Emergence occurs when there is a conceptual discontinuity between two descriptions targeting the same phenomenon. This does not mean that emergence is a purely subjective phenomenon — only that scientific ‘double coverage’ may be a good place to look for emergent phenomena.
For example, in the case of starling murmuration, there is an aggregate description of individual birds, and a description of the flock as a unified entity. The latter phenomenon invites description in terms of concepts from fluid dynamics, but descriptions of individual birds, however detailed, typically do not.
In the case of phase transitions in physics, the description of one phase of matter, such as gas, does not fully map onto descriptions of the other phases. Surface tension, for example, is not defined for gases, since gases do not have surfaces. In the transition from gas to liquid, a qualitatively new attribute not only emerges, it becomes a defining feature of the post-transition system. From a different perspective we can say that it is the emergent qualitative property that enables us to determine that the transition has occurred in the first place. Quantitative readings of some control variable (such as temperature or pressure) cannot themselves be used to mark out ‘events’ — they can only be used to index them.
A common type of theoretical disjunction involves mismatch between descriptions of parts and wholes. A description of micro-level constituents in terms of atomic properties does not lead in any smooth way to descriptions in terms of thermodynamics, hydrodynamics, or solid state physics. In physics, the lack of smoothness in transitioning from one theoretical domain of discourse to another is not always apparent, since the two domains are often well specified mathematically, while the (often ad hoc) linking assumptions enabling the transition are neglected in popular (and even introductory textbook-level) explanations.
Emergence and ontology
The existence of such theoretical non-smoothness is not in doubt. Why then might emergence be considered controversial? Two reasons suggest themselves. The first has to do with the ontological commitments of science in general, and the second has to do with the status of human agency.
As long as scientific theories are understood as descriptions that have restricted domains of usefulness, emergent phenomena and their accompanying theories pose no great threat to science, which in such a framework only has an instrumental goal. But subscribers to a scientific worldview often make a more ambitious claim: that the best theories are isomorphic with the fundamental nature of the universe. In such a framework, a commitment to the unity of scientific theories amounts to a commitment to the integrity and intelligibility of the universe as such. Disjunctions between theories are unpalatable in isomorphic framings of the science-nature relationship, since they seem to suggest that reality itself is not fully law-governed, and therefore contains some irrational or ineffable component. Examples of apparent emergence must then be understood as cases of incomplete theorizing or insufficient computation.
Reductionism can be understood as a combination of (1) the claim that the intelligibility of the universe depends on the unity of scientific theories, and (2) the idea that the most fundamental scientific theories describe the smallest physical parts of the universe. Reductionism can also be relativized to specific scientific disciplines, so that the parts of one discipline (say, cells in the context of organ-level biology) are the wholes of another discipline (cells in the context of molecular biology). Since reductionism has had only infrequent success as a research methodology (especially outside of physics), it must be understood primarily as a metaphysical commitment. Despite its limited usefulness as a guide to scientific practice, reductionism is a powerful cultural idea. We might call it the Lego-block conception of reality: only the Lego blocks are real, so ‘fundamental’ science involves identifying what the blocks are and how they interact, while ‘applied’ science involves discovering the right combination or permutation of blocks that accounts for the phenomenon in question. This perspective is captured in jokes that circulate in physics departments, such as Ernest Rutherford’s line “Science consists of physics and stamp-collecting.”
An emergent phenomenon can always be understood in a reductionist ontology as a situation in which the ‘true’ reductionist theory simply hasn’t been discovered yet, or (if it is understood as already existing) is too computationally intensive to be applied to macro-scale phenomena. Thus from the reductionist perspective, emergence can only ever be “weak” or “epistemic”. Emergentism as a scientific stance must then be understood as making an ontological claim that goes beyond the mere occurrence of apparent emergence: that the intelligibility of the universe does not depend on isomorphism between the universe and the (as yet undiscovered) fully reductionist theory. Emergentism instead suggests the possibility that part-whole relationships may not be fully explainable using causal chains proceeded from parts to wholes. Emergentism therefore raises a possibility that sounds radical and perhaps even meaningless from the reductionist perspective: that parts and wholes have ‘equal’ ontological priority, with the wholes constraining the parts just as much as the parts constrain the wholes. Along with this ontology that departs from strict hierarchy, emergentism may draw attention to the possibility that the universe is in some sense open to novel phenomena that cannot be perfectly anticipated using any scientific theory, but, once present, can still be studied using scientific methods. In other words, emergentism suggests that even our best quantitative theories cannot always tell us when qualitative changes will occur.
The ontological debate between reductionism and emergentism takes on cultural importance in the context of explaining human agency. If phenomena associated with living agents (humans and other biological entities) are ultimately to be explained in terms of their constituent parts, then can causation be attributed to an emergent agent? In other words, do humans cause their actions, or must the ultimate causes be sought at the level of molecular biology? If the latter perspective is taken then human agency becomes a kind of illusion or epiphenomenon, as it restricts causality to ‘fundamental’ entities. Emergentism in this context can therefore be understood as an attempt to create ‘ontic space’ for scientifically coherent forms of human agency.
To explore the implications of ontology on agency, we will need to make explicit our intuitions about what causality is. This challenging and, unsurprisingly, contested topic will* be dealt with in a future essay.
* Assuming my self-model of future behavior is smooth. Which is a big assumption. :P
Image: Eight different allotropes of carbon.
This post grew out of an ongoing journal club on emergence. In this post I haven’t really provided any references, but hopefully in future I’ll do short posts explaining where these ideas came from.
EDIT: the subsequent post is an annotated bibliography of the readings we covered in the journal club.