During sensitive periods opportunity exists for optimal development, when it is easy to learn. Learning during the sensitive period is as thorough and complete as it ever can be. Without the right stimulation at the right time, the child's development suffers what Maria Montessori called a dropped stitch. The garment of the child's development is not quite as strong and well-developed as it might have been.
E.M. Standing (Maria Montessori: Her Life and Work, 1957) lists these as the characteristics of normalization: love of order, love of work, spontaneous concentration, attachment to reality, love of silence and of working alone, sublimation of the possessive instinct, power to act from real choice, obedience, independence and initiative, spontaneous self-discipline, and joy. Montessori believed that these are the truly "normal" characteristics of childhood, which emerge when children's developmental needs are met.
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Maria Montessori was trained as a scientist. She observed children with an open mind, unprejudiced by educational dogmas of the day. When she saw instances of profound concentration and seemingly endless repetition, she began to realize that children were working to enhance their own development, not only to accomplish the outer goal of getting the cylinders in the correct hole or the blocks forming the right pattern.
Jean-Jacques Rousseau - Wikipedia
Only "normalised" children, aided by their environment, show in their subsequent development those wonderful powers that we describe: spontaneous discipline, continuous and happy work, social sentiments of help and sympathy for others. . . . An interesting piece of work, freely chosen, which has the virtue of inducing concentration rather than fatigue, adds to the child's energies and mental capacities, and leads him to self-mastery. . . . One is tempted to say that the children are performing spiritual exercises, having found the path of self-perfectionment and of ascent to the inner heights of the soul. (Maria Montessori, The Absorbent Mind, 1949)