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 in the language of more branches of science. The central mistake of previous attempts to make this notion precise, I think, lies in thinking of this “nestedness” as hierarchical in the traditional linear sense: of there being strict pyramidal structure to the relationship between the various branches of science. In Oppenheim and Putnam’s formulation, for instance, physics was at the bottom of the pyramid, then chemistry, then biology, then psychology, then sociology. The assumption lurking behind this model is that all systems described by chemistry can also be described by physics (true enough, but only in virtue of the fact that the goal of physics is to describe all systems), all systems described by biology can also be described by chemistry (probably also true), that all systems that can be described by psychology can also be described by biology (possibly not true), and that all systems described by sociology can also be described by psychology (almost certainly not true). The last two moves look particularly suspect, as they rule out a priori the possibility of non-biological systems that might be usefully described as psychological agents, or the possibility of systems that cannot be treated by psychology, and yet whose behavior can be fruitfully treated by the social sciences.

Dynamical complexity escapes from this problem by relaxing the pyramidal constraint on the relationship between the various branches of science. As I argued in Chapter One, the intersections between the domains of the various sciences are likely to be messy and complicated: while many psychological systems are in fact also biological systems, there may well be psychological systems which are not—the advent of sophisticated artificial intelligence,

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