Simon Stevin (1558/59–1620) was a mathematician, physicist and engineer with an extremely wide spectrum of interest. His music treatise titled Vande spiegeling der Singconst was written in ca. 1600 but remained unpublished until the end of the 19th century. This was the first European writing which defined the equal temperament with mathematical exactness, involving the ratio 12√2 for half-tones and chromatic steps. This paper tries to show parallels between his music theory and his general way of thinking. It describes the highly contradictory and changing practical requirements and demands a theory of tone systems and temperaments had to meet; and it explains how Stevin, sacrificing much of the practical needs of the musician of his day for the simplicity of theoretical construction, discovered ingeniously equal temperament which was justified and generally accepted only two centuries later, with the development of musical style.