This heading includes all those manifold cases in which mental presentation is accepted as actual, whether they are concerned with genuine memory pictures or merely some mental content worked up for the occasion. When a fever patient sees an absent friend bodily before him, we call this imagination as well as when he seems to see absurd or grotesque things. The distinguishing feature is whether the illusion appears as a substitute for reality, as in dreams, delirium, hypnosis, and insanity, or as the product of conscious self-deception (K. Lange’s “bewusste Selbsttäuschung,” P. Souriau’s “illusion volontaire”), where the knowledge that we have ourselves produced the illusion prevents actual substitution, as in play and art. Transition from one to the other of these states is easy. The dreamer or fever patient may have the feeling that the fantasy in which he lives and suffers is, after all, an unreal thing; and, on the other hand, illusion is often so strong for playing children and artists that it forms a perfect substitute for reality. Just now we are concerned with conscious illusion only. In inquiring how far experimentation is involved in it we must bear in mind that there are two sides to all illusion, one which has reference to an internal image, and the other blending with external phenomena. It is a distinction similar to that between hallucination and illusion in the narrower pathological sense. The illusion which depends on internal images can, as we have seen, elevate actual memories as well as convertible mental contents to the appearance of reality. So we see that the two kinds of mental activity included under the name imagination are intimately and variously related, while neither alone covers the entire ground. Enjoyment of play with memory pictures which are more than ordinarily faithful to fact is practised almost exclusively by adults, and more especially by the aged. The psychological condition of this is that by means of strong concentration of attention on the mental picture (we are reminded again of hypnosis) the actual present is thrown very much into the background, and the past thus conjured up loses many of the usual characteristics of a past, since the memory picture, from lacking the usual projection, assumes the expression of reality. The following is a beautiful example of this distinction between mere reflective memory and playful illusion where the differentiation was gradually built up. When Goethe as a mature man took up his Faust manuscript, he said to himself, “I thought over this subject a great deal ten years ago; but that would be only a memory.” Yet as he lost himself in the joyful or painful memories connected with that period, he came to ignore the fact that they were long past, and more and more substituted them for the present, which in its turn became gradually submerged. These words reveal the play of his imagination: