INTRODUCTION The work to be described in this paper is a direct continuation of the work done by the author in 1897-1898 and described in Monograph Supplement No. 8 of the _Psychological Review_ under the heading, ‘Animal Intelligence; an Experimental Study of the Associative Processes in Animals.’[23] This monograph affords by far the best introduction to the present discussion, and I shall therefore assume an acquaintance with it on the part of my readers. It will be remembered that evidence was there given that ordinary mammals, barring the primates, did not infer or compare, did not imitate in the sense of ‘learning to do an act from seeing it done,’ did not learn various simple acts from being put through them, showed no signs of having in connection with the bulk of their performances any mental images. Their method of learning seemed to be the gradual selection of certain acts in certain situations by reason of the satisfaction they brought. Quantitative estimates of this gradualness were given for a number of dogs and cats. Nothing has appeared since the ‘Experimental Study’ to negate any of these conclusions in the author’s mind. The work of Kline and Small[24] on rodents shows the same general aspect of mammalian mentality. Adult human beings who are not notably deficient in mental functions, at least all such as psychologists have observed, possess a large stock of images and memories. The sight of a chair, for example, may call up in their minds a picture of the person who usually sits in it, or the sound of his name. The sound of a bell may call up the idea of dinner. The outside world also is to them in large part a multitude of definite percepts. They feel the environment as trees, sticks, stones, chairs, tables, letters, words, etc. I have called such definite presentations ‘free ideas’ to distinguish them from the vague presentations such as atmospheric pressure, the feeling of malaise, of the position of one’s body when falling, etc. It is such ‘free ideas’ which compose the substance of thought and which lead us to perhaps the majority of the different acts we perform, though we do, of course, react to the vaguer sort as well. I saw definitely in writing the last sentence the words ‘majority of the different acts’ and thought ‘we perform’ and so wrote it. I see a bill and so take check book and pen and write. I think of the cold outside and so put on an overcoat. This mental function ‘having free ideas,’ gives the possibility of learning to meet situations properly by thinking about them, by being reminded of some property of the fact before us or some element therein.