That is one thing I've learned, that it is possible to really understand things at certain points, and not be able to retain them, to be in utter confusion just a short while later. I used to think that once you really knew a thing, its truth would shine on forever. Now it's pretty obvious to me that more often than not the batteries fade, and sometimes what you knew even goes out with a bang when you try to call on it, just like a lightbulb cracking off when you throw the switch.
