Most social research today fails to show in just which ways the processes studied are social in their very nature. Thus, we may ask, "In which way are thinking and reasoning about an observed phenomenon social as opposed to individual in nature?" The general answer will be that children or students participate in the 'social construction" of some discourse or knowledge and then internalize the result of the external process. Here, the social is explained in a trivial sense, as a context within which some but not necessarily all higher functions develop. Vygotsky thought differently, suggesting that all higher psychological functions always exist as social relations. This chapter shows how cultural practices such as reasoning first appear in the life of a person as social relations with others before they become part of the personal repertoire of behaviors. That is, reasoning first is a form of collective behavior in which the child already takes part and only later is observable as individual behavior. Case materials from a second-grade mathematics class are used for the purpose of grounding and exemplifying the proposed approach.