The best possible sort of system into which to weave an object, mentally, is a rational system, or what is called a ‘science.’  Place the thing in its pigeon-hole in a classificatory series; explain it logically by its causes, and deduce from it its necessary effects; find out of what natural law it is an instance,—­and you then know it in the best of all possible ways.  A ‘science’ is thus the greatest of labor-saving contrivances.  It relieves the memory of an immense number of details, replacing, as it does, merely contiguous associations by the logical ones of identity, similarity, or analogy.  If you know a ‘law,’ you may discharge your memory of masses of particular instances, for the law will reproduce them for you whenever you require them.  The law of refraction, for example:  If you know that, you can with a pencil and a bit of paper immediately discern how a convex lens, a concave lens, or a prism, must severally alter the appearance of an object.  But, if you don’t know the general law, you must charge your memory separately with each of the three kinds of effect.

A ‘philosophic’ system, in which all things found their rational explanation and were connected together as causes and effects, would be the perfect mnemonic system, in which the greatest economy of means would bring about the greatest richness of results.  So that, if we have poor desultory memories, we can save ourselves by cultivating the philosophic turn of mind.

There are many artificial systems of mnemonics, some public, some sold as secrets.  They are all so many devices for training us into certain methodical and stereotyped ways of thinking about the facts we seek to retain.  Even were I competent, I could not here go into these systems in any detail.  But a single example, from a popular system, will show what I mean.  I take the number-alphabet, the great mnemonic device for recollecting numbers and dates.  In this system each digit is represented by a consonant, thus:  1 is t or d; 2, n; 3, m; 4, r; 5, l; 6, sh, j, ch, or g; 7, c, k, g, or qu; 8, f or v; 9, b or p; 0, s, c, or z.  Suppose, now, you wish to remember the velocity of sound, 1,142 feet a second:  t, t, r, n, are the letters you must use.  They make the consonants of tight run, and it would be a ‘tight run’ for you to keep up such a speed.  So 1649, the date of the execution of Charles I., may be remembered by the word sharp, which recalls the headsman’s axe.

Apart from the extreme difficulty of finding words that are appropriate in this exercise, it is clearly an excessively poor, trivial, and silly way of ‘thinking’ about dates; and the way of the historian is much better.  He has a lot of landmark-dates already in his mind.  He knows the historic concatenation of events, and can usually place an event at its right date in the chronology-table, by thinking of it in