Silsilah

In him the lost perfection is remanifested. It corresponds to the culmination of the flow of the wave, the point from which the ebb begins. We have already seen that the ideal is 'earthly plenitude' combined with 'readiness to leave' and it is to this perfection, poised between flow and ebb, that the initial aspiration of the mystic must be directed. The Divine Messenger enters and leaves this world by the celestial gate towards which all mysticism is oriented. But the mystic himself, like other men, has entered this world through a gate that is merely cosmic; and to avoid ebbing back through such a gate, his little individual wave of entry must reach the culminating point of the great wave in order that its own relatively feeble current may be overpowered by the great current and drawn along with it. Not that the mystic could ever reach this central point of perfection by his own efforts. But the Prophet himself is always present at this centre, and to those who are not, he has the power to throw out a 'life-line', that is, a chain (silsilah) that traces a spiritual lineage back to himself. Every Sufi order (tariqah) is descended from the Prophet in this way, and initiation into a tariqah means attachment to its particular chain. This confers a virtual centrality, that is, a virtual reintegration into the Primordial State which has then to be made actual.

From "what is sufism" by Martin Lings, in the chapter "The Messenger"