In this paper, the author maintains that the term “ideal border”, used by Roman Ingarden several times in his writing on musical perception, has more to offer than its face value suggests. The notion can have multiple meanings and at fi rst it may seem that it only underlines incoherence of the philosopher’s concept. However, the author of this paper thinks that the notion of “ideal border” points towards diffi culties impossible to overcome in defi ning a musical work — provided that the notion is taken seriously, i.e. as a vital element of Ingarden’s theory, and not as a fi tting metaphor. The notion of “ideal border” in relation to a musical work and its perception, as the author believes, refl ects an ambiguous state of the ontological grounding of a musical work, while confi rming the proposed character of Ingarden’s overall theory in an inspiring way.