系 is read xì here and means 'attached': the statements were attached to the hexagrams. The text is one of the Ten Wings (
十翼), the commentaries that tradition credits to Confucius, and it comes in two halves, so citations name the half: 《
周易·
系辞上》 or 《
周易·
系辞下》. It is the source of
触类旁通,
物以类聚 and
仁者见仁，
智者见智.
