The manner in which trains of imagery and consideration follow each other through our thinking, the restless flight of one idea before the next, the transitions our minds make between things wide as the poles asunder, transitions which at first sight startle us by their abruptness, but which, when scrutinized closely, often reveal intermediating links of perfect naturalness and propriety--all this magical, imponderable streaming has from time immemorial excited the admiration of all whose attention happened to be caught by its omnipresent mystery. And it has furthermore challenged the race of philosophers to banish something of the mystery by formulating the process in simpler terms. The problem which the philosophers have set themselves is that of ascertaining, between the thoughts which thus appear to sprout one out of the other, _principles of connection_ whereby their peculiar succession or coexistence may be explained. But immediately an ambiguity arises: which sort of connection is meant? connection _thought-of_, or connection _between thoughts_? These are two entirely different things, and only in the case of one of them is there any hope of finding 'principles.' The jungle of connections _thought of_ can never be formulated simply. Every conceivable connection may be thought of--of coexistence, succession, resemblance, contrast, contradiction, cause and effect, means and end, genus and species, part and whole, substance and property, early and late, large and small, landlord and tenant, master and servant,--Heaven knows what, for the list is literally inexhaustible. The only simplification which could possibly be aimed at would be the reduction of the relations to a small number of _types_, like those which some authors call the 'categories' of the understanding. According as we followed one category or another we should sweep, from any object with our thought, in this way or in that, to others. Were _this_ the sort of connection sought between one moment of our thinking and another, our chapter might end here. For the only summary description of these categories is that they are all thinkable relations, and that the mind proceeds from one object to another by some intelligible path. =Is it determined by any laws?= But as a matter of fact, What determines the particular path? Why do we at a given time and place proceed to think of _b_ if we have just thought of _a_, and at another time and place why do we think, not of _b_, but of _c_? Why do we spend years straining after a certain scientific or practical problem, but all in vain--our thought unable to evoke the solution we desire? And why, some day, walking in the street with our attention miles away from that quest, does the answer saunter into our minds as carelessly as if it had never been called for--suggested, possibly, by the flowers on the bonnet of the lady in front of us, or possibly by nothing that we can discover?