George Airy, the person nominated to choose the clockmaker and the specifications of the clock wrote something which was believed to be impossible. "The Great Clock should be so accurate that the first strike for each hour shall be accurate to within ONE second of time.” This kind of accuracy was unheard of and unnecessary in those days. And hence thought of to be impossible.
The person who thought it to be possible was a lawyer who also was a hobby clock-maker called Edmund Beckett. Anything's possible with lawyers.