[22] I have recently (1886) propounded these views in a pamphlet _Beiträge zur Analyse der Empfindungen_. Avenarius, with whom I recently became acquainted, approaches my point of view (_Philosophie als Denken der Welt nach dem Princip des kleinsten Kraftmasses_, 1876). Hering, too, in his treatise upon Memory (_Almanach der Wiener Akademie_, 1870, p. 258; also published in Nos. 6 and 7 of _The Open Court_), and J. Popper in his beautiful book, "The Right to Live and the Duty to Die" (Leipsic 1878, p. 62), have advanced similar thoughts. Compare also my paper _Ueber die ökonomische Natur der physikalischen Forschung_ (_Almanach der Wiener Akademie_, 1882, p. 179, note). Finally let me also refer here to the introduction to W. Preyer's _Reine Empfindungslehre_, and to Riehl's _Freiburger Antrittsrede_, p. 14. I should probably have to cite much additional matter that is in some way related to my line of thought if I possessed a more extensive bibliographical knowledge. XIII. Science always arises through a process of adaptation of thoughts to a certain department of experience. The results of this process are thought-elements, which represent the entire department. The result, of course, is different according to the character and extent of the province surveyed. If the province of experience in question is extended, or if several provinces hitherto separated become united, the traditional, familiar thought-elements no longer suffice for the province thus extended. In the struggle of acquired habit with the effort after adaptation, _problems_ arise, which disappear when the adaptation is completed, to give way to others that have sprung up in the mean time.