"The heart is first of all the center of our being on all the different levels of our existence, not only the corporeal and emotive, but also the intellectual and spiritual. It is what connects the individual to the supra-individual realms of being. In fact, if in modern society heart-knowledge is rejected, it is because modernism refuses to see man beyond his individual level of existence. The heart is not a center of our being; it is the supreme center, its uniqueness resulting from the metaphysical principle that for any specific realm of manifestation there must exist a principle of unity. The heart is the barzakh or isthmus between this world and the next, between the visible and invisible worlds, between the human realm and the realm of the Spirit, between the horizontal and vertical dimensions of existence. In the same way that the vertical and horizontal lines of the cross, itself the symbol not only of Christ in Christianity but also of the Universal Man (al-insan al-kamil) in Islam, meet at only one point, there can be only one heart for each human being, although this single reality partakes of gradations and levels of being. The heart, then, is our unique center, the place where the supreme axis penetrates our microcosmic existence, the place where the All-Merciful resides, and also the locus for the Breath of God. Hence the profound relation that exists between invocatory prayer carried out with the breath and the heart."