What is the material, the mental content, out of which imagination
builds its structures?

IMAGES THE STUFF OF IMAGINATION.--Nothing can enter the imagination the
elements of which have not been in our past experience and then been
conserved in the form of images. The Indians never dreamed of a heaven
whose streets are paved with gold, and in whose center stands a great
white throne. Their experience had given them no knowledge of these
things; and so, perforce, they must build their heaven out of the images
which they had at command, namely, those connected with the chase and
the forest. So their heaven was the "happy hunting ground," inhabited by
game and enemies over whom the blessed forever triumphed. Likewise the
valiant soldiers whose deadly arrows and keen-edged swords and
battle-axes won on the bloody field of Hastings, did not picture a
far-off day when the opposing lines should kill each other with mighty
engines hurling death from behind parapets a dozen miles away. Firearms
and the explosive powder were yet unknown, hence there were no images
out of which to build such a picture.

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.