A single strand of argument with unfounded conclusion

Key (2016) describes the neural system involved in human pain experience in an excellent fashion but then suggests that only that complete system can generate the experience of pain. Thus animals without all components will not feel pain. This argument has been refuted in the past by analogy to vision where it is clear that a broad range of taxa, vertebrate and invertebrate, have good visual abilities albeit with completely different central nervous systems and receptors. This known counterargument to Key’s main idea is not mentioned in the target article. Further criteria that might indicate pain and studies examining these criteria in fish and other animals also get scant attention. The function of pain in fitness terms is not considered, yet that might provide clues as to when and how often pain might have evolved