This theoretical text focuses on the development of written language as a higher mental function. It aims to highlight the ideational representation of the word as a minimum unit of analysis for understanding the development of written language. By affirming that writing is to write ideas, this analysis is supported by the historical-cultural perspective of human development. Throughout this essay, and taking as its axis the periodization of the formation of concepts in Vygotsky, we analyze the appropriation of writing from the concept of word. We point out that the psychic act embodied in writing is not based on the treatment or on the external use that the child makes of the word orally, but in its representation as a concept. We argue, therefore, that the formation of the concept of word, both in its semantic dimension and in its morphology and phonetics, is imposed as tasks of a developmental education from early childhood education, thus building the substrate for the appropriation of writing itself, in the subsequent school years.