One of the principal differences between Theravada and Mahayana is the doctrine of Buddha Nature.
This takes a number of forms–and Dogen has a unique position on it–but generally, it is the idea that we have Buddha Nature as a kind of foundational ground or potentiality.
The doctrine probably derives from the Tathāgata Garbha tradition. Tathāgata is Buddha; Garbha means both embryo and womb.
But who is giving birth to whom?
We might be inclined to see the embryo as our latent Buddha nature, but perhaps it’s the other way around. Perhaps the doctrines, the ritual, the lineage, the traditions; everything, is the womb which enables us to give birth to ourselves.
It is as if Buddhism is a plaster-cast on something broken. When the body is healed, when the body is whole, Buddhism is no longer needed.
When we have crossed the river, do we still require to carry the boat?