From the Devil’s Dictionary, by Ambrose Bierce:
DIARY, n. A daily record of that part of one's life, which he can relate to himself without blushing.
Hearst kept a diary wherein were writ All that he had of wisdom and of wit. So the Recording Angel, when Hearst died, Erased all entries of his own and cried: "I'll judge you by your diary." Said Hearst: "Thank you; 'twill show you I am Saint the First" -- Straightway producing, jubilant and proud, That record from a pocket in his shroud. The Angel slowly turned the pages o'er, Each stupid line of which he knew before, Glooming and gleaming as by turns he hit On Shallow sentiment and stolen wit; Then gravely closed the book and gave it back. "My friend, you've wandered from your proper track: You'd never be content this side the tomb -- For big ideas Heaven has little room, And Hell's no latitude for making mirth," He said, and kicked the fellow back to earth.
"The Mad Philosopher"
(It’s true, I keep a diary and consistently omit all the things which I would blush to have others read--- or my mind whitewashes the memory for me. So instead of the stupid or thoughtless or rather, course things we do that pass the small moments of the day, my diary merely records what I think about things when the nib meets the paper: of course its a departure from reality, and an exercise in self-gratification to boot.
But in the end, what should I strive to record in my diary? Certainly not just important things, since life is mainly made up of ordinary, boring moments. Certainly not every mundane thing that comes to mind, since that would be of no interest. Perhaps, what I like to read in journals that have come down to us from the past: interesting things, and the flavor of daily life. Especially the latter, since in time all our world will fade away, and be replaced by new things.)