Concerning the nature of time, Augustine famously wrote:
"What, then, is time? If no one ask of me, I know; if I wish to explain to him who asks, I know not." (Confessions, Bk XI, ch.XIV).
A recent article on personhood by bioethics writer Virginia Hughes discusses an analogous definition of personhood. Hughes draws upon the work of academics Martha Farah and Andrea Heberlein, who in 2007 argued that personhood "is a concept that everyone feels they understand but no one can satisfactorily define".
Hughes surveys the 'hard science' of personhood. Much of recent neuroscientific research attempts to explain our intuitions about the 'personhood' through reference to features of the brain that structure our experience of the world. It's a kind of curious hybrid of materialism and Kantianism.
Hughes discusses a number of neuroscientific discoveries in the past century. Structures like the fusiform face area and the superior temporal sulcus are activated when we are looking at faces or moving bodies. We also can't help but anthropomorphise inaniminate structures when they display animate characteristics ( consider the 'bullying triangle' from a famous 1940's experiment ). In addition, babies are able to process facial expressions at an extremely early stage, despite their scant real world experience. All this seems to suggest that personhood, rather than having some objective existence in the world, is a direct product of neurophysiological phenomena in the human brain.
However, even with all the scientific evidence describing the neurophysiological substrate of the concept 'personhood', there seems to be something left unexplained. As Farah and Heberlein note, science has been able to offer an objective definition of plants - organisms that get their energy through photosynthesis - but it hasn't be able to do the same for the category of personhood.
Some might pounce on the claim and use it to justify a bioethical human exceptionalism. Others may argue the opposite. If personhood is a flaky, undefinable concept, it loses its moral force in bioethical debates.
Whilst not directly engaging in this debate, Hughes suggests that personhood is the categorical foundation of human social life:
"Here's why I think the personhood notion so valuable. We are people. Our people-centric minds evolved for a reason (namely, our species depends on social interactions) and our people-centric minds dictate how our society works. So maybe personhood is not based in reality. It's the crux of our reality."
There are a number of assumptions here, some more controversial than others. Whilst Hughes provides a useful summary of the topic, the need for extended scholarly treatment of the topic is evident.