 Although introspective psychologists were able to build scientific consensus on some issues concerning sense experience—issues such as the limits of sensory perception in various modalities and some of the contours of variation in sensory experience—by the early 20th century it was becoming clear that on many issues consensus was elusive. The most famous dispute concerned the existence of “imageless thought” (see the discussion of the imageless thought controversy in the entry  mental imagery;  see also Humphrey 1951; Kusch 1999); but other topics proved similarly resistant such as the structure of emotion or “feeling” (James 1890 [1981]; Külpe 1893 [1895]; Wundt 1896 [1902]; Titchener 1908 [1973]) and the experiential changes brought about by shifts in attention (Wundt 1896 [1902]; Pillsbury 1908; Titchener 1908 [1973]; Chapman 1933).