ABSTRACT
OUR historical task falls into two parts which must be kept strictly apart. The first consists of a presentation of the scientific and practical utilization of the fiction in the course of the historical development of science, the second of a description of the gradual discovery and theory of these fictions. In the introduction to this book we laid down the principle that logical theory must take its empirical data from scientific practice. Our enumeration of the fictions found in present-day science showed quite clearly that they do actually occur. It is, however, also one of the tasks of logic to follow, in connection with the general history of science, the gradual evolution of scientific practice in general and that of each scientific method in particular. Hitherto this task has always been confused with another, that of describing the gradual development of logical theory. The slow development of a cognitive instrument presents features of the greatest interest, and a history of method is consequently an indispensable complement and preliminary to any methodology.
