Anatomical conditions: various hypotheses. Obscurity of the question. Flechsig's theory.--Physiological conditions: are they cause, effect, or accompaniment? Chief factor: change in cerebral and local circulation.--Attempts at experimentation.--The oddities of inventors brought under two heads: the explicable and inexplicable. They are helpers of inspiration.--Is there any analogy between physical and psychic creation? A philosophical hypothesis on the subject.--Limitation of the question. Impossibility of an exact answer. 65 CHAPTER V. THE PRINCIPLE OF UNITY. Importance of the unifying principle. It is a fixed idea or a fixed emotion.--Their equivalence.--Distinction between the synthetic principle and the ideal, which is the principle of unity in motion: the ideal is a construction in images, merely outlined.--The principal forms of the unifying principles: unstable, organic or middle, extreme or semi-morbid.--Obsession of the inventor and the sick: insufficiency of a purely psychological criterion. 79 SECOND PART. THE DEVELOPMENT OF THE IMAGINATION. CHAPTER I. IMAGINATION IN ANIMALS. Difficulties of the subject.--The degree of imagination in animals.--Does creative synthesis exist in them? Affirmation and denials.--The special form of animal imagination is motor, and shows itself through play: its numerous varieties.--Why the animal imagination must be above all motor: lack of intellectual development.--Comparison with young children, in whom the motor system predominates: the roles of movements in infantile insanity. 93 CHAPTER II. IMAGINATION IN THE CHILD. Division of its development into four principal periods.--Transition from passive to creative imagination: perception and illusion.--Animating everything: analysis of the elements constituting this moment: the role of belief.--Creation in play: period of imitation, attempts at invention.--Fanciful invention. 103 CHAPTER III. PRIMITIVE MAN AND THE CREATION OF MYTHS.