Philosophers wrestle with emergence

The concept of emergence has received significant attention in philosophical circles, from early in the twentieth century. Here, it is important to make the distinction between "weak" emergence and "strong" emergence. Philosophers such as Broad and Alexander were advocates of what would be now called "strong" emergence. A position of strong emergence holds that emergent properties cannot, even in principle, be deduced from the properties of the constituents of the system. Vitalism, the notion that living matter, is not purely physical, is an example of such a view.

A reductionist would claim we can always understand all the properties of a complex system in terms of the properties of its constituent particles and their interactions. Given sufficient computational resources we can predict properties of the whole system. In contrast, someone advocating "weak" emergence acknowledges it may be possible "in principle" to deduce emergent properties from properties of the constituents of the system. However, they would emphasize that in practice (at least, at this point in time) we cannot make such deductions. Furthermore, such deductions do not necessarily provide significant insight or allow one to deduce the organizing principles of the system under study.

Sometimes a "strong" emergence position is associated with "top-down causation" or "downward causation", in contrast to the notion of "bottom-up causation" which a reductionist advocates.

Silberstein and McGeever discuss how the distinction between "weak" and "strong" emergence can also be viewed as a distinction between epistemological emergence and ontological emergence:

A property of an object or system is epistemologically emergent if the property is reducible to or determined by the intrinsic properties of the ultimate constitutents of the object or system, while at the same time it is very difficult for us to explain, predict or derive the property on the basis of the ultimate constituents.

Ontologically emergent features are neither reducible to nor determined by more basic features. Ontologically emergent features are features of systems or wholes that possess causal capacities not reducible to any of the intrinsic causal capacities of the parts nor to any of the (reducible) relations between the parts.

They claim that the existence of entangled states in quantum mechanics provides the most conclusive evidence for the existence of ontological emergence and that this completely explodes the ontological picture of reality as divided into a `discrete hierarchy of levels’ and they quote Humphreys statement ,

even if the ordering on the complexity of structures ranging from those of elementary physics to those of astrophysics and neurophysiology is discrete, the interactions between such structures will be so entangled that any separation into levels will be quite arbitrary

However, this argument overlooks the fact that entangled quantum states are very fragile and their interaction with the environment can quickly "decohere" them and destroy the entanglement. This is how classical mechanics emerges from quantum mechanics. It is possible to show that in biomolecules this decoherence occurs in less than a picosecond, which is why large scale quantum entanglement does not play a role in biochemical phenomena.

In considering the relationship between chemistry and physics, Lombardi and Labarca reject the separation of ontological and epistemological emergence. They work within the framework of Hilary Putnam’s internal realism, which aims to find middle ground between metaphysical realism and radical relativism. Conceptual schemes and descriptions are required to define objects, even though reality exists independent of our subjective description.