What is changing, however, is the notion that personhood is not something that one is simply born into by virtue of their species, or something that's dependant on one's level of sociopolitical engagement. Rather, it's something that comes about by virtue of the presence of certain cognitive psychological, and emotional capacities.

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One of the better attempts to define a person came from the bioethicist Joseph Fletcher who presented a list of fifteen "positive propositions." His attributes included such things as self-awareness, self-control, a minimum level of intelligence, a sense of time (including a sense of the past and future), concern for others, curiosity, and so on.

Fletcher's list did not come without controversy. By virtue of his criteria, a person in a permanently vegetative state and with no brain activity would not be considered a person. And at the same time, certain nonhuman animals would have to be considered persons. Put another way, Fletcher's list meant that not all humans are persons, and not all persons are humans.

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The subsequent challenge facing scientists and bioethcists has been in proving that nonhuman animals have these capacities. But as the signatories of the Cambridge Declaration conceded, there is a tremendous amount of data emerging in support of the idea that animals are conscious to the degree that humans are.

"Convergent evidence indicates that non-human animals have the neuroanatomical, neurochemical, and neurophysiological substrates of conscious states along with the capacity to exhibit intentional behaviors," they wrote in the Declaration.

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Shook, on the other hand, takes great exception to the idea that personhood can be defined by ticking off a checklist of criteria.

"First, let's at least assume that assigning personhood to all species in the animal kingdom is not optimal," he says "Therefore, some criteria must be chosen which has both these properties: (a) these criteria select out only those "higher" species that seem sensible (for example, the criteria forbids insects but includes birds; or forbids birds but includes aardvarks; or forbids aardvarks but includes lions), and also (b) every human being possesses all these criteria — after all, its not a good result to leave some humans out of personhood."

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Next, Shook asks us to consider the factual trouble: "any criteria capable of including all humans will also include far too many nonhuman species. If comatose humans are in, then frogs surely are in. If six month old fetuses are in, then birds are in. If newborn babies are in, then gophers are in." Shook says the same problem arises for a similar category like 'rights-bearer.'

"Biting the bullet here and saying that only a subset of humans are really persons so that horses or foxes can be persons will arouse far more controversy than protecting animals from cruelty," he says. "Have animal rights activists intrigued by personhood yet agreed on exactly when a human fetus becomes a person in the womb, or perhaps their criteria for personhood suggest that the line is drawn six months after birth? At this stage, it instead seems wiser to think about how to use new science about sentience and suffering to arouse human compassion for many other species than to risk limiting personhood to fewer humans."

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It's what brains do

Marino, on the other hand, is not convinced that paternalistic rights will suffice. Moreover, her personal definition of personhood is far less strict than that that of Fletcher's and other personhood theorists.

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"My definition of personhood is very practical and very context driven," she says, "I don't see this as a black and white issue — it is more of an issue of bringing the existing evidence to bear on the efforts."

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She agrees that there is an overwhelming amount of scientific evidence indicating that many other species, such as cetaceans, elephants, great apes, and some birds, share those basic characteristics that define personhood in our own species. But she takes it even further than that.

"On a personal level, I view all other animals with a brain as persons, she says. "They are not human but they are other persons. But regarding the NhRP we are letting the science lead our legal definition."

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Marino doesn't believe that there is a clear-cut line in nature that differentiates conscious from nonconscious animals. "From a neuroanatomical point of view it is reasonable to accept the premise that all animals with a central nervous system are conscious," she says. "This is what brains do for a living — they provide a way for the animal to process information and respond appropriately, and that's true whether you are an aplysia or a chimpanzee."

She concedes that the empirical evidence tells us there are differences across species. "Some readily recognize themselves in mirrors, for instance, and others just don't get it," she says. But when we look at the complete set of data in the literature, she argues, it demonstrates that consciousness is a dimensional phenomenon.

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"Some animals may be capable of more complex and profound levels of awareness than others," she says, "But all are conscious."

Images: Shutterstock/Rahmo/Villiers Steyn/FineShine/VogueHouse/Vittorio Bruno/Ivan Cholakov/Miles Away Photography. NP/DOMINIQUE FAGET/AFP/Getty Images, AWF.