 Relations form the group of objects whose study belongs to the general theory of relations. This theory represents an important part of logic and formal ontology, and its task in fact lies in the study of the laws governing the formal relations between objects and parts of objects. This aspect of the general theory of relations is comparable to Husserl’s pure logic. The relations of structure between psychical functions and the hierarchy within the domain of simple and complex functions obey what Stumpf refers to as the “laws of structure” between parts or aspects of a whole. Its task is to establish relations between phenomena, functions, and formations and to investigate the origins of complex relations in simple relations, fixed by means of definition, and to determine the structural laws to which they obey. To these Grundverhälnisse or fundamental relations belong, namely, the concepts of analogy, equality and fusion that Stumpf examines succinctly in the second volume of Tonpsychologie, as being the relations of logical and real dependency, the part-whole relations as well as the relations of real dependency between psychical functions. These relations are immanent to phenomena and are noticeable directly and without intermediary. Sensory contents are noticed or remarked in themselves, and each of these relations is, to again use the Scholastic expression, an ens rationis cum fundamento in re. The notion of fusion designates one of the forms of relations that structure the sensory material. It is defined as the relation between two phenomena (or contents of sensation) by virtue of which they do not merely form a sum, but a whole. Stumpf argues that the elements which fuse into the whole, for example quality and intensity of sounds in a melody, are not changed, but this relation of fusion establishes between these contents a narrower unity which is perceptible as a Gestalt. To use the well-known case of the melody, moments of quality and of intensity form in that case a unitary whole which is perceptible as a “quality of form,” and it is precisely the unitary character of this perception, the fact that it is perceptible in one stroke and immediately as unitary form, that Stumpf tries to account for with the aid of the notion of relation of fusion.