As James Ladyman and Karoline Wiesner put it,
the most basic kind of emergence is the existence of laws and properties at the level of the whole System that do not exist at the level of the constituent parts.Ladyman & Wiesner (2020), 121
The classic example for this kind of emergence is the property of being wet: While water (the “whole system”) is wet, i.e. has this property, individual water (H₂O) molecules aren’t – they don’t have the property. Wetness is emergent.
In more technical terms,
there is a lower dimensional effective dynamics within a higher dimensional State Space. … Emergent properties and laws massively reduce the number of degrees of freedom needed to describe and predict the behaviour of the system.ibid., 74 ff
An example, temperature and expansion of a gas can be described without computing the trajectories of all its constituent molecules.
Describing emergent patterns of behaviour is therefore often much more efficient than calculating the behaviour of a system from the behaviour of its components. (This is one reason why we should Start system descriptions top-down).
In Complex Systems, these patterns are not fully predictable, though. This is because in these systems, emergence is the result of (lots of) interactions of (lots of) components pursuing their own goals.
This also means the behaviour of complex systems is not predefined, but evolves dynamically as the relationships between their components change.
What’s more, it cannot be derived from isolated descriptions of the
components. Reducing a complex system to its parts, i.e. ignoring
their relationships, makes the behaviour disappear. This is why
reductionist science often “destroys” the very things it wants to
explain.See Sapolsky (2010) for a very good explication of
this.
References
- Ladyman & Wiesner (2020): What Is a Complex System?
- Sapolsky (2010), “Chaos and Reductionism”