There are certain facts that can be learned only “from one point of view” — from a subjective perspective. So argues Thomas Nagel in his landmark 1974 paper “What Is It Like to Be a Bat?” Such subjective facts are what philosophers call raw feels, phenomenal properties, or qualia (from Latin for “of what kind”). They are facts about sensation, or what it is like to have some experience. Qualia are the basic units of experience: redness, pain, the wetness of water, the taste of sweetness. Nagel argues that qualia define consciousness itself: “fundamentally an organism has conscious mental states if and only if there is something that it is like to be that organism.”

The general approach of dualists is to demonstrate that qualia are an additional set of properties of the world, over and above its physical traits. The general approach of physicalists is to deny that there is anything extra about qualia. To the physicalist, it is only a mystical belief in the soul or in human uniqueness, abetted by the current incompletion of science, that creates the illusion that qualia cannot be accounted for by biology, chemistry, and physics. But just as we now know that the wet stuff in rivers and the fluffy stuff in the sky are all the same molecule made of hydrogen and oxygen, so too will sensations someday be shown to be the same thing as certain physical events.

With this sketch in mind, we can consider the most important lines of criticism of the knowledge argument.