on irreversibility

a dear friend has this quote in his email signature:

A mind that has been stretched will never return to its original dimension!
- Oliver Wendell Holmes

spiritual teachers and philosophers (including tnh and the gita) speak along these lines as well, and i must say it has brought me boundless solace to realize this truth. true that we may sometimes forget, sometimes lose track, but knowledge is never lost in completeness. and once we have acquired a degree of self-knowledge, the process is not reversible. no progress we make along our spiritual journey is ever in vain. indeed, there is no "back to square one". every step takes us a step further in our growth. every depth we transcend to is a depth we cannot unknow. the only direction we can proceed in, then, is forward. reassuring, is it not?