Another view is that only humans can be persons because only humans have the ability to use language and reason in abstract and sophisticated ways. Kiko and Tommy can do many impressive things, including communicate with others and pursue goals in a creative, intelligent manner. But they cannot do these things in the same kinds of ways that many humans can.

However, this view of personhood is unacceptably exclusionary. We all lack the ability to use language and reason in abstract and sophisticated ways early in life, some of us lose these abilities later in life, and some of us never develop them. Yet while humans might not have moral or legal duties when we lack these abilities, we can clearly still have moral and legal rights. This is why many judges and legal experts now rightly reject this exclusionary view of personhood as fundamentally at odds with contemporary standards of human rights.

But now suppose we accept a more inclusive view of personhood, according to which humans are persons because we have some or all of the features mentioned before: conscious experience, emotionality, a sense of self or bonds of care or interdependence. This view is more plausible than the opposing view, in part because it includes all humans within the scope of personhood. At the same time, it includes some nonhumans too. For example, Kiko and Tommy are conscious, emotional, intelligent, social beings whose lives are deeply entangled with our own, their current state of isolation notwithstanding. As a result, they count as persons on any view inclusive enough to meet contemporary standards of human rights.

The idea of nonhuman personhood does raise difficult questions. One question is which rights nonhumans can have. For instance, if Kiko and Tommy can have the right to liberty, can they also have the right to property? What about the right to free expression or association, or the right to political representation or participation?

Another question is which nonhumans can have rights. For instance, if Kiko and Tommy can have rights, can bonobos and gorillas have rights too? What about cats, dogs and fish? What about chickens, cows and pigs? What about ants or sophisticated artificial intelligence programs?

These questions are unsettling. They are also reasonable to ask. After all, we might think that we need to draw the line somewhere. So if we decide not to draw the line at species membership — if we decide to accept that at least some nonhumans can have at least some rights — then it is not immediately clear where to draw it instead, or even, on reflection, whether to draw this particular kind of line at all.