In connection with this part of our subject, it may here be necessary to remark, that as the experience of all teachers may not be alike in the _first working_ of a newly applied principle,--the principle itself, when fully ascertained, is not on that account to be either belied or abandoned. There are many concurring circumstances, which may make an exercise that succeeds well in the hands of one person, fail in the hands of another; but to refuse credence to the principle itself, because he cannot as yet successfully apply it, is neither prudent nor wise. There are chemical experiments so exceedingly nice, and depending on so many varying circumstances, that they frequently fail in the hands of even good operators. But the chemical principles upon which they rest remain unchanged, although individual students may have not been able successfully to apply them. If their professor has but _once_ fairly and undoubtedly succeeded in ascertaining the facts on which the principle is based, their failure for a thousand times is no proof that the ascertained principle is really a fallacy. In like manner, any important principle in education, if once satisfactorily ascertained, is a truth in the science, and will remain a truth, whoever may believe or deny it. If it has been proved to produce certain effects in certain given circumstances, it will in all future times do the same, when the circumstances are similar. The inability, therefore, of a parent or teacher, to produce equal effects by its means, may be good enough proof of his want of skill, but it is no proof of the want of inherent power in the principle itself. The rings of Saturn which my neighbour's telescope has clearly brought to view, are not blotted from the heavens because my pocket glass has failed to detect them. It has been by attention to these, and similar rules, that all the secular arts have advanced to their present state; and the art of teaching must be perfected by similar means. There ought therefore to be a distinct object in view on the part of the teacher,--a specific end which he is to endeavour to arrive at in his intercourse with his pupil. For the attainment of this end, he must employ the best and the surest means that are in his power; for the same purpose, he ought honestly and fairly to apply the successive discoveries of science as they occur; and should never allow himself to abandon an exercise founded upon ascertained principles, merely because he at first finds difficulty in putting it in operation. CHAP. IV. _On the Establishment of Sound Principles in Education._ The application of the foregoing remarks to our present purpose, is a matter of great practical importance. It has indeed been owing chiefly to their having been hitherto overlooked, that education has been left in the backward state in which we at present find it.