The intention [of this project] is to furnish a psychology that shall be a natural science.
Sigmund Freud, 1895
When Freud began this manuscript project, psychology was just emerging as a distinct scientific field. Mental phenomena--long the province of philosophers--were now being approached in much the same way natural scientists approached physical phenomena.
By “natural science,” Freud meant disciplines like biology and physics that deal with matter by using measurement and experimentation.