Which psychologist believed it was impossible to understand the mind of a child without carefully considering the child's social and cultural interactions?

Lev Vygotsky.

The work of Lev Vygotsky has become the foundation of what has become Social Development Theory. His theories emphasize the fundamental role of social interaction in the development of cognition, as he believed that community plays a central role in the process of "making meaning". In other words, social learning tends to precede development.