On the model presented here, the brain is organised as a supervenience hierarchy, such that higher levels of organisation emerge from interactions at lower levels of organisation during development and experience. There is no interaction between levels; for example, higher levels of organisation do not control lower levels — ‘downward causation’ — any more than a clock controls the molecules on which it supervenes as a level of organisation, because levels of organisation are complementary perspectives on the same phenomenon. On the TNGS model, the notion of ‘control’ is better reinterpreted in terms of selectional interaction between systems at the same level of organisation, if only because it is a ‘category error’ to map a hierarchy of degrees of control onto a hierarchy of organisational levels.
In considering interactions between systems at the same level of organisation, each system is necessary but not sufficient for the function it performs, just as a specific gene, as “for” iris colour, is necessary but not sufficient for its function (phenotypic expression); a gene only functions in the context of (the functions of) other genes, and its function is distinguished by contrast with the functions of other genes in the genome. Similarly, neurological functions are carried out in the context of other functions and each function is distinguished by contrast with those other functions. Absence or disruption of a specific function results from the absence or disruption of a necessary condition for its performance, as the absence or mutation of a gene results in the absence or variation of its phenotypic expression. By identifying loss of brain function with localised anatomical damage, some have argued that those functions are carried out in those areas, as if such areas are sufficient for the function. However, as brain imaging shows, even for something as “simple” as reciting digits, neural activity is distributed over many regions the brain, and the precise locations of activity vary from one individual to the next.