Random, in short.
The name of the author is the first to go followed obediently by the title, the plot, the heartbreaking conclusion, the entire novel which suddenly becomes one you have never read, never even heard of, as if, one by one, the memories you used to harbor decided to retire to the southern hemisphere of the brain, to a little fishing village where there are no phones. Long ago you kissed the names of the nine Muses goodbye and watched the quadratic equation pack its bag, and even now as you memorize the order of the planets, something else is slipping away, a state flower perhaps, the address of an uncle, the capital of Paraguay. Whatever it is you are struggling to remember it is not poised on the tip of your tongue, not even lurking in some obscure corner of your spleen. It has floated away down a dark mythological river whose name begins with an L as far as you can recall, well on your own way to oblivion where you will join those who have even forgotten how to swim and how to ride a bicycle. No wonder you rise in the middle of the night to look up the date of a famous battle in a book on war. No wonder the moon in the window seems to have drifted out of a love poem that you used to know by heart.