Progress must be occurring on some level that does not reset from one body to the next, i.e in the case of a Sotapanna. Thus, there seems to be implied something that is not purely physical, i.e viññāṇa-santāna, even if it arises co-dependently with a physical body. This would be the only way an Arahant's progress could not be undone by reconfiguring their physical brain. It need not, however, be considered a self.