Not so the instructor of youth. The material operated on here is of a nature too subtle to be shaped and fashioned by the undeviating routine of any such mechanical operations. The process necessary to sharpen one intellect may terrify and confound another. The means which in one instance serve to convince, serve in other cases to confuse. The illustration which to one is a ray of light, is to another only "darkness visible." Mind is not, like matter, fixed and uniform in its operations. The workman who is to operate upon a substance so subtle and so varying must not be a man of _one idea_--who knows one thing, and nothing more. It is not true in mind, as in matter, that perfection in the knowledge of one particular point is gained by withdrawing the attention from every other point. All truth and all knowledge are affiliated. The knowledge of arithmetic is increased by that of algebra, the knowledge of geography by that of astronomy, the knowledge of one language by knowing another. As no one thing in nature exists unconnected with other things, so no one item in the vast sum of human knowledge is isolated, and no person is likely to be perfectly acquainted with any one subject who confines his attention with microscopic minuteness to that subject. To understand thoroughly one subject, you must study it not only in itself, but in its relations. To know one thing well you must know very many other things. Let us return then to the point from which we set out, namely: that one important difference between the teacher and other operatives arises from the difference in the objects on which they operate. The one operates upon matter, the other upon mind. The one attains perfection in his art by a process which in the other would produce an ignoramus, a bungler, a narrow-minded, conceited charlatan. Hence the necessity on the part of those who would excel in the profession of teachers, of endeavoring continually to enlarge the bounds of their knowledge. Hence the error of those who think that to teach anything well it is necessary to know only that one thing. That young woman who undertakes to teach a primary school, or even an infant class, has mistaken her calling if she supposes that because she has to teach only the alphabet or the "table card," she has therefore no need to know many other things. There are some things which every teacher needs. Every teacher needs a cultivated taste, a disciplined intellect, and that enlargement of views which results only from enlarged knowledge.