It is also here worthy of remark,--and it is perhaps another proof of the efficiency of the several exercises before enumerated as imitations of Nature,--that they all, more or less, embody a portion of this principle of double duty performed by the mind. In each of them, when properly conducted, the pupil is compelled to speak, and to think at the same moment. Not a little of their efficiency and value indeed, may be attributed to this circumstance. In the catechetical exercise, for example, it is not difficult to trace its operation. For in the attempt of the child to answer a question previously put to him, the teacher will be at no loss to perceive the mind gradually acquiring an ability to think of the original question and of the ideas contained in the subject from which he has selected his answer, at the very moment he is giving it utterance. And a knowledge of the fact should excite teachers in general, so to employ this exercise as to produce this effect.--The analytical exercise also, in its whole extent, calls into operation the working of this principle, whether employed synthetically or analytically. When children are employed with the analytical exercise proper,--as in tracing a practical lesson backwards to the subject or circumstance from which it has been drawn, and in attaching that circumstance to the story or class of truths to which it belongs; or when, as in the "Analysis of Prayer," a text of Scripture has to be classified according to its nature, among the several parts into which prayer is divided;--in all these cases, there is this same double operation of the mind, searching and comparing one set of ideas, while the pupil is employed in giving expression to others.