D. Kellerman (s.v. twylk, Theological Dictionary of the Old Testament, ed. G. Johannes Botterwech and Helmer Ringgren [Grand Rapids, MI: Eerdmans, 1974–2001], vol. 7, p. 181) suggests that heart and kidneys together are meant to encompass the total interior life by representing both the upper and lower parts of the trunk and (respectively) the rational and emotional faculties.
