Difficult vs Impossible

It occurred to me that when you say a particular task or goal is 'impossible', you effectively remove any obligation to try. I know I often think certain things are impossible when really they are just difficult. There's quite a distinction between the two.

One state of mind requires no effort what-so-ever, while the other you have to try, train, pray, problem-solve and persevere. I know I have to be really careful in mindlessly substituting the latter for the former. In one sense they're similar words, but the distance between them is incredible.