The mechanistic details of the Samsara doctrine vary within the Buddhist traditions. Theravada Buddhists assert that rebirth is immediate while the Tibetan schools hold to the notion of a bardo (intermediate state) that can last at least forty-nine days before the being is reborn. In Mahayana Buddhist philosophy Samsara and Nirvana are seen as the same. According to Nagarjuna, an ancient Indian philosopher, and a teacher of Mahayana Buddhism, "Nothing of Samsara is different from Nirvana, nothing of Nirvana is different from Samsara. That which is the limit of Nirvana is also the limit of Samsara, there is not the slightest difference between the two."