Montessori observed four distinct periods, or “planes”, in human development, extending from birth to 6 years, from 6 to 12, from 12 to 18, and from 18 to 24.

The first plane extends from birth to around six years of age. During this period, Montessori observed that the child undergoes striking physical and psychological development. The first-plane child is seen as a concrete, sensorial explorer and learner engaged in the developmental work of psychological self-construction and building functional independence. Montessori introduced several concepts to explain this work, including the absorbent mind, sensitive periods, and normalization.

Montessori described the young child’s behavior of effortlessly assimilating the sensorial stimuli of his or her environment, including information from the senses, language, culture, and the development of concepts with the term “absorbent mind”. She believed that this is a power unique to the first plane, and that it fades as the child approached age six. Montessori also observed and discovered periods of special sensitivity to particular stimuli during this time which she called the “sensitive periods”. In Montessori education, the classroom environment responds to these periods by making appropriate materials and activities available while the periods are active in each individual young child. She identified the following periods and their durations

  • Acquisition of language—from birth to around 6 years old
  • Interest in small objects—from around 18 months to 3 years old
  • Order—from around 1 to 3 years old
  • Sensory refinement—from birth to around 4 years old
  • Social behavior—from around 2½ to 4 years old

Finally, Montessori observed in children from three to six years old a psychological state she termed “normalization”. Normalization arises from concentration and focus on activity which serves the child’s developmental needs, and is characterized by the ability to concentrate as well as “spontaneous discipline, continuous and happy work, social sentiments of help and sympathy for others.”