Exegesis
Fremantle (2001: p. 53-54) charts the development of the bardo concept through the Himalayan tradition:
Originally bardo referred only to the period between one life and the next, and this is still its normal meaning when it is mentioned without any qualification. There was considerable dispute over this theory during the early centuries of Buddhism, with one side arguing that rebirth (or conception) follows immediately after death, and the other saying that there must be an interval between the two. With the rise of mahayana, belief in a transitional period prevailed. Later Buddhism expanded the whole concept to distinguish six or more similar states, covering the whole cycle of life, death, and rebirth. But it can also be interpreted as any transitional experience, any state that lies between two other states. Its original meaning, the experience of being between death and rebirth, is the prototype of the bardo experience, while the six traditional bardos show how the essential qualities of that experience are also present in other transitional periods. By refining even further the understanding of the essence of bardo, it can then be applied to every moment of existence. The present moment, the now, is a continual bardo, always suspended between the past and the future.
However, as shown above, Fremantle's idea that it was originally only "between one life and next" was not how it was used by the Sarvāstivāda school from the outset. Also, the idea that the ascendancy of this idea was due to the Mahāyāna is unfounded, and it is much more likely that it was due to the Sarvāstivāda influence, several centuries before the Mahāyāna had any real influence.
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