If you declare something impossible, you are also judging that you have considered every possible factor. You have walled off the things you cannot see. You have dismissed all supernatural outcomes. You are trusting that you have reasoned not just correctly, but perfectly. There is no guesswork involved, you have determined truth.
How many instances in our lives can we state this truthfully and confidently? Do we ever know other people well enough to render a perfect opinion on them now and forever? Can we do the same for ourselves?
When we see a large lake for the first time, how easy it is to assume we are on a sea. Only when we travel around or over it do we gain the perspective and experience to know that it is not so vast. Likewise with our lives. We cannot see how small our opportunities are by a common vantage point. Only by walking around our lives and seeing over them do we begin to understand that most of our choices are capricious and that even the most careful person assumes it is fate when those outcomes glide forth.