The Sufis often add the element of sirr, the innermost part of the heart in which the divine revelation is experienced. Jafar introduced, in an interesting comparison, reason, aql, as the barrier between nafs and qalb -- "the barrier which they both cannot transcend" (Sūra 55:20), so that the dark lower instincts cannot jeopardize the heart's purity. Each of these spiritual centers has its own functions, and Amr al-Makkī has summed up some of the early Sufi ideas in a myth: