Emergence
Why the parts do not always explain what the whole does, and why that need not be a problem.
More is different. Under that title the physicist Philip Anderson published an essay in 1972 that is still an important point of reference in thinking about complexity. A whole can be more than the sum of its parts, and that more is, in our impression, often precisely what we want to understand.
Emergence is the phenomenon that properties appear at a higher level of description which are not present at the lower levels. The magnetism of a piece of iron does not exist in a single atom, a traffic jam is not a property of one car, and consciousness is not a property of one neuron. Yet they arise reliably from the interaction between those parts.
Weak and strong emergence
Philosophers usually distinguish two kinds. Weak emergence is the "innocent" variant: the higher level is a useful description, but in principle reducible to the lower one. A traffic jam is ultimately a collection of cars, each with a speed and a position. That we describe the jam as a thing is a convenient shorthand and not an ontological claim.
Strong emergence goes further: the higher level has a causal power of its own that cannot be fully reduced to the parts. A predator responds to the flock and not to the individual birds, so that the flock as a whole influences the behaviour of the birds. This is called downward causation: the whole influences its parts. Whether such examples really count as strong emergence is a matter of debate among philosophers. For psychology, at any rate, the idea is less esoteric than it sounds. A depression can be understood as more than the sum of its symptoms, because it in turn helps to determine which symptoms reinforce one another.
"Science is possible at many different levels of description without our having to fully understand the lower levels." (Van der Maas, 2024, after Fodor, 1974)
Why this matters for mental health care
Much clinical thinking implicitly follows a reductionist logic. We look for the "real" cause of complaints at a lower level: an imbalance in neurotransmitters, a genetic predisposition, an abnormality in the brain. Sometimes that yields something. But in depression, anxiety, personality problems and eating disorders we often find no unambiguous cause at that lower level that fully explains the complaint.
From the perspective of emergence, this need not be a problem. Much psychopathology can be understood as an emergent property, arising from the interaction between biology, cognition, behaviour, context and relationships. The depression then "exists" at a level of description that is real, even though there is no single biological marker, in the same way that a traffic jam is real without there being a "jam particle" to be found in any individual car.
This has consequences for diagnostics and treatment. We do not first have to find the "real" level of the complaint in order to be able to do something about it. Interventions at a higher level (behaviour, social context, meaning-making) are then not a surrogate for the "real" treatment, but interventions at a level at which the problem actually takes shape.
A reassurance and a challenge
Emergence can be seen as an argument for psychology as a science in its own right. We need not wait for neuroscience to be "finished" before thinking about depression or anxiety. At the same time, thinking in terms of emergence asks more of us: we will have to learn to work with several levels at once, and to judge which level offers the most insight for which problem.
Complexity science therefore does not stand in opposition to biology or neuroscience. It is rather a complement: a way of doing justice to the fact that psychological problems, like traffic jams, flocks and consciousness, often exist on several levels at once and can also be approached on several levels.