Another common misconception of Mahayana doctrine is in regard to the eternality of the Buddha, which does not refer to Shakyamuni Buddha as a historical person. Nagarjuna and others made clear that the Buddha, in entering final Nirvana, is beyond existence and non-existence, just as the Buddha described in the Pali scriptures:
What is eternal is the Dharma-body of the Buddha, which is the one and the same Dharma-body in all buddhas, that all buddhas share:
Enough, Vakkali! What is there to see in this vile body? He who sees Dhamma, Vakkali, sees me; he who sees me sees Dhamma. Truly seeing Dhamma, one sees me; seeing me one sees Dhamma.
The Dharma-body is also referred to as the Buddha-nature in all beings, our innate potential for enlightenment. In the Lotus Sutra, for example, the Buddha says that all other buddhas are a manifestation of himself. This is in reference to the Dharma-body of the Buddha, rather than Shakyamuni as a particular historical person.
A common phrase in Mahayana Buddhism is that there is no Buddha outside the mind. This means that if you are looking for the Buddha, as in the historical Buddha who passed away into Nirvana 2,500 years ago, you will not find him. The living Buddha is the Buddha within your own body and mind, waiting to be awakened as your own Buddha-nature.
The celestial buddhas and bodhisattvas, such as Amitabha and Avalokitesvara, are symbolic of our own Buddha-nature and the Dharma-body in all things, rather than literal historical persons like Shakyamuni Buddha.
The Eternal Buddha, the Dharma-body, is an eternal principle rather than a personal being.