I found this quote on slide 31 of 35 in Paul Kedrosky's ppt presentation given at the Ottawa Venture & Technology Summit.
Most of the things worth doing in the world had been declared impossible before they were done.
- Louis Brandeis
This seems like very good advice for anyone starting any project be it a start-up company, personal goals, volunteer work, politics, travel, etc. Its something many of us forget as we try to be too smart or too practical.