"A child's different inner sensibilities enable him to choose from his complex environment what is suitable and necessary for his growth. They make the child sensitive to some things, but leave him indifferent to others. When a particular sensitiveness is aroused in a child, it is like a light that shines on some objects but not others, making of them his whole world."
The Secret of Childhood p. 42, Chap 7
A sensitive period refers to a transient state that children go through that is focussed upon one particular area.
Montessori had read about these periods of sensitivity in the development of animals, but soon realised that she was seeing similar qualities in the interests of the children.
"A child learns to adjust himself and make acquisitions in his sensitive periods. These are like a beam that lights interiorly or a battery that furnishes energy." (The Secret of Childhood p40)
She saw that during these periods the child could learn at a particularly intense rate and that such learning appeared to come very easily.
"At such a time everything is easy; all is life and enthusiasm. Every effort marks an increase in power." (Ibid p40).
The sensitive periods that she noted were not linear, i.e., they did not follow one after the other; some overlapped and some were continuous.
They included a sensitive period for order, refinement of the senses, language acquisition, walking and movement, small objects and involvement in social life.
Montessori teachers were therefore alerted to the existence of these periods of sensitivity and encouraged to observe them in the activities of the children.
Quotations
"A sensitive period refers to a special sensibility which a creature acquires in its infantile state, while it is still in a process of evolution. It is a transient disposition and limited to the acquisition of a particular trait. Once this trait, or characteristic, has been acquired, the special sensibility disappears."
The Secret