In many subjects, the difficulties of fixing upon the proper Text-book are not inconsiderable. The mere reputation of a book may be great, and well-founded; and yet the merits may not be of the kind that fits it for the commencing student. Such conditions as the following must be taken into account. The Form or Method should be of a high order: this we shall have occasion to illustrate under the next head. It should be abreast: of the time, on its own subject. It should be moderately full, without being necessarily exhaustive in detail. It is on this point that the cheap primers of the present day are mainly defective. They state general ideas, and lay down outlines; but they do not provide sufficiently expanded illustration to stamp these on the mind of the learner. A shilling primer is really a more advanced book than one on a triple scale, that should embrace the same compass of leading ideas. As a farther condition, the work chosen should not have so much of individuality as to fail in the character of representing the prevailing views. The greatest authors often err on this point; and, while a work of genius is not to be neglected, it may, for this reason, have to take the second place in the order of study. Newton's _Principia_ could never be a work suited for an early stage of mathematical study. Lyell's Geology has been a landmark in the history of the subject; but it is not cast in the form for a beginner in Geology. It is, in its whole plan, argumentative; setting up and defending a special thesis in Geology; the facts being arrayed with that view. Many other great works have assumed a like form; such are Malthus on Population, Grove's Correlation of Physical Forces, Darwin's Origin of Species. Even expressly didactic works are often composed more to bring forward a peculiar view, than from the desire to develop a subject in its due proportions. Locke's Essay on the Understanding does not propose to give a methodical and exhaustive handling of the Powers of the Mind, or even of the Intellect. That was reserved for Reid. The question as between old writers and new, would receive an easy solution upon such grounds as the foregoing, were it not for the sentiment of veneration for the old, because they are old. If an ancient writer retains a place by virtue of surpassing merits, as against all subsequent writers, his case is quite clear. In the nature of things, this must be rare: if there be an example, it is Euclid; yet his position is held only through the mutual jealousy of his modern rivals.