Second-language scholars 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 way that would be productive for second-language acquisition (SLA) research. Article argues that this is futile, not only because concepts are unrelatable, but also because they are rooted in incommensurable theoretical discourses. (Author/JL)