When does personhood begin?

Personhood, properly understood, is not a species-specific concept; to believe otherwise is to commit the intellectual sin of speciesism, as well as ignore the raft of science-fiction shows as well as the work on AI that demonstrate the intellectual possibility (if not yet the physical actuality) of non-human persons.

That being said, not all humans are persons - fetuses, for example, are clearly not - rather, they are potential persons; beings which, if their developmental changes go well, will eventually become persons, but do not have such status yet. Analogously, my students are potential college graduates, but not actual college graduates; they have begun a process that, if all goes well and certain changes occur (like passing my class!), will result in their attaining the status of college graduate; but they do not have such status now, and may never do so, if things do not go well.

It is clear that the abilities which one must attain in order to become a person are mental; no body type or specific physical ability, like walking or eyesight, are required. An ability to pass the Turing Test - to convince an interlocutor that one could communicate indistinguishably from a normal human adult - seems sufficient, but perhaps not necessary. The kind of mental autonomy that we consider required for moral responsibility - for holding a person responsible for their actions - seems a better bet for a necessary and sufficient condition. Let's call that agency - the capacity for the rational exercise of free will.

Given the 'ought implies can' principle of ethics, along with the principle of alternative possibilities, a person can thus be defined as a being that has two or more courses of action open to them, and has rational control over which of the courses of action they pursue.

So when do humans make the transition from potential personhood to full fledged personhood? Under normal circumstances, at the age of two; and recent research has reinforced the finding that toddlers of that age have a social intelligence (of the sort required for such agency) missing from our most intelligent species relatives, chimps and orangutans.

So a normal neonate is not a person, but a normal two-year old has become one. One becomes a person (no matter what the species) when one becomes an agent. And although it is certainly conceptually possible to have members of many different species demonstrate agency, at the moment, only one species has convincingly demonstrated what it takes to be a person - Homo sapiens sapiens. (Although it is VERY interesting, to me anyway, to speculate on whether Neanderthal man had what it takes - I tend to believe (s)he did, and so two different species of persons co-existed for a while - perhaps until our species wiped them out, perhaps even eating them.)

So while I doubt very much that any extraterrestrial persons exist, it isn't because they are conceptually impossible - not at all. Rather, it's because I suspect that once persons evolve in any particular biosphere, they rapidly destroy themselves and the ecosystems required for their survival. In other words, I suspect many species across the Universe have attained personhood in the past - and none of them have lasted, or else we would be aware of them (or members of them!) now. It is the nature of intelligent civilizations to destroy themselves. Alas, that's another version of the Argument for Doomsday - from the Fermi Paradox. Hopefully I'll publish it in more detail soon... before it's too late!