ABSTRACT
According to the hypothesis explored in this chapter, the evolution of conscious sensation started with the most primitive sensory receptors-the unmyelinated, high threshold, free nerve endings known as heat nociceptors, pressure nociceptors, and chemical nociceptors, which, when innervated by harmful heat or harmful pressure or harmful chemicals, manifest the irreducible subjective quality of pain. Throughout the course of evolutionary history, on this account, some heat nociceptors evolved into temperature receptors manifesting subjective qualities of pain-free heat, and others evolved into retinal receptors that respond to airborne or waterborne heat (i.e., light) and that manifest subjective qualities like visual brightness. Similarly, some pressure nociceptors evolved into touch receptors possessing subjective qualities of pain-free pressure, and others evolved into cochlear receptors that are responsive to airborne or waterborne pressure (i.e., sound) and that are paralleled by subjective qualities like auditory loud - ness. And likewise, some chemical nociceptors evolved into gustatory chemical
receptors with pain-free qualities of taste, and others evolved into olfactory receptors that respond to airborne chemicals and manifest subjective qualities of smell. Consideration of this evolutionary history must begin, therefore, with consideration of the subjective quality of pain.
