well, as explained in the rest of the post and links: pain is a subjective experience and there is no objective means to measure it (to date) we can measure limited aspects of pain in a person however, the primary reason I marked "The question doesn't make sense" is that I don't believe the term "the quality of pain" makes sense as you don't state if the quality refers to either the level of pain, the psychological measurement of pain, the physiological measurement of pain or the impact said pain has on the subject or it's lifestyle. If you've included all of that then it's too vague of a statement and it should have been left to "pain" rather than adding an adverb "quality". So, IMHO, the term isn't specific enough unless you wish to argue a point of philosophy, which is different than a discussion from science. well, are you wanting a scientific theory of pain or a philosophical one? there seems to be an oft-used reference for the definition of pain that I can't find a copy of "Pain: definition and properties of the unit for sensory reception" but there are other definitions, such as the 1952 study by J. Allan Walters "Pain and Suffering" the medical dictionary I have is similar to the free medical dictionary online so, even in the scientific literature you can see that there are subjective qualities of pain and an attempt to measure it. The studies are ongoing, but here is a decent read called "New Perspectives on the Definition of Pain" so from what I can tell, pain is not only a subjective percieved sensation but it is actual stimulus, or overstimulus, of specific type neurons for the purpose of eliciting emotional or physical responses for actual or potential injury to the body. all of the above links and references, and in my previous post, discuss the scientific theories of pain and their foundation, and some include that there is a subjective aspect to pain, but I don't think there is a singular specific all-encompassing Theory as there isn't even a way to objectively measure pain (to date).