Finally, there is (5). What reason is there for thinking that is true? Well, we can compare it with a number of similar principles to get the general idea. In moral philosophy, there is a principle sometimes called Hume's law that says it is not possible to derive a normative judgement from a descriptive judgement. A normative judgement is a judgement about what is good or bad, right or wrong, and a descriptive judgement is basically anything else. Another way this is stated is that you cannot deduce an "ought" from an "is": you can't derive what ought to be the case solely on the basis of what is the case. This principle is almost universally acknowledged. And it is merely part of a more general pattern. For example, you can't derive a statement describing distances from any set of statements that don't describe distances. You can not derive a statement about colors from any set of non-color statements. You can't derive geometrical statements from non-geometrical ones. And generally, if you have an inference in which the conclusion talks about one thing and the premises talk about something else, the inference is invalid. In the same way, it is a conceptual truth that you cannot derive a mental description from a physical description. After all, just consider some physical concepts, such as spatial/geometrical properties, mass, force, and electric charge. Is it plausible that there is any way that these concepts could be used to explain what it feels like to be in pain? Say whatever you like about masses, positions, and forces of particles, you will not have ascribed any mental states to anything.
