 In the century after Stewart’s Elements the diversity among the phenomena that attention was expected to explain continued to grow, and it continued to include phenomena from across the psychological spectrum from perception, to thought, to action. By the end of the nineteenth century, in what was a crucial period in the development of scientific psychology, there were some psychologists, such as E.B. Titchener, who took the role of attention in perception and in ‘sensory clearness’ to be its most essential feature (see Titchener, 1908, 1910); others, such as Alexander Bain, who thought that the essential feature of attention was its role in action, (Bain, 1888); and a third group, of whom G.F. Stout was the most prominent example, who argued that the primary job for a theory of attention was to explain attention’s role in reflective thought (see Stout, 1891).