Field of Science

What makes me human? On personhood and levels of emergence

Ce n'est pas une machine
There is a wonderful episode of Star Trek (The Next Generation) titled “The Measure of a Man” which tackles an issue that all of us take for granted. The episode asks if Data – the highly intelligent and indispensable android on the USS Enterprise – has self-determination. In fact, Data faces an even greater challenge: he has to prove that he is equivalent to a person. If he cannot do this he faces a grim fate.

We might think it’s easy enough to decide if an android is a person or not, partly because we think we know how we define ourselves as persons. But is it really that simple? Can we actually ascribe unique, well-defined qualities to ourselves that lend themselves to a singular definition of “personhood”? Can we be sure that these qualities distinguish us from cats and jellyfish? Or mountains and computers for that matter?

Let’s throw down the gauntlet right away in the form of a question: A tree and I are both composed of the same kinds of atoms – carbon, hydrogen, oxygen and a handful of others. So what makes me a person and the tree a mere tree, a non-person?

To a simple first approximation, the difference lies in the arrangement of those atoms. Clearly the arrangement is different in a tree, in a lion and in me. But it’s not just the arrangement of the parts, it’s the connections between them. If you delete enough connections between the neurons in a human brain, at some point the person possessing that brain would clearly cease to be a sentient human being. The same goes for the connections between the cells in a tree.

Thus, simply from a physical standpoint, a person is an object that presents an arrangement of atoms in a particular configuration. But that definition comes no closer to telling us exactly what makes that particular arrangement special. To get some insights into these reasons, it’s worth thinking about the concept of emergence.

Emergence is a very common phenomenon, and in its basic incarnation it simply means that the whole is different from the sum of the parts; or, as the physicist Philip Anderson put it in a seminal article in 1972, “More is Different”. Going back to our example, the human brain may be composed of the same atoms as a tree, but because of the unique arrangement of these atoms and the connections between them, the sum total of these atoms possesses properties that are very different from those of the individual atoms. Just as an example, a carbon atom in our brain can be uniquely defined by what are called quantum numbers – atomic parameters related to properties like the spin of the atom’s electrons, the energy levels on which the electrons lie and their angular momentum. And yet it’s downright absurd to talk about these properties in the context of the brain which these atoms make up. Thus it’s the emergent properties of carbon and other atoms that contribute to the structure and function of the human brain.

Emergent properties of individual atoms don’t uniquely make a human person, however, since even the brains of cats and dogs exhibit these properties. We don’t yet have a perfect understanding of all the qualities that distinguish a cat’s brain from a human’s, but we do know for certain that at least some of those qualities pertain to the size and shape of parts of the brains in the two species and the exact nature of the connections between their neurons. For instance, the cerebral cortex in a human brain is bigger and far more convoluted than in a cat. In addition the human brain has a much greater density of neurons. The layering of neurons is also different.

Taking our idea of emergence further, each one of these qualities is an emergent property that distinguishes cat brains from human brains. There are thus different levels of emergence. Let’s call the emergence of properties arising when individual atoms coalesce into neurons Level I emergence. This level is very similar for humans and cats. However, the Level II emergence which arises when these neurons connect differently in humans and cats is very different in the two species. The Level III emergence that arises when these connections give rise to modules of a particular size and shape is even more different. The interactions of these modules with themselves and with the environment presumably give rise to the unique phenomenon of human consciousness: Level IV emergence. And finally, the connections that different human brains form with each other, giving rise to networks of families, friends, communities and societies, constitute an overarching Level V emergence that truly distinguishes persons not just from cats but also from every other creature that we can imagine.

This idea of thinking in terms of different levels of emergence is useful because it captures both similarities and differences between persons and non-persons, emphasizing the common as well as the distinct evolutionary roots of the two kinds of entities. Cats and human beings are similar when defined in terms of certain levels of emergence, but very different when defined in terms of ‘higher order’ levels.

The foregoing discussion makes it sounds as if a simple way to distinguish persons from non-persons would be to map different levels of emergence on each other and declare something to be a non-person if we are successfully able to map the lower levels of emergence but not the higher ones. I think this is generally true if we are comparing animals with human beings. But the analogy is actually turned on its head when we start to compare humans with a very important and presumably non-person object: a sophisticated computer. In that case it’s actually the higher order emergent functions that can be mapped on to each other, but not the lower order ones. 

A computer is built up of silicon rather than carbon, and silicon and carbon are different emergent entities. But as we proceed further up the hierarchy, we start to find that we can actually simulate primitive forms of human thinking by connecting silicon atoms to each other in specific ways. For instance, we can teach the silicon-based circuitry in a computer to play chess. Chess is presumably a very high level (Level VIIXXI?) emergent human property, and yet we can simulate this common higher order property from very different lower order emergent properties. Today computers can translate languages, solve complex mathematical puzzles and defeat Go champions. All of these accomplishments constitute higher levels of emergent behavior similar to human behavior, arising from lower levels that are very different from those in human.

In fact it is precisely this kind of comparison that allowed Captain Jean-Luc Picard to secure personhood for Data. Data is a human-machine hybrid that is built up from a combination of carbon and non-carbon atoms. His underlying molecular structure is thus very different from those of persons. But the higher order emergent functions he exhibits – especially free will allows Picard to make a convincing case for Data to be treated as a person. This crucial recognition of emergent functions in fact saves Data’s life. It's what compels the man who is trying to dismantle him to address him as "he" instead of "it".

Whether it’s Data or a dolphin, a computer or a catfish, while it’s probably not possible to give a wholly objective and airtight definition of personhood, framing the discussion in terms of comparing different levels of emergent functions and behaviors provides a useful guide.

More is indeed different.
This piece was published yesterday in an issue of '3-Hours', the online magazine of Neuwrite Boston.


  1. "One day the AIs are going to look back on us the same way we look at fossil skeletons on the plains of Africa... an upright ape living in dust with crude language and tools, all set for extinction."

    1. indeed! We'll be like those apes gaping at the black stone in "2001".


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