for sure that it [i.e., I] feels, and also to know what it feels, i.e., what that feeling feels like. [It might feel quite different from anything I am currently able to feel, being me.]