Between the various types of imagination hitherto studied we have shown great differences as regards their external conditions. While the so-called forms of pure imagination, whence esthetic, mythic, religious, mystic creations arise, can realize themselves by submitting to material conditions that are simple and not very exacting, the others can become embodied only when they satisfy an _ensemble_ of numerous, inevitable, rigorously determined conditions; the goal is fixed, the materials are rigid, there is little choice of the appropriate means. If there be added to the inflexible laws of nature unforeseen human passions and determinations, as in political or social invention, or the offensive combination of opponents, as in commerce and war; then the imaginative construction is confronted with problems of constantly growing complexity. The most ingenious inventor cannot invent an object as a whole, letting his work develop through an immanent logic:--the early plan must be continually modified and readapted; and the difficulty arises not merely from the multiple elements of the problem to be solved, but from ceaseless changes in their positions. So one can advance only step by step, and go forward by calculations and strict examination of possibilities. Hence it results that underneath this thick covering of material and intellectual conditions (calculation, reasoning), spontaneity (the aptness for finding new combinations, "that art of inventing without which we hardly advance"[136]) reveals itself to few clear-sighted persons; but, in spite of everything, this creative power is everywhere, flowing like subterranean streams, a vivifying agency.