Aum is complete in itself. Aum is chanted whether one is praying to the Nirgun or to the Sagun. Since ages, man by nature has been praying to one or the other. Its this obeisance, which makes man evolved and different from the other living beings. But if a person has to get out of the cycle of life, death and rebirth, then he must do meditation of the supreme Aum. In fact, in this universe, only Aum is complete in all respects and forms.