 A different line of argument (Horgan 1994; Loar 1990; Lycan 1990; Hill 1997, Hill and McLaughlin 1999, Balog 1999) maintains that, while generally the conceivability of a scenario entails its possibility, scenarios involving zombies stand as important exceptions. The difference is that the phenomenal, or “what it's like”, concepts used to describe the properties of experience that we conceive zombies to lack are significantly different from the third-personal, discursive concepts of our common sense and scientific theories such as mass, force or charge; they comprise a special class of non-discursive, first-personal, perspectival representations of those properties. Whereas conceptually independent third-personal concepts x and y may reasonably be taken to express metaphysically independent properties, or modes of presentation, no such metaphysical conclusions can be drawn when one of the concepts in question is third-personal and the other is phenomenal, since these concepts may merely be picking out the same properties in different ways. Thus, the conceivability of zombies, dependent as it is on our use of phenomenal concepts, provides no evidence of their metaphysical possibility.