The passage from the imaginative to the rational phase may be slow or sudden. "For eight months," says Kepler, "I have seen a first glimmer; for three months, daylight; for the last week I see the sunlight of the most wonderful contemplation." On the other hand, Hauey drops a bit of crystallized calcium spar, and, looking at one of the broken prisms, cries out, "All is found!" and immediately verifies his quick intuition in regard to the true nature of crystallization. We have already indicated[114] the psychological reasons for these differences. Underneath all the reasoning, inductions, deductions, calculations, demonstrations, methods, and logical apparatus of every sort, there is something animating them that is not understood, that is the work of that complex operation--the constructive imagination. To conclude: The hypothesis is a creation of the mind, invested with a provisional reality that may, after verification, become permanent. False hypotheses are characterized as imaginary, by which designation is meant that they have not become freed from the first state. But for psychology they are different neither in their origin nor in their nature from those scientific hypotheses that, subjected to the power of reason or of experiment, have come out victorious. Besides, in addition to abortive hypotheses, there are dethroned ones. What theory was more clinging, more fascinating in its applications, than that of phlogiston? Kant[115] praised it as one of the greatest discoveries of the eighteenth century. The development of the sciences is replete with these downfalls. They are psychological regressions: the invention, considered for a time as adequate to reality, decays, returns to the imaginative phase whence it seems to have emerged, and remains pure imagination. IV Imagination is not absent from the third stage of scientific research, in demonstration and experimentation, but here we must be brief, (1) because it passes to a minor place, yielding its rank to other modes of investigation, and (2) because this study would have to become doubly employed with the practical and mechanical imagination, which will occupy our attention later. The imagination is here only an auxiliary, a useful instrument, serving: (1) In the sciences of reasoning, to discover ingenious methods of demonstration, stratagems for avoiding or overcoming difficulties. (2) In the experimental sciences for inventing methods of research or of control--whence its analogy, above mentioned, to the practical imagination. Furthermore, the reciprocal influence of these two forms of imagination is a matter of common observation: a scientific discovery permits the invention of new instruments; the invention of new instruments makes possible experiments that are increasingly more complicated and delicate.