Impossible is nothing. ~Muhammad Ali
"Impossible is just a big word thrown around by small men who find it easier to live in the world they've been given than to explore the power they have to change it. Impossible is not a fact. It's an opinion. Impossible is not a declaration. It's a dare. Impossible is potential. Impossible is temporary. Impossible is nothing." ~Muhammad Ali
"It is impossible to sell animal stories in the U.S.A."
— Rejection slip for George Orwell's "Animal Farm"
That quote works on many levels. :)
"Always push your limits. Always push your comfort zone." ~ Aleksey Vayner, Impossible is Nothing (video resume)
This became an Internet meme, which was parodied by Michael Cera.
Also parodied by Barney Stinson. Go for Barney!
PS I remember a few years ago when Elon Musk had a lot of critics. It seems they all faded away now that he's built some cars and some rocket ships. :)