Traditionally, people have believed that the world is full of qualitatively different kinds of things; that is, there are a multitude of distinct species. In the realm of living things, there was seen to be a vast gulf between plants and animals, and between irrational animals and humans. These categories were based on what were seen as the characteristic activities of each type of organism. Plants do things like absorb nutrients and grow. Animals do “vegetative” things like this too, but they also sense their environment and move. Humans do both vegetative and animal activities, but we also engage in abstract thought. Of course, it was also generally believed that there are many different species within the plant and animal kingdoms as well.
Since Linnaeus, modern biologists have put far more thought and precision into taxonomy than the ancients ever did. Ironically, the farther these efforts have gone, the more skeptical moderns have become over whether the thing can really be done in a non-arbitrary way at all. First, as more species become known, the separation between them gets smaller. It becomes more difficult to say exactly where one species ends and another begins. Two equally reasonable definitions might give slightly different answers. Even if intermediates between clearly distinct species don’t exist now, oftentimes they did in the past, and we have fossils to prove it. This leads one to ask, “Isn’t this whole exercise arbitrary to begin with? Shouldn’t we just think of a continuum of animal properties, with no qualitative jumps?” At the same time, some biologists seem to take a postmodernist’s delight in “problemetizing” the old categories, e.g. showing how plants can—in some sense—move or sense, or showing how animals can—in some sense—reason. I often see articles written for a popular audience making great claims for animal intelligence. Dogs use logic, apes communicate, etc. One very popular claim now is that animals have morality, by which is usually meant empathy and self-sacrifice, but sometimes also a sense of fairness. The implication of all this is clear, but in case you’re not clever enough to see it, these ardent biologists never miss a chance to point it out for you: human beings are qualitatively no different from animals. It’s not that we have some quality that other animals lack entirely; the differences are all just quantitative. Your capacity for abstract reason is, say, 40% higher than that of your pet dog; your moral sense is perhaps 20% more developed. What would it mean to accept that humans are not different from other animals? The conclusions we’re usually asked to draw are these: we should treat animals more like we treat humans, and we should think of humans more like the way we think of animals. The first consequence, of giving animals what we think of as “human” rights, would at worst be an inconvenience. The second consequence, however, would be devastating, as it would invalidate the idea that human beings have a unique calling, a qualitatively higher state of perfection, and therefore a unique dignity.