the child [themselves], since [their] eye is already taught to recognize differences of dimension, will see the error if only the objects are of fixed dimensions and highly colored. for this reason the succeeding objects contain a control of error in their very size and vivid colorings. a control of error of quite a different kind and of a much higher order is found in the material used for the multiplication table where the control consists in comparing the work itself with the answer, a comparison which necessitates a marked effort of the child's intellect and will and which henceforth places him amid true conditions of a conscious auto-education. the seeming distraction is revealed in its real essence by the happy expression of the chidren's serious faces animated by the keenest joy. the child, to all appearances, does nothing, but only for a minute: shortly [they] will speak and will tell us what is taking place within [them] and then an outburst of activity will carry [them] on a round of continuous explorations and discoveries. [they] is saved.

on the other hand, here are other children who experienced the same primitive phenomena, but they were surrounded with too many objects. at the moment of maturity, they felt themselves seized, forced, actually "bound with cords" to earth. a diminution of the intensity of the attention given to new objects, instability, and hence wariness are made manifest by the cessation of inner activity. the child gives way to lower tendencies, foolish laughter, and disorderly acts. [they] ask for more objects and still more objects, because [they] have remained imprisoned in the "vicious whirl of vanities" and they no longer feel the need of gaining relief from their ennui.
such is also the fate of an adult who, in life's chaos has committed a like error—they become undisciplined, weak,  and "is in danger of losing themselves." if someone does not help them and, tearing away all unnecessary objects, point out to them "their heaven," it will be difficult for him to have the energy to attain it by themselves.

(...) the "too much" weakens and retards progress. this has been proven again and again by all my collaborators. if, on the other hand, the material is insufficient and the natural auto-exercises are unable to lead up to that state of maturity which raises one, there is no outburst of that spontaneous phenomenon of abstraction which is the second step in that auto-education which goes forward in infinite progression.

***

discurso "my system of education"  dado por maria montessori en 1915.