It is quite generally recognized that imagination is indispensable in all sciences; that without it we could only copy, repeat, imitate; that it is a stimulus driving us onward and launching us into the unknown. If there does exist a very widespread prejudice to the contrary--if many hold that scientific culture throttles imagination--we must look for the explanation of this view first, in the equivocation, pointed out several times, that makes the essence of the creative imagination consist of images, which are here most often replaced by abstractions or extracts of things--whence it results that the created work does not have the living forms of religion, of art, or even of mechanical invention; and then, in the rational requirements regulating the development of the creative faculty--it may not wander at will. In either case its end is determined, and in order to exist, i.e., in order to be accepted, the invention must become subject to preestablished rules. This variety of imagination being, after the esthetic form, the one that psychologists have best described, we may therefore be brief. A complete study of the subject, however, remains yet to be made. Indeed, we may remark that there is no "scientific imagination" in general, that its form must vary according to the nature of the science, and that, consequently, it really resolves itself into a certain number of genera and even of species. Whence arises the need of monographs, each one of which should be the work of a competent man.