Emergency, causality and realism

The origin of the modern concept of emergence can be traced to the mid-nineteenth century, when realist philosophers first began to ponder the deep dissimilarities between causality in the fields of physics and chemistry. The classic example of causality in physics is a collision between two molecules or other rigid objects in which the overall effect is a simple addition. However, when two molecules interact chemically, an entirely new entity may emerge, as when hydrogen and oxygen interact to form water. The fact that novel properties and capacities emerge from a causal interaction was believed to have important philosophical implications for the nature of scientific explanation. In particular, the absence of novelty in physical interactions meant that explaining their effects could be reduced to deduction from general principles or laws. However, the synthesis of water does produce something new, something that emerges from the interacting entities acting as causes. This led some philosophers to the erroneous conclusion that emergent effects could not be explained, or, what amounts to the same thing, that an effect is emergent only so long as a law from which it can be deduced has not been found. This line of thought went on to become a full-fledged philosophy in the early twentieth century, based on the idea that emergence was intrinsically inexplicable. This essay argues that while the first wave of "emergentist" philosophers correctly saw that the concept of emergence was a powerful way to block reductionism and, therefore, to give fields other than physics their due respect, they were wrong about its inherent inexplicability: the emergent properties of a whole arise from the causal interactions between its parts, and these interactions constitute an explanatory mechanism for those properties.