According to Dr. Avraham Elqayam of Bar-Ilan University, it is difficult to grasp what the core of mysticism is. In its extreme form, mysticism is a rejection of knowledge in the conventional sense, since its practitioners see knowledge as an ability to differentiate between one object and another and therefore as a hindrance to their pursuit of unity within the world. It is therefore difficult to talk about mystical "realizations," which many mystics say are ineffable. However its practice is essentially based around meditation. Sufi meditation, Elqayam points out, takes two main forms. In silent meditation, the practitioner puts one hand on the heart and pushes his tongue to the roof of this mouth. The idea is that one concentrates on what Elqayam calls the "words of your heart," the names of Allah, of which there are 99. This technique is called Dhikr Khafi. Then there is loud extrovert meditation. In traditions such as the Rumi, groups sing and dance accompanied by loud beats in a tradition reminiscent of Greek Bacchic rituals. Both techniques, Elqayam says, aim at the same goal - to lose one's sense of self. Silent meditation aims to cut one off from the material world, and ultimately one's own body, through the power of isolation. The communal technique aims at immersing oneself into a collective. Underneath the noise is a strictly regulated breathing system and set of prayer, which ensures that the group experiences the chaotic ritual in synchronicity.