According to Jewish historian Joseph Telushkin in his A Code of Jewish Ethics, Volume 1, this description is attributed to Rabbi Chaim Soloveitchik, an early twentieth century Rabbi.
The story is told of a young scholar who approached the early twentieth-century rabbi Chaim Soloveitchik and asked the rabbi to grant him rabbinical ordination (semicha). Since ordination is normally given after testing the applicant's knowledge of the Shulchan Aruch, the sixteenth-century code of Jewish law, the rabbi began the examination by asking the young man to name the Shulchan Aruch's five volumes. Confused, the young student protested, "But there are only four volumes in the Shulchan Aruch." "No," the rabbi answered. "There is a fifth, unwritten volume. It is called common sense (seichel), and unless you know this volume, your knowledge of the other four volumes will not help you at all."
In an asterisk note, Telushkin notes that "the story, which may be apocryphal, is also attributed to other rabbis."