ing exercise becomes a factor of development. The experiment is
repeated with a number of children, and thus the dimensions of a
series of objects are established.

It is the same with colors and with every kind of _quality_. In order
that a quality should be felt to such a degree as to fix the
attention, a certain extension and a certain intensity of the stimulus
are necessary, which may be _determined_ by the degree of psychical
reaction shown by the child; as, for instance, the minimum chromatic
extension sufficient to attract the attention to the colored tablets,
etc. Quality, therefore, is determined by a psychical experiment
demonstrating the activity it produces in a child, who will continue
the exercise with the same object for a long time, thus elaborating a
phenomenon of internal development, of self-formation.

Among the characteristics of the objects, one must be pointed out,
which demands the highest degree of activity in the intelligence: they
contain in themselves _control of error_.

To make the process one of self-education, it is not enough that the
stimulus should call forth activity, it must also direct it. The child
should not only persist for a long time in an exercise; he must
persist without making mistakes. All the physical or intrinsic
qualities of the objects should be determined, not only by the
immediate reaction of attention they provoke in the child, but also by
their possession of this fundamental characteristic, the control of
error, that is to say the power of evoking the effective collaboration
of the highest activities (comparison, judgment). For instance, one of
the first objects which attract the attention of the child of three
years old, the solid insets (a series of cylinders of various
dimensions to be placed in or taken out of a block with corresponding
holes) contains the most mechanical control, because if a single
mistake be made in placing the cylinders, one of these must be left
out at the end of the exercise. Hence a mistake is an obstacle only to
be overcome by correction, for without it the exercise cannot be
completed. On the other hand, the correction is so easy that the child
makes it himself. The little problem suddenly presenting itself to the
child, almost like the unexpected object of a jack-in-the-box, has
"interested" him.

It is, however, noteworthy that the "problem" thus presented is not in
itself the stimulus to interest; it is not that which incites to