If a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.
— Arthur C. Clarke
I’d like to have a more accurate reference but presently lack the inertia to track it down. Not that I necessarily doubt that he said or wrote that, but I’d like to attribute it more accurately to a speech, paper, or book.
Also, I will never assert that something is impossible.