Emergence

What it is

Emergence is the appearance of properties in a whole that none of its parts possess. A single water molecule is not wet. A single starling does not flock. A single neuron does not decide anything. Put enough of them together in the right arrangement and something new shows up.

Everything turns on a distinction that popular usage almost always skips.

Weak emergence means the property is genuinely novel and genuinely surprising, but follows from the parts and their interactions — derivable in principle, even if no human could do the derivation in practice. Traffic jams emerge from cars. Termite mounds emerge from termites. A sufficiently patient calculation would get you there from the components.

Strong emergence means the property is not derivable even in principle from a complete knowledge of the parts, and brings genuinely new causal powers into the world — the whole does something the parts, however arranged, could not account for.

Weak emergence is everywhere and uncontroversial. Strong emergence is contested, has no agreed example, and is what people usually mean without realising it when they say consciousness "emerges" from the brain.

Where it came from

The word in this sense is owed to G.H. Lewes, who coined "emergent" in 1875, and the idea was developed into a full philosophical programme by the British Emergentists of the early twentieth century — C. Lloyd Morgan's Emergent Evolution (1923), Samuel Alexander, and most carefully C.D. Broad in The Mind and Its Place in Nature (1925).

Their story is the most instructive thing in this article, and it is a warning.

The British Emergentists needed a clear case of strong emergence, and they had one they considered unassailable: chemistry. The properties of a chemical compound, they argued, simply could not be deduced from the properties of its constituent elements — you could know everything about sodium and everything about chlorine and never derive table salt. Broad argued this at length and with care. It seemed obvious.

Then quantum mechanics arrived, and through the late 1920s and 1930s chemical bonding was explained in terms of physics. The flagship example of irreducibility was reduced. The movement collapsed and the word fell out of respectable use for decades.

The lesson is not that strong emergence is impossible. It is that "we cannot currently derive X from Y" is a statement about the state of knowledge, and it has been mistaken for a statement about the structure of reality before, by careful people arguing in good faith.

How it is used

Complexity science uses weak emergence productively and rigorously — in flocking models, in economics, in cellular automata, in the study of phase transitions. That work is real and the concept does genuine explanatory labour there.

In writing about consciousness and spirituality, "emergence" frequently does the opposite job. "Consciousness emerges from the complexity of the brain" has the cadence of an explanation while being, on inspection, a placeholder for one. The dilemma is sharp:

• If the emergence is weak, then consciousness is derivable in principle from neural facts — in which case the hard problem is a temporary ignorance rather than a deep one, and saying "emergence" has not addressed it, only named the homework
• If the emergence is strong, then something appears in the world that the physical facts do not account for, which is a substantial metaphysical claim requiring defence rather than a way of avoiding one

Either way the word has not done the work it appears to be doing. It is worth noticing when it is being used as a full stop rather than a question.

What we can and cannot say

We can say weak emergence is real, well understood, and everywhere. Nothing controversial attaches to it.

We can say strong emergence has no uncontested example. It faces a serious structural objection in Jaegwon Kim's causal exclusion argument: if every physical event already has a sufficient physical cause, then an emergent property with genuinely new causal powers has nothing left to do, unless physical causal closure is false. Either the emergent property is redundant, or it overdetermines its effects, or physics is incomplete. Each option costs something.

We can say — and this is the honest complication — that the failure of the British Emergentists cuts in more than one direction. It shows the argument from present underivability is weak. It does not show that everything reduces, only that the one example they trusted did.

What we cannot say is that emergence explains consciousness. It might. Nobody has shown how, and using the word as though someone had is the single most common way of appearing to have solved the hard problem without touching it.

Further reading

• C.D. Broad, The Mind and Its Place in Nature (1925) — the careful statement of the position that history then falsified, which makes it more worth reading rather than less.
• Jaegwon Kim, Mind in a Physical World (1998) — the causal exclusion argument.
• Mark Bedau, "Weak Emergence" (1997) — where the weak/strong distinction gets its modern precision.