Much of what is attributed to imagination in very young children rests essentially on the formation of such vague concepts, on the inability to combine constant qualities into sharply defined concepts. When, in the twenty-third month, the child holds an empty cup to his mouth and sips and swallows, and does it repeatedly, and with a serene, happy expression, this "play" is founded chiefly on the imperfect notion "filled cup." The child has so often perceived something to drink, drinking-vessel, and the act of drinking, in combination with one another, that the one peremptorily demands the other when either appears singly; hence the pleasure in pouring out from empty pitchers into empty cups, and in drinking out of empty cups (in second to fifth years). When adults do the same in the play of the theatre, this action always has a value as language, it signifies something for other persons; but with the child, who plays in this fashion entirely alone, the pleasure consists in the production of familiar ideas together with agreeable feelings, which are, as it were, crystallized with comparative clearness out of the dull mass of undefined perceptions. These memory-images become real existences, like the hallucinations of the insane, because the sensuous impressions probably impress themselves directly--without reflection--upon the growing brain, and hence the memory-images of them, on account of their vividness, can not always be surely distinguished from the perceptions themselves. Most of the plays that children invent of themselves may be referred to this fact; on the other hand, the play of hide-and-seek (especially in the seventeenth and eighteenth months), and, nearly allied to this, the hunting after scraps of paper, bits of biscuit, buttons, and other favorite objects (in the fifteenth month), constitute an intellectual advance. By practice in this kind of seeking for well-known, purposely concealed objects, the intelligence of little children can easily be increased to an astonishing degree, so that toward the end of the second year they already understand some simple tricks of the juggler; for example, making a card disappear. But after I had discontinued such exercises for months, the ordinary capacity for being duped was again present.