Kuhn further developed his ideas regarding incommensurability in the 1980s and 1990s. In his unpublished manuscript "The Plurality of Worlds", Kuhn introduces the theory of "kind concepts:" sets of interrelated concepts that are characteristic of a time period in a science and differ in structure from the modern analogous kind concepts. These different structures imply different “taxonomies” of things and processes, and this difference in taxonomies constitutes incommensurability. This theory is strongly naturalistic and draws on developmental psychology to “found a quasi-transcendental theory of experience and of reality.”