“All vertebrates have similar physiological pain receptors .” This might be true, but doesn’t really decide the issue. Pain receptors are in the peripheral nervous system, but pain is mediated by the brain. In humans, there are many cases where we have activation in pain receptors without consciously feeling pain (soldiers during battle), and similarly there are many cases where we consciously feel pain without any activation in pain receptors (phantom limb pain, chronic pains). Also, it’s worth noting that the link you provide refers to a table in Gary Varner’s 1998 book. I’m a former grad student of his, and I don’t think even then he would have said the argument for pain is “just as strong,” in other groups as in mammals, but he does have an updated table in his 2012 book “Personhood, Ethics, and Animal Cognition” that incorporates some of the points I mentioned above. And, in particular, he writes (p. 123) that ”...the argument by analogy for pain in non-human animals is strongest in the case of our fellow mammals, and weaker for all of the other taxa.” He does still conclude that the best place to draw the line is between vertebrates and invertebrates, but he’s also not saying the arguments are equally strong in al cases.
All vertebrates have similar physiological pain receptors (http://philosophyforprogrammers.blogspot.com/2010/11/who-feels-pain.html), and it seems like there’s only a (possible) significant difference in ability to feel pain when you get down to invertebrates, like insects.
“All vertebrates have similar physiological pain receptors .” This might be true, but doesn’t really decide the issue. Pain receptors are in the peripheral nervous system, but pain is mediated by the brain. In humans, there are many cases where we have activation in pain receptors without consciously feeling pain (soldiers during battle), and similarly there are many cases where we consciously feel pain without any activation in pain receptors (phantom limb pain, chronic pains). Also, it’s worth noting that the link you provide refers to a table in Gary Varner’s 1998 book. I’m a former grad student of his, and I don’t think even then he would have said the argument for pain is “just as strong,” in other groups as in mammals, but he does have an updated table in his 2012 book “Personhood, Ethics, and Animal Cognition” that incorporates some of the points I mentioned above. And, in particular, he writes (p. 123) that ”...the argument by analogy for pain in non-human animals is strongest in the case of our fellow mammals, and weaker for all of the other taxa.” He does still conclude that the best place to draw the line is between vertebrates and invertebrates, but he’s also not saying the arguments are equally strong in al cases.