How vain it is to sit down to write when you have not stood up to live.
- Henry David Thoreau (1817-1862)
Writing is not necessarily something to be ashamed of, but do it in private and wash your hands after wards.
- Robert Heinlein (1907-1988)
And of course:
Things are only impossible until they are not
- Jean Luc Picard (commanded or the star ship Enterprise NG)