 According to Kepler, it is necessary to distinguish “sensible” from “pure” harmony. The first is to be found among natural, sensible entities, like sounds in music or rays of light; both could be in proportion to one another and hence in harmony. He resolves this matter by combining three of the Aristotelian categories: quantity, relation and, finally, quality. Through the function of the category of relation Kepler passes over to the active function of the mind (or soul). It turns out that two things can be characterized as harmonic if they can be compared according to the category of quantity. But the fact that at least two things are needed shows that the property of “being harmonic” is not a property of an isolated thing. Furthermore, the relationship between the things cannot be found in the things themselves either; rather, it is produced by the mind: “in general every relation is nothing without mind apart from the things which it relates, because they do not have the relation which they are said to have unless the presence of some mind is assumed, to relate one to another” (KGW 6, p. 212; Engl. trans., p. 291). This process takes place through the comparison of different sensible things with an archetype (archetypus) present in the mind.