You will notice, as you read on in the book, that back references become numerous. Be advised to look these references up! They send you, in every case, to a particular page, so that their finding is easy, and you can refresh your memory without any great loss of time; though, for that matter, it will do no harm to glance over the section in which they occur. If you, on your part, want to refer to some past discussion, consult the index; it has been made fairly full, and is meant to be used. Questions and Exercises Many of the books to which you will be referred, now and later, have appeared in numerous editions, library and popular, English and American. The references are made so complete that you will easily find the corresponding passages in editions other than those used by the author. (1) Discuss the following definitions of science. If you have access to the books, read the passages in which the definitions occur; if not, do the best you can with your present knowledge. Try to see a reason even for the definitions that you cannot accept. (_a_) Science is perfected common sense (Huxley). The definition accords with the view of Spencer that science and ordinary knowledge are allied in nature, and that the one is but a perfected and extended form of the other. What is there in the common interests of these two men, or in the period in which they lived, to account for such a definition? (_b_) Reduced to its lowest terms, science is the observation of phenomena and the colligation of the results of observation into groups (Hill). (_c_) When may any subject be said to enter the scientific stage? I suppose when the facts of it begin to resolve themselves into groups; when phenomena are no longer isolated experiences, but appear in connection and order; when, after certain antecedents, certain consequents are uniformly seen to follow; when facts enough have been collected to furnish a basis for conjectural explanation, and when conjectures have so far ceased to be utterly vague, that it is possible in some degree to foresee the future by the help of them (Froude). (_d_) Mechanics is the science of motion; and its problem is to describe the motions that occur in nature _completely_ and _in the simplest way_ (Kirchhoff). Can this definition of mechanics be generalised, so that it applies to science at large? T. H. Huxley, Science Primers: Introductory, 1880, 18 f.; H. Spencer, The Genesis of Science, in Essays, ii., 1891, 8; A. Hill, Introduction to Science, 1900, 3; J. A. Froude, The Science of History, in Short Studies on Great Subjects, First Series, i., 1901, 13 f.; G. R. Kirchhoff, Vorlesungen über mathematische Physik: Mechanik, 1883, 1.