Ever since William James employed the phrase “the _stream_ of thought” to describe mental phenomena, the whole trend of psychology has been altered. Not only did the wide-spread use of that phrase result in the giving up of many traditional beliefs in regard to things mental, but it also stimulated the most profitable investigations in psychological science. One of the most ancient of the beliefs which it demolished was the belief in Ideas as eternal, external, and immutable. According to the notion of a stream of consciousness (itself a variable conception), ideas are sensory images which show individual variations, which never are exactly repeated, and which have meaning only when they refer or lead to some concrete reality. This view, which finally replaced the ancient Platonic conception, is now held by practically all psychologists. Moreover, the influence of James’ teaching was such as to show that will, intellect, feeling, and the like were not separable parts of a mind (i. e., faculties) which were capable of acting independently, but were merely handy words to indicate the various qualities of the stream of thought. The stream flows swiftly,—call that impulsiveness; it flows again broadly and deeply with many glancing eddies,—call that deliberation; it flows once more with swift descent and foam,—call that strong feeling. Nevertheless, the stream of thought is one stream, and mental phenomena one and all specify its labile and fluid consistency. And now let us see to what further developments this striking conception has led.