Most important single concept?

The GPA is three dimensional: socio-cultural, cognitive and temporal. In cognitivist theories of second  language acquisition  I would say that the idea of a "mental L2 grammar" is the most important idea. "Acquisition" means modifying that mental object on the basis of experience. In reality, exceedingly little is known with any confidence regarding how that happens. Of course, the cognitive dimension the GPA is concerned not with this speculated mental entity, but with processes that convert acoustic cues into situation models and discursive acts, and also, processes concerned with producing speech which will have these effects in the interlocutors. We'd love to know as much about those processes as possible. Unlike mental grammars, we can be certain that such processes exist and must somehow develop, or be mimicked, as part of growing participation.

However, the cognitive dimension is secondary in the GPA to the sociocultural dimension. Communities ("cultures") use brains to construct and maintain themselves, but the brains, or at least their dispositions resulting from the moulding effects of community, are secondary to the social realities of communities. Thus, prior to cognitive processes are communities characterized by shared practices.

Here's a question: Given the primacy of the sociocultural dimension, what is the most important single concept of the GPA ?