When envisaging new digital instruments, designers do not have to limit themselves to their sonic capabilities (which can be absolutely any), not even to their algorithmic power; they must be also especially careful about the instruments' conceptual capabilities, to the ways instruments impose or suggest to their players new ways of thinking, new ways of establishing relations, new ways of interacting, new ways of organizing time and textures; new ways, in short, of playing new musics. This article explores the dynamic relation that builds between the player and the instrument, introducing concepts such as efficiency, apprenticeship and learning curve It aims at constructing a framework in which the possibilities and the diversity of music instruments as well as the possibilities and the expressive freedom of human music performers could start being evaluated.