You are human, and so am I. We can both agree on that. But what does it mean to say of someone that they are human?  

Many people believe that science has already answered this question. They think that it’s been scientifically proven that being human is the same as being a member of the species Homo sapiens. It’s true that this is what some scientists claim, but others tell a different story. Have a look at the literature on human genetics, evolution, and paleoanthropology, and you’ll find that some scientists equate being human with being a member of genus Homo, others cast the net much wider and include every member of our evolutionary lineage since our common ancestor with the chimpanzee, and still others restrict humanity to the subspecies Homo sapiens sapiens.  

Very strikingly, scientists rarely if ever tell us why they choose one or another of these options. In fact, the only way to figure out their views on the matter is to read between the lines of their writings and notice (for example) that some describe Homo erectus as “human” whereas others describe the species as “pre-human.”

The reason for this lack of clarity has got nothing to do with science and everything to do with philosophy. The problem isn’t that there aren’t enough data. There are plenty of data. The problem is that the data are irrelevant. Strange as it may sound, even if scientists knew everything there is to know about our biological lineage, this still wouldn’t settle the question of what it means to be human.

Science excels at discovering, predicting, and explaining facts about the world around us, and it has an ever-expanding arsenal of tools and techniques for accomplishing these things. But not every question concerns matters of fact, and scientific methods are not helpful when trying to answer questions of this sort. It turns out that “What does it mean to be human?” isn’t really a scientific question: it’s a philosophical one.  It’s a question about concepts rather than one about facts.

Let me explain….

We make sense of the world by classifying the things around us. We sort them into categories—mental “boxes” if you will. Some categories are scientific ones. Chemical elements, subatomic particles, and biological species are all examples of scientific categories. Mostly, though, we use non-scientific or folk categories. Folk-categories correspond to our ordinary, everyday ways of classifying things. Categories like “furniture,” “frying pans,” and “days of the week” are crucial for getting along in life, but they don’t play any role in scientific discourse. Furniture, frying pans, and days of the week don’t feature in any scientific laws or theories. Of course, scientists can study frying pans, but they investigate them as lumps of matter (physics) or objects with a certain sort of molecular composition (chemistry). The fact that they are frying pans is strictly irrelevant to science. Their “frying pan-ness” (to coin an awful term) falls out of the picture when we approach them from a scientific angle.

Sometimes, scientific categories correspond to folk categories precisely. Consider the stuff that we call “water.” Anything that’s a bucket of water (a folk category) is also a bucket of H2O (a scientific category), and vice versa. Philosophers express this relation by saying that water is reducible to H2O, which means that water is the very same stuff as H2O.  

Not all folk categories are reducible to scientific ones. Many of them don’t have scientific counterparts (and vice versa). Consider the category of plants that we call “weeds.” It’s crucial for gardeners to distinguish weeds from other sorts of plants, and they can do so because they’ve learned to categorize certain plants as weeds. But “weed” isn’t a biological category. It isn’t reducible to any scientifically legitimate taxonomic category (there aren’t any biological characteristics that set weeds apart from other sorts of plants).  Imagine that there’s an alien super-botanist from a distant galaxy who has who knows all there is to know about the biological properties of Earth plants—their morphology, physiology, ecology, life cycles, evolution, and so on. Even with all this knowledge at her disposal, the alien botanist wouldn’t be able to distinguish weeds from other plants. To do that, she would have to learn how Earthlings talk about plants in non-scientific contexts (perhaps by getting some practical lessons in gardening).

“Human” is a folk category, not a scientific one, and science can’t settle the question of what kinds of beings are human for the same reason that it can’t settle the question of what kinds of plants are weeds. Scientists can’t answer these questions because neither of them is a question about facts. When we ask what it means to be human, we’re asking a question about what should be included in a folk category, and when we try to answer it scientifically, we’re trying to match the folk category ‘human’ with a scientific one (for example, ‘genus Homo’), with the unspoken assumption that the former is reducible to the latter.  

Once we realize that we are on philosophical rather than scientific terrain, the problem becomes clearer and more tractable. In particular, it becomes easier to question the unspoken assumption that the category of the human is reducible to a scientific category. We can open our minds to the possibility that being human is more like being a weed than it is like being water.

What, then, can philosophy tell us about what it means to be human? Can it deliver the answer that we seek? I’ll tackle this issue in my next blog posting.