But it may occur here, just as in other gelatinous substances, that an impression will vibrate throughout the brain, and send waves into other parts of it. In cases of this sort, although the immediate impression may fade out quickly, it does modify the cerebral mass; for the paths it makes there may remain, and become so many avenues through which the impression may be reproduced if they ever get excited again. And its liability to reproduction will depend of course upon the variety of these paths and upon the frequency with which they are used. Each path is in fact an associated process, the number of these associates becoming thus to a great degree a substitute for the independent tenacity of the original impression. As I have elsewhere written: Each of the associates is a hook to which it hangs, a means to fish it up when sunk below the surface. Together they form a network of attachments by which it is woven into the entire tissue of our thought. The 'secret of a good memory' is thus the secret of forming diverse and multiple associations with every fact we care to retain. But this forming of associations with a fact,--what is it but thinking _about_ the fact as much as possible? Briefly, then, of two men with the same outward experiences, _the one who thinks over his experiences most_, and weaves them into the most systematic relations with each other, will be the one with the best memory. But, if our ability to recollect a thing be so largely a matter of its associations with other things which thus becomes its cues, an important paedagogic consequence follows. _There can be no improvement of the general or elementary faculty of memory: there can only be improvement of our memory for special systems of associated things_; and this latter improvement is due to the way in which the things in question are woven into association with each other in the mind. Intricately or profoundly woven, they are held: disconnected, they tend to drop out just in proportion as the native brain retentiveness is poor. And no amount of training, drilling, repeating, and reciting employed upon the matter of one system of objects, the history-system, for example, will in the least improve either the facility or the durability with which objects belonging to a wholly disparate system--the system of facts of chemistry, for instance--tend to be retained. That system must be separately worked into the mind by itself,--a chemical fact which is thought about in connection with the other chemical facts, tending then to stay, but otherwise easily dropping out.