The thesis consists of a critical edition of a previously unpublished treatise by the celebrated Muslim mystic and thinker, Muhyi 'd-Din b. 'Arabi (1165-1240), entitled "Mahiyat al-Qalb", or "The Quiddity of the Heart", thus adding yet another of his many works to the comparatively short list of those already published. In this way it is hoped to increase, if only in a small way, the corpus of material available, not only to the student of Ibn 'Arabi's thought, but also to the student of Sufism in general.