In last month’s blog, I used the example of PETA’s proposal to erect a memorial to the thousands of lobsters that died in a road accident in Brunswick, Maine, to address the subject of the Great Chain of Being—the notion that the biosphere is partitioned into ranks, with humans at the top, and every other organism at some inferior position. The rank that a being occupies corresponds to its intrinsic value—the value that it has in and of itself. The higher an organism is ranked, the greater its worth, and the more consideration we owe it. We humans regard ourselves as supremely valuable—indeed, as Immanuel Kant put it, infinitely valuable—beings. So, we have demanding moral obligations to members of our own kind, but less, if any, to the “lower” creatures that are ranked “beneath” us.

According to the philosopher and intellectual historian Arthur O. Lovejoy, whose 1936 book The Great Chain of Being remains the best resource on this subject, the Great Chain of Being was a Western invention. It was a philosophical theory forged from ideas mined from the works of Plato and Aristotle during late antiquity, that blossomed during the middle ages before fading away in the late 18th century to make room for better, more scientific accounts.

That’s the official story. But as I explained last month, it just can’t be right. One reason is that it never went away. The concepts of human exceptionalism and of “higher” and “lower” animals are still very much with us—not just in ordinary discourse, but also, surprisingly enough, in the writings of biological scientists. The second reason is that it’s never been limited to the Western intellectual tradition, but can be found in the belief-systems and philosophical traditions of the civilizations of China, India, Mexico, West Africa, and elsewhere. Wherever the idea of the Chain of Being came from, it wasn’t the works of Plato and Aristotle.

There’s clearly something much bigger going on here than Lovejoy’s story would have us believe. What is it?

I’ve been thinking about this question for a long time, because it’s closely bound up with the phenomenon of dehumanization, which is the subject that I do most of my research on. So let me to share with you what I think is the best explanation of the origin and persistence of the Great Chain of Being. The full story is more complicated than I can do justice to in a blog posting, but I can sketch out some of the main ideas (I’ll be delving into the details in my next book).

Any explanation of the hierarchical conception of nature has got to engage with human psychology. The Great Chain of Being isn’t just a theory that’s found in abstruse philosophical texts. It’s a way of thinking about the world that structures our everyday values and perceptions, and one that’s incredibly difficult, if not impossible, to dislodge. The claim human lives matter more than lobster lives seems unquestionable. Most of us have a strong conviction that it’s true, and we couldn’t be persuaded that it isn’t true, but we don’t have a clue why we think that.

To find out, let’s start with some facts about human nature. We Homo sapiens are a hugely social species. In fact, there’s no other mammal that comes anywhere near to our feverishly intense level of sociality. We’re also extremely intelligent creatures who carve up the world with concepts and are able to think about our own thoughts. Thanks to these two traits we invented culture—a device for self-engineering that gives us vastly greater behavioral flexibility than is available to any other animal.

These traits have, for better or for worse, allowed human beings to dominate the planet. But they’ve also posed some serious challenges to us. One such challenge is the problem of how to sustain our social way of life. How does such an adaptable animal, who’s not rigidly tethered to biological instincts, keep its societies from falling apart? In particular, how does such an animal manage the problem of aggression?

Let me flesh these questions out.

All social animals have to inhibit aggression against members of their own community. That’s because if these inhibitions weren’t there, it would be impossible for them to sustain a social way of life. Animal societies would fray, and life would be, as Thomas Hobbes famously put it “solitary, poor, nasty, brutish, and short.” That’s why most social animals have an instinctive aversion to in-group violence, which was installed in them over evolutionary time by natural selection. But in our case, things are more complicated. We aren’t equipped with a suite of instincts to keep our societies intact. That’s why we need systems of moral rules to keep our societies in working order.

The problem of violence—especially lethal violence—is at the very center of morality. Whatever else must be regulated to make the social gears mesh smoothly, putting the brakes on violence is by far the most important. But herein lies a dilemma. Just as killing our own kind is profoundly antagonistic to the social order, killing others is absolutely necessary for maintaining that order. Life feeds on life. Animals have to kill and dismember other organisms in order to keep on living, and those who don’t participate in acts of violence are doomed to extinction. Arthur Schopenhauer was spot on when he observed that the natural world is “a scene of tormented and agonized beings, who only continue to exist by devouring each other, in which, therefore, every ravenous beast is the living grave of thousands of others, and its self-maintenance is a chain of painful deaths.” Schopenhauer obviously had predators in mind when he wrote these words, but even herbivores must kill and dismember plants in to survive.

We, as social animals, were confronted with a problem. We had to refrain from killing and harming our own kind, while also killing and harming nonhuman others. Imagine that moment in the distant past when our forebears began to reflect on their own behavior. They had a system of rules—the thou shalt not kills and the thou shalt kills—but they needed some justification to underwrite them. They must have asked themselves why it’s permissible to kill the animals and plants that they consumed, and from whose body parts they fashioned tools, and ornaments, and clothing, but not permissible to kill members of their community.

At first, they may have dealt with this problem by adopting the belief that the animals that they hunted and the plants that they consumed offered themselves up to be killed and eaten, and that being killed was, in some sense, beneficial to them (there are examples of present-day hunter-gatherer societies that hold such beliefs). But at some point—probably around the time of the emergence of agriculture and the domestication of livestock—they adopted a new ideology. Killing and exploiting nonhuman animals was justified by their low rank on a cosmic hierarchy—a hierarchy that mirrored (and thereby justified) the rigid social hierarchies that were endemic to these societies. And there was an added bonus. The idea of the Great Chain of Being could be used to justify killing and enslaving other human beings by dehumanizing them. If others humans are imagined as subhuman animals it becomes legitimate, or even mandatory, to treat them accordingly.

The idea of the Great Chain of Being has been immensely successful ideology. It performed its function amazingly well, and that’s why it’s still with us today. Like our ancient ancestors, we cling to it because we are moral animals who are condemned to kill, and it does the job of reconciling these contradictory facets of our being. With its help, we can drop living lobsters into pots of boiling water because they’re only lobsters.