In physics, we use the convention that the suffix "-on" denotes a quantized unit of something. For example, in classical physics, there are electromagnetic waves. But in the quantum version of the theory, originating with Einstein's 1905 Nobel Prize winning work, we know that under certain circumstances, it is more precise to regard electromagnetic radiant energy as being distributed in particles called photons. This "wave-particle duality" is the underpinning of modern physics: not just photons, but a zoo of what were once called elementary particles that include protons, neutrons, pions, mesons, and of course the Higgs boson. (Neutrino? It's a long story …).

And what about you? You are a person. Are you a quantum of something too? Well, clearly there are no fractional humans, and we are trivially quantized. But elementary particles, or units, are useful conceptually because they can be considered in isolation, devoid of interactions, like point particles in an ideal gas. You would certainly not fulfil that description, networked, online, and cultured as you undoubtedly are. Your strong interactions with other humans mean that your individuality is complicated by the fact that you are part of a society, and can only function properly in such a milieu. We could go further and say that you are a quantum of a spatially-distributed field, but one that describes the density around each point in space of humans, rather than the electromagnetic field intensity. This description turns out to be technically very powerful for describing the behavior of ecosystems in space and time, particularly to describe extinction, where discontinuous change is important. It seems apt to invoke here the strangely oxymoronic term "Indivi-duality", a counterpart to wave-particle duality.

The notion of individual has several other connotations. It can mean discrete or single, but its etymology is also reminiscent of "indivisible". Clearly we are not indivisible, but are constituted from cells, themselves constituted of cytoplasm, nucleic acids, proteins etc., themselves constituted of atoms, which contain neutrons, protons, electrons, all the way down to the elementary particles which themselves are now believed to be products of string theory, itself known now not to be a final description of matter. In other words, it's "turtles all the way down", and there are no indivisible units of matter, no meaning to the notion of elementary particle, no place to stop. Everything is made of something, and so on ad infinitum.

However, this does not mean that everything is simply the sum of its parts. Take the proton for example, made up of three quarks. It has a type of intrinsic angular momentum called spin, which was initially expected to be the sum of that of its constituent quarks. Yet experiments carried out over the last 20-30 years have shown clearly that this is not the case: the spin arises out of some shared collective aspect of the quarks and short-lived fluctuating particles called gluons. The notion of individual quarks is not useful when the collective behavior is so strong. The proton is made of something, but its properties are not found by adding up the properties of its parts. When we try to identify the something, we discover that, as with Los Angeles, there is no "there" there.

You probably already knew that naïve reductionism is often too simplistic. However, there is another point. It's not just that you are composite, something you already knew, but you are in some senses not even human. You have perhaps a hundred trillion bacterial cells in your body, numbering ten times more than your human cells, and containing a hundred times as many genes as your human cells. These bacteria are not just passive occupants of the zoo that is you. They self-organize into communities within your mouth, guts and elsewhere; and these communities—microbiomes—are maintained by varied, dynamic patterns of competition and cooperation between the different bacteria, which allow us to live.

In the last few years, genomics has given us a tool to explore the microbiome by identifying microbes by their DNA sequences. The story that is emerging from these studies is not yet complete but already has led to fascinating insights. Thanks to its microbes, a baby can better digest its mother's milk. And your ability to digest carbohydrates relies to a significant extent on enzymes that can only be made from genes not present in you, but in your microbiome. Your microbiome can be disrupted, for example due to treatment by antibiotics, and in extreme cases can be invaded by dangerous monocultures, such as Clostridium difficile, leading to your death. Perhaps the most remarkable finding is the gut-brain axis: your gastrointestinal microbiome can generate small molecules that may be able to pass through the blood-brain barrier and affect the state of your brain: although the precise mechanism is not yet clear, there is growing evidence that your microbiome may be a significant factor in mental states such as depression and autism spectrum conditions. In short, you may be a collective property arising from the close interactions of your constitutents.

Now, maybe it is true then that you are not an individual in one sense of the word, but how about your microbes? Well, it turns out that your microbes are a strongly interacting system too: they form dense colonies within you, and exchange not only chemicals for metabolism, but communicate by emitting molecules. They can even transfer genes between themselves, and in some cases do that in response to signals emitted by a hopeful recipient: a bacterial cry for help! A single microbe in isolation does not do these things; thus these complex behaviors are a property of the collective, and not the individual microbes. Even microbes that would seem to be from the same nominal species can have genomes which differ in content by as much as 60% of their genes! So much for the intuitive notion of species! That’s another too-anthropomorphic scientific idea that does not apply to most of life.

Up to now I talked about connections in space. But there are also connections in time. If the stuff that makes the universe is strongly connected in space, and not usefully thought of as the aggregate sum of its parts, then attributing a cause of an event to a specific component may also not be meaningful. Just as you can't attribute the spin of a proton to any one of its constituents, you can't attribute an event in time to a single earlier cause. Complex systems have neither a useful notion of individuality nor a proper notion of causality.