Vygotsky's zone of proximal development and Krashen's i + 1: Incommensurable constructs; incommensurable theories

Second language scholars, in public research and in public discussions, have suggested that Krashen's construct of i + 1 is similar to Vygotsky's zone of proximal development and that it might therefore be feasible to integrate the two constructs in a way that would be productive for second language acquisition (SLA) research. After surveying publications relevant to the issue, we argue that this enterprise is futile, not only because the concepts are unrelatable, but also because they are rooted in incommensurable theoretical discourses. We also propose a way in which SLA research and theory might deal with incommensurability.