This article is devoted to examining the ontological foundations of computer-generated music scores. Specifically, we focus on the categorial question, i.e., the inquiry that aims to determine the kind of ontological category that musical works belong to. This task involves considerations concerning the existence and persistence conditions for musical works, and it has consequences for the determination of what it is to compose a musical work. Our contention is that not all the possible answers to the categorial question in the ontology of music are equally compatible with the hypothesis that creative music systems compose musical works. The thesis defended here is that musical Platonism is the proposal that best accommodates this hypothesis. We claim that musical Platonism is the answer to the categorial question that offers the most straightforward explanation for the possibility of considering creative music systems as genuinely composing musical works. Moreover, we uphold that the notion of creative-evaluative discovery as the characterization of what it is to compose a musical work entailed by Platonism is the simplest explanation of the process developed by a computer in producing musical works. For this purpose, we will take as empirical data the features of the Iamus computer, a system that produces musical works autonomously using evolutionary algorithms and following an evo-devo strategy. The works generated by this computer have been recorded by the London Symphony Orchestra and other renowned international soloists, and its impact has been notable in the literature (Ball, 2012; Coghlan, 2012; Berger, 2013).