Variations
Usage of the notion "emergence" is often subdivided into two perspectives: "weak emergence" and "strong emergence". Philosopher David Chalmers writes that emergence often causes confusion in philosophy and science due to a failure to demarcate weak and strong emergence, which are "quite different concepts".
Both "weak" and "strong" positions hold that emergent properties are dependent on lower-level phenomena while nevertheless being in some sense autonomous from them.
Weak emergence describes scenarios in which emergent properties, while autonomous, do not introduce novel forces or causes "beyond" their constituent components. A property might count as autonomous if it is the result of interaction rather than aggregation (for example, how the behaviour of an ant colony is produced by exchanges among individuals); if it is sufficient to explain and predict a system (for example, how the path of a tornado can be deduced without reference to its molecular components); or if it can be multiply realised by systems possessing different components (for example, how similar mental states can be produced by different brains). Philosopher Mark Bedau writes that in cases of weak emergence the emergent property is amenable to computer simulation or similar forms of after-the-fact analysis (for example, the formation of a traffic jam, the structure of a flock of starlings in flight, or the formation of galaxies).Although new properties arise in systems as a result of the interactions at a fundamental level, the properties can be determined by observing or simulating the system as a whole, without requiring a reductionist analysis. At large enough scales, seemingly chaotic, hard-to-predict behaviour can emerge, while at a microscopic scale the behaviour of the constituent parts can be fully deterministic.
Strong emergence, by contrast, obtains when a high-level emergent system is autonomous by virtue of being novelly causal, such that it exerts downward influence on its constituent parts. This means that the emergent entity can act on the world in such a way that cannot be deduced from an analysis of the interactive operations of its components. As Chalmers writes, in cases of strong emergence "truths concerning that phenomenon are not deducible even in principle from truths in the low-level domain." Bedau argues that for strongly emergent properties no simulation of the system can exist, for such a simulation would itself constitute a reduction of the system to its constituent parts. The system will evolve in a way that is fundamentally unpredictable, rather than merely difficult to predict.
Weak and strong emergence are often described as compatible with "subjective" and "objective" accounts of emergence respectively (sometimes termed "epistemic" and "ontological" emergence). Subjective accounts maintain that emergence requires the impossibility in practice, rather than in principle, to explain the whole in terms of the parts. This position is defended by its advocates on the grounds that it does not imply the appearance of mysterious forces, but simply reflects the limits of individuals' capabilities. Physicist James Crutchfield regards the properties of complexity and organization of any system as subjective qualities determined by the observer: Defining structure and detecting the emergence of complexity in nature are inherently subjective, though essential, scientific activities. Despite the difficulties, these problems can be analysed in terms of how model-building observers infer from measurements the computational capabilities embedded in non-linear processes. An observer's notion of what is ordered, what is random, and what is complex in its environment depends directly on its computational resources: the amount of raw measurement data, of memory, and of time available for estimation and inference. The discovery of structure in an environment depends more critically and subtly, though, on how those resources are organized. The descriptive power of the observer's chosen (or implicit) computational model class, for example, can be an overwhelming determinant in finding regularity in data.An observer who could see the precise micro-scale dynamics of every system - such as Laplace's Demon - would not see properties as emergent, as it would understand the exact dynamics giving rise to them. Cognitively limited beings like humans, however, must rely on "coarse-grained" descriptions of the world, and these descriptions can be both predictive and explanatory.
Subjective accounts of emergence are closely related to probability as described in statistical mechanics and Information theory, in which the emergent tendency of entropy to increase over time is a reflection of observers' ignorance of the total system.
Sean Carroll and Achyuth Parola propose a taxonomy that classifies emergent phenomena by how the macro-description relates to the underlying micro-dynamics.
Type‑0 (Featureless) Emergence: A coarse-graining map Φ from a micro state space A to a macro state space B that commutes with time evolution, without requiring any further decomposition into subsystems.
Type‑1 (Local) Emergence: Emergence where the macro theory is defined in terms of localized collections of micro-subsystems. This category is subdivided into:
Type‑1a (Direct) Emergence: When the emergence map Φ is algorithmically simple (i.e. compressible), so that the macro behavior is easily deduced from the micro-states.
Type‑1b (Incompressible) Emergence: When Φ is algorithmically complex (i.e. incompressible), making the macro behavior appear more novel despite being determined by the micro-dynamics.
Type‑2 (Nonlocal) Emergence: Cases in which both the micro and macro theories admit subsystem decompositions, yet the macro entities are defined nonlocally with respect to the micro-structure, meaning that macro behavior depends on widely distributed micro information.
Type‑3 (Augmented) Emergence: A form of strong emergence in which the macro theory introduces additional ontological variables that do not supervene on the micro-states, thereby positing genuinely novel macro-level entities.