1. From the above facts we can readily ascertain the cause, why some exercises employed in education are so much relished by the young, and so efficient in giving strength and elasticity to the mind; while others, on the contrary are so inefficient, so irksome, and sometimes so intolerable. Every exercise that tends to produce active thought,--the "reiteration of ideas,"--is natural, and therefore, not only promotes healthful mental vigour, but is also exciting and delightful; while, on the contrary, whenever the mind is fettered by the mere decyphering of words, or the repeating of sounds, without reiterating ideas, the exercise is altogether unnatural, and must of course be irritating to the child, and barren of good. 2. By a due consideration of the above principles, we see the reason why mental arithmetic, though it may not communicate any knowledge, is yet productive of considerable mental vigour. These exercises compel the young to a species of voluntary thought, the reiteration in the mind of the powers of numbers; and although the result of the particular calculations which are then made, may never again be of any service to the pupil, yet the consequent exercise of mind is beneficial. It should never be forgotten, however, that this exercise of mind upon _numbers_ is altogether an artificial operation, and is on this account, neither so efficient nor so pleasant as the reiteration of moral or physical truths. The same degree of mental exercise, brought into operation upon some useful fact, where the imagination as well as the understanding, can take a part, would at once be more natural, more efficient, more pleasant, and more useful. 3. From the nature and operation of the above principle, also, we can perceive in what the efficiency of Pestalozzi's "Exercises on Objects," consists.--When a child is required to tell you the colour and the consistence of milk, qualities which have all along been familiar to him, it conveys to him no knowledge; but it excites to observation and active thought,--to the "reiteration of ideas;"--and for this reason it is salutary. But it is still equally true, as in the former case, that the same degree of mental exercise, brought into operation upon some useful practical truth, would be at least equally useful as a mental stimulant, and much more beneficial as an educational exercise.