_a._ In the difficulty of dictation and manipulation arising
from the different character of the faces of the bricks, and
the many positions which each brick can assume.
_b._ In the necessity of perfect balance.
_c._ In a clearer illustration of dimension. In the third
gift the parts were equal in height, breadth, and thickness;
in the fourth they are unequal, and therefore each dimension
is emphasized.
As to progression, the increase of difficulty suits the increase in
the child's power of comprehension and receptivity. He is being
developed thus far, not by rapid changes in material or greater
exercise in number, but by practice with differing forms, each one
bringing with it new knowledge and experience. The organs of
perception are being constantly made to grow by exercise with
intention. We are forming the scientific eye which can detect
differences ever after at a glance.