TODAY'S understanding of what constitutes music “theory” in the years between 1520 and 1640 differs somewhat from that of sixteenth- and seventeenth-century musicians themselves. We tend to include under the heading “Theory, 1520–1640” all books about music written during the period. Contemporaries, however, would be much more likely to label this literature simply “music” (musica) and to consider “theoretical or speculative music” (musica theorica/theoretica or speculativa) to constitute only a part of “music,” namely, the liberal-art part dedicated to the scientific contemplation of the universe of sound (what today we would call “acoustics”), that is, dealing principally with the measurement of intervals and closely related issues in the physics and metaphysics of music, the legacy of the Boethian curriculum of medieval universities. Following the Aristotelian distinction between “contemplating” (theoria) and “acting” (praxis), “theoretical music” was contrasted with “practical music” (musica practica) dedicated to various aspects of the craft of making music (what today we would call, in a curious reversal of the traditional usage, “music theory”), that is, dealing principally with the modal organization of the tonal material, counterpoint, mensural rhythm and notation, and closely related issues in the composition and performance of music, the legacy of the Guidonian curriculum of medieval cathedral choir schools. In 1537, a widely-read German theorist, Nicolaus Listenius, introduced a further category, that of “poetic music” (musica poetica), modelled on the Aristotelian notion of “making” (poiesis), to distinguish the theory of composition from that of performance. Poetic music, Listenius explained, “consists in making or producing, that is, in such labor that even afterwards, when the craftsman is dead, leaves a complete and fully finished work.” This usage gained considerable currency in Germany, where “poetic music” was considered to be either a third subdivision of “music” (in which case “practical music” would refer to the theory of performance), or as a subordinated part of “practical music.”