The person think impossible is a negative word, but he forget that the word impossible itself says I m possible.
If we see anything in a positive manner it is going to work in a positive way, it is we, who have to think positive, who have to react positive, and who have to believe in positivity, without this three thing everything is going to be impossible, and thinking that impossible is a negative word, that itself is an negativity.
So rather than thinking impossible as a negative word, think it as a I am Possible positive word, that will be a good sign of making yourself understand that, you are thinking in a positive way.