§ 123. (1) For the most, the best, and the mightiest things that we know we are indebted to Life itself. The sum of perceptions which a human being absorbs into himself up to the fourth or fifth year of his life is incalculable; and after this time we involuntarily gain by immediate contact with the world countless ideas. But especially we understand by the phrase "the School of Life," the ethical knowledge which we gain by what happens in our own lives. ——If one says, _Vitæ non scholæ discendum est_, one can also say, _Vita docet_. Without the power exercised by the immediate world our intelligence would remain abstract and lifeless.—— § 124. (2) What we learn through books is the opposite of that which we learn through living. Life _forces_ upon us the knowledge it has to give; the book, on the contrary, is entirely passive. It is locked up in itself; it cannot be altered; but it waits by us till we wish to use it. We can read it rapidly or slowly; we can simply turn over its leaves——what in modern times one calls reading;——we can read it from beginning to end or from end to beginning; we can stop, begin again, skip over passages, or cut them short, as we like. To this extent the book is the most _convenient_ means for instruction. If we are indebted to Life for our perceptions, we must chiefly thank books for our understanding of our perceptions. We call book-instruction "dead" when it lacks, for the exposition which it gives, a foundation in our perceptions, or when we do not add to the printed description the perceptions which it implies; and the two are quite different. § 125. Books, as well as life, teach us many things which we did not previously intend to learn directly from them. From foreign romances e.g. we learn, first of all, while we read them for entertainment, the foreign language, history or geography, &c. We must distinguish from such books as those which bring to us, as it were accidentally, a knowledge for which we were not seeking, the books which are expressly intended to instruct. These must (_a_) in their consideration of the subject give us the principal results of any department of knowledge, and denote the points from which the next advance must be made, because every science arises at certain results which are themselves again new problems; (_b_) in the consideration of the particulars it must be exhaustive, i.e. no essential elements of a science must be omitted. But this exhaustiveness of execution has different meanings according to the stand-points of those for whom it is made. How far we shall pass from the universality of the principal determinations into the multiplicity of the Particular, into the fulness of detail, cannot be definitely determined, and must vary, according to the aim of the book, as to whether it is intended for the apprentice, the journeyman, or the master; (_c_) the expression must be precise, i.e. the maximum of clearness must be combined with the maximum of brevity.