How similar is similar?

In the first part of the paper a theoretical discussion is presented regarding the fundamental concept of similarity and its relation to cue abstraction and categorisation. It is maintained that similarity is by definition context-dependent and strongly interrelated to cue abstraction and categorisation. Emphasis is given to determining the musical surface that can act as a musically pertinent lowest level of abstraction on which similarity between musical entities can be measured. Then, each of these concepts is examined in more detail with respect to a number of research studies presented in the recent special issue of Musicæ Scientiæ on musical similarity (Discussion Forum 4A, 2007). Views claiming that a geometric piano-roll-like representation is the most appropriate choice for polyphonic pattern matching, or that musical repetition is structurally significant if at least fifty percent of a pattern is equivalent (i.e. if it is more similar than dissimilar), or that dramatic disparities between musical similarities and corresponding categories can be found in empirical studies, are critically re-examined with a view to clarifying the fundamental concept of similarity