This article discusses C. Stumpf's concept of "Verschmelzung", as part of his philosophical and psychological theories including consonance since these are not generally known today. In particular, it is shown how Stumpf had elaborated his concept in certain respects from the Tonpsychologie (Stumpf, 1883, 1890) to his final writings. To allow a more adequate understanding of Stumpf, also his ideas on concordance and discordance are briefly summarized. Further, it is argued that "Verschmelzung" is related to, yet not identical with modern views of tonal fusion. In connection with Stumpf's criteria for integral hearing ("einheitliches Hören"), issues and experimental findings on tonal fusion are surveyed, and are interpreted also with respect to neurophysiological mechanisms that are possibly relevant to perception of fusion. It can be hypothesized that fusion — as being prerequisite to the experience of consonance — is a natural rather than a cultural phenomenon. Finally, Stumpf's assertion according to which the sensational attribute of roughness, and the perceptual quality of dissonance, can be separated from each other, is reexamined on the basis of experimental data.