I do not mean that your imagination cannot construct an object which has never before been in your experience as a whole, for the work of the imagination is to do precisely this thing. It takes the various images at its disposal and builds them into _wholes_ which may never have existed before, and which may exist now only as a creation of the mind. And yet we have put into this new product not a single _element_ which was not familiar to us in the form of an image of one kind or another. It is the _form_ which is new; the _material_ is old. This is exemplified every time an inventor takes the two fundamental parts of a machine, the _lever_ and the _inclined plane_, and puts them together in relations new to each other and so evolves a machine whose complexity fairly bewilders us. And with other lines of thinking, as in mechanics, inventive power consists in being able to see the old in new relations, and so constantly build new constructions out of old material. It is this power which gives us the daring and original thinker, the Newton whose falling apple suggested to him the planets falling toward the sun in their orbits; the Darwin who out of the thigh bone of an animal was able to construct in his imagination the whole animal and the environment in which it must have lived, and so add another page to the earth's history. THE TWO FACTORS IN IMAGINATION.--From the simple facts which we have just been considering, the conclusion is plain that our power of imagination depends on two factors; namely, (1) _the materials available in the form of usable images capable of recall_, and (2) _our constructive ability_, or the power to group these images into new _wholes, the process being guided by some purpose or end_. Without this last provision, the products of our imagination are daydreams with their "castles in Spain," which may be pleasing and proper enough on occasions, but which as an habitual mode of thought are extremely dangerous.