What does it mean to be human? Or, putting the point a bit more precisely, what are we saying about others when we describe them as human? Answering this question is not as straightforward as it might appear. Minimally to be human is to be one of us, but this begs the question of the class of creatures to which “us” refers.

Can’t we turn to science for an answer? Not really. Some paleoanthropologists identify the category of the human with the species Homo sapiens, others equate it with the whole genus Homo, some restrict it to the subspecies Homo sapiens sapiens, and a few take it to encompass the entire hominin lineage. These differences of opinion are not due to a scarcity of evidence. They are due to the complete absence of evidence − or, to put the point with greater clarity, the absence of any conception of what sort of evidence can settle the question of which group or groups of primates should be counted as human. Biologists aren’t equipped to tell us whether an organism is a human organism because “human” is a folk-category rather a scientific one. 

Some folk-categories correspond more or less precisely to scientific categories. To use a well-worn example, the folk-category “water” is coextensive with the scientific category “H2O”. In the philosophical jargon, water is said to be reducible to H2O, which means that H2O is nothing over and above water, and therefore any statement that is true of water is also true of H2O. But not every folk category is even approximately reducible to a scientific one. Consider the category “weed.” Weeds don’t have any biological properties that distinguish them from non-weeds. In fact, one could know everything there is to know biologically about a plant, but still not know that it is a weed. So, at least in this respect, being human is more like being a weed than it is like being water.  

If this sounds strange to you, it is probably because you are already committed to one or another conception of the human (for example, that all and only members of Homo sapiens are human). However, claims like “an animal is human only if it is a member of the species Homo sapiens” are stipulated rather than discovered. In deciding that all and only Homo sapiens are humans, one is expressing a preference about where the boundary separating humans from non-humans should be drawn, rather than discovering where such a boundary lays.  

If science can’t give us an account of the human, why not turn to the folk for an answer?  

Unfortunately, this strategy multiplies the problem rather than resolving it. When we look at how ordinary people have used the term “human” and its equivalents across cultures and throughout the span of history, we discover that often (maybe even typically) members of our species are explicitly excluded from the category of the human. It’s well-known that the Nazis considered Jews to be non-human creatures (Untermenschen), and somewhat less well-known that fifteenth century Spanish colonists took a similar stance towards the indigenous inhabitants of the Caribbean islands, as did North Americans toward enslaved Africans (my 2011 book Less Than Human: Why We Demean, Enslave, and Exterminate Others, gives many more examples). Another example is provided by the seemingly interminable debate about the moral permissibility of abortion, which almost always turns on the question of whether the embryo is a human being. 

At this point, it looks like the concept of the human is hopelessly confused. But looked at in the right way, it’s possible to discern a deeper order in the seeming chaos. The picture only seems chaotic if one assumes that “human” is supposed to designate a certain taxonomic category across the board (‘in every possible world’ as philosophers like to say). But if we think of it as an indexical expression – a term that gets its content from the context in which it is uttered—a very different picture emerges. 

Paradigmatic indexical terms include words like “now,” “here,” and “I.” Most words name exactly the same thing, irrespective of when, where, and by whom they are uttered. For instance, when anyone anywhere correctly uses the expression ‘the Eiffel Tower,’ they are naming one and the same architectural structure. In contrast, the word “now” names the moment at which the word is uttered, the word “here” names the place where it is uttered, and the word “I” names the person uttering it. If I am right, the word “human” works in much the same way that these words do. When we describe others as human, we are saying that they are members of our own kind or, more precisely, members of our own natural kind.  

What’s a natural kind? The best way to wrap one’s mind around the notion of natural kinds is to contrast them with artificial kinds. Airplane pilots are an artificial kind, as are Red Sox fans and residents of New Jersey, because they only exist in virtue of human linguistic and social practices, whereas natural kinds (for example, chemical elements and compounds, microphysical particles, and, more controversially, biological species) exist ‘out there’ in the world. Our concepts of natural are concepts that purport to correspond to the structural-fault-lines of a mind-independent world. In Plato’s vivid metaphor, they ‘cut nature at its joints.’ Weeds are an artificial kind, because they exist only in virtue of certain linguistic conventions and social practices, but pteridophyta (ferns) are a natural kind because, unlike weeds, their existence is insensitive to our linguistic conventions.

Philosophers distinguish the linguistic meaning of indexical expressions from their content. The content of an indexical is whatever it names. For example, if you were to say ‘I am here’, the word ‘here’ names the spot where you are sitting. Its linguistic meaning is ‘the place where I am when I utter the word “here”.’ If  ‘human’ means ‘my own natural kind,’ then referring to a being as human boils down to the assertion that the other is a member of the natural kind that the speaker believes herself to be. This goes a long way towards explaining why a statement of the form ‘x is human,’ in the mouth of a biologist might mean ‘x is a member of the species Homo sapiens’ while the very same statement in the mouth of a Nazi might mean ‘x is a member of the Aryan race.’ That's what it means to be human. 

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