The difference between the two types is found in the nature of the need or desire stimulating the invention, and secondly in the nature of the materials employed. Others have confounded two distinct things—liberty of imagination, which belongs rather to esthetic creation, and quality and power of imagination, which may be identical in both cases.
I have questioned certain inventors very skillful in mechanics, addressing myself to those, preferably, whom I knew to be strangers to any preconceived psychological theory. Their replies agree, and prove that the birth and development of mechanical invention are very strictly like those found in other forms of constructive imagination. As an example, I cite the following statement of an engineer, which I render literally:
"The so-called creative imagination surely proceeds in very different ways, according to temperament, aptitudes, and, in the same individual, following the mental disposition, the milieu.
"We may, however, as far as regards mechanical inventions, distinguish four sufficiently clear phases—the germ, incubation, flowering, and completion.
"By germ I mean the first idea coming to the mind to furnish a solution for a problem that the whole of one's observations, studies, and researches has put before one, or that, put by another, has struck one.
"Then comes incubation, often very long and painful, or, again, even unconscious. Instinctively as well as voluntarily one brings to the solution of the problem all the materials that the eyes and ears can gather.
"When this latent work is sufficiently complete, the idea suddenly bursts forth, it may be at the end of a voluntary tension of mind, or on the occasion of a chance remark, tearing the veil that hides the surmised image.
"But this image always appears simple and clear. In order to get the ideal solution into practice, there is required a struggle against matter, and the bringing to an issue is the most thankless part of the inventor's work.
"In order to give consistence and body to the idea caught sight of enthusiastically in an aureole, one must have patience, a perseverance through all trials. One must view on all sides the mechanical agencies that should serve to set the image together, until the latter has attained the simplicity that alone makes invention viable. In this work of bringing to a head, the same spirit of invention and imagination must be constantly drawn upon for the solution of all the details, and it is against this arduous requirement that the great majority of inventors rebel again and again.
"This is then, I believe, how one may in a general way understand the genesis of an invention. It follows from this that here, as almost everywhere, the imagination acts through association of ideas.