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To address this problem when assessing the capacity of other species to experience pain, argument-by-analogy is used.
This is based on the principle that if an animal responds to a stimulus in a similar way to ourselves, it is likely to have had an analogous experience.
At the turn of the century, studies were published showing that arthritic rats self-select analgesic opiates.
and in 2015, it was reported in the science journal Pain, that several mammalian species (rat, mouse, rabbit, cat and horse) adopt a facial expression in response to a noxious stimulus that is consistent with the expression of pain in humans.
Nociception: The reflex arc of a dog with a pin in her paw.
Note there is no communication to the brain, but the paw is withdrawn by nervous impulses generated by the spinal cord.
Sometimes a distinction is made between "physical pain" and "emotional" or "psychological pain".
Arguing by analogy, Varner claims that any animal which exhibits the properties listed in the table could be said to experience pain.
Initially, this was based around theoretical and philosophical argument, but more recently has turned to scientific investigation.
The idea that non-human animals might not feel pain goes back to the 17th-century French philosopher, René Descartes, who argued that animals do not experience pain and suffering because they lack consciousness. " Peter Singer, a bioethicist and author of Animal Liberation published in 1975, suggested that consciousness is not necessarily the key issue: just because animals have smaller brains, or are ‘less conscious’ than humans, does not mean that they are not capable of feeling pain. federal laws regulating pain relief for animals, writes that researchers remained unsure into the 1980s as to whether animals experience pain, and veterinarians trained in the U. before 1989 were taught to simply ignore animal pain.
It has been argued that only primates and humans can feel "emotional pain", because they are the only animals that have a neocortex – a part of the brain's cortex considered to be the "thinking area".
However, research has provided evidence that monkeys, dogs, cats and birds can show signs of emotional pain and display behaviours associated with depression during or after a painful experience, specifically, a lack of motivation, lethargy, anorexia, and unresponsiveness to other animals.