Qualia theory

Qualia (singular form: quale) are defined as individual instances of subjective, conscious experience. The term qualia derives from the Latin neuter plural form (qualia) of the Latin adjective "quālis" meaning "of what sort" or "of what kind" in a specific instance, such as "what it is like to taste a specific apple, this particular apple now".

Examples of qualia include the perceived sensation of pain of a headache, the taste of wine, as well as the redness of an evening sky. As qualitative characters of sensation, qualia stand in contrast to propositional attitudes, where the focus is on beliefs about experience rather than what it is directly like to be experiencing.

A similar argument holds that it is conceivable (or not inconceivable) that there could be physical duplicates of people, called "philosophical zombies", without any qualia at all. These "zombies" would demonstrate outward behavior precisely similar to that of a normal human, but would not have a subjective phenomenology. It is worth noting that a necessary condition for the possibility of philosophical zombies is that there be no specific part or parts of the brain that directly give rise to qualia: The zombie can only exist if subjective consciousness is causally separate from the physical brain.