The legacy of defectology, Vygotsky's theoretical work on children with disabilities, still offers a useful approach in disability research. Vygotsky understood disability as an incongruence between the individual's psychological structure and the structure of cultural forms. The incongruence describes a dialectical relation between the person with a disability and the surrounding society. The aim of this study is to explore Vygotsky's concept of incongruence. A case example is presented. Through video observation of a child with severe cerebral palsy (CP) and cognitive visual impairment, the incongruence is analysed in two settings: in school together with a teacher and at home together with his mother. Use of vision, arm movement and a Rolltalk (a computer for communication) were the categories selected for analysis. The results demonstrate how the impact of the boy's cognitive visual impairment and CP differed depending on the interaction with the environment and on how his teacher and mother interacted with him. The dialectical dynamics of incongruence in each of the two settings created different levels of abilities/disabilities. It is argued that the dialectical psychological understanding of disability offers a useful approach to overcome dualistic understandings of the person with a disability and the surrounding society. (Contains 1 table.)