It may be that the overworn analogy between the history of philosophy and a river system has still a value, and if so, it is a value beyond that usually claimed for it. The grosser likenesses between the stream of history and the stream of geography have been, it is true, too often pointed out, while the subtler similarities are neglected. For instance, there are points in both kinds of stream where it runs deep, others where it is shallow; at one point it may run swiftly, as through a gorge, and at another the valley may be wide and the stream broad, and sluggish. However, whereas in the stream of science the running waters often purify themselves, in that of philosophy there are many stagnant pools where the current of criticism has never set in. Further, there are discernible here and there canalizations by which arid plains are artificially irrigated. And it must be admitted that, at other points, outhouses have been built over the stream and its waters sullied.