The main aim of this article-based dissertation is to construct and articulate a dialectical metatheoretical architecture for the study of second language development termed Purposive-Historical Systems Theory (PHiST) invoking, inter alia, the axiomatic imperatives of dynamic systems theory (DST), Vygotskian cultural-historical theory (CHT), and Pepperian root metaphor theory (RMT). Specifically, it primarily purports, first, to demonstrate the tenability of a dialectical synthesis of DST and CHT; second, to philosophize on the fundamental contours of PHiST; third, to apply a novel metatheoretical perspective, as a quintessential example, to reconceptualizing the seminal construct of the Zone of Proximal Development (ZPD). It is argued that the prevailing schism in the second language acquisition (SLA) discipline between the social and cognitive families of theories partly stems from the dearth of a systematic and principled scrutinization of the bedrock assumptions, primitive predicates, and guiding principles anchoring SLA theories. This research shows the commensurability of the attentive axioms and predicates underpinning DST and CHT. It is suggested that a dialectical integration of CHT and DST - which by implication mandates consilience of the world hypotheses of organicism and contexualism propounded by RMT - eschews the dualistic fallacy of ontologically reducing L2 development wholesale to the social (i.e., objective) or to the mental (i.e., subjective) reality. It is proposed that constitutive relationalism between DST and CHT lends credence to PHiST for nuancing the sociogenesis and psychogenesis of L2 development as a dynamic, complex, emergent, and purposive human-culture-centered process co-constructed across the lived timescales of an L2 learner. It is contended that the causal heteronomy of the L2 developmental system on the agentive L2 learner’s purposive significations, affective impressions, and experiential ideations, coupled with the realized idiosyncrasies of the socioculturally-fashioned umwelt and its meaning-saturated affordances, engenders, and is actualized qua, dialogic speeching activities. Thereby, a speeching activity concatenates lived and perceived past timescales and prospective and conceived future timescales. The proposed metamodel has far-flung implications for theoretical and empirical problems pertaining to language learning, teaching, and assessment.