There was a nice little quote that came out of the film "Postcards from the Edge." It went something like this, "The main problem we face is the voice within that says 'it can't be done'".
Many a mountain has moved through the determination of a determined will, while many a stone was left standing. It is certainly not possible to do the impossible, but often our impossible is quite possible if we hadn't said "it's impossible".
Positive thinking, or possibility thinking, is a way of viewing life. We can say to ourselves that life is beyond us and that we can't make much of an impression - all is lost, the game is up, the circumstances beyond us. Or we can look at life in a positive frame of mind and say to ourselves that everything is possible - the game isn't over until they blow the final whistle. It is just a way of looking at life. The circumstances we face day-by-day can be viewed as either hostile events which are likely to defeat us, or new possibilities to experience and grow through. It's all in the viewing, the attitude.
Of course, positive thinking can become a little unreal if we start believing that what is actually impossible is possible. This, connected with a religious faith, can have us believing that the impossible is possible if we have enough faith. Jesus said that with faith as small as a mustard seed we could move mountains, but he did go on to explain what he meant by faith and what the actual mountains were. Faith is trust in the revealed will of God; it's not some hyped-up religious feeling. The mountains are the promises of God, not the wild intentions, dreams and aspirations of we mere mortals. So, the truly impossible remains impossible. Norman Vincent Peel, and later his disciple, Robert Schullar of the Crystal Cathedral fame, were both exponents of possibility thinking. At least under their ministry the impossible barrier has moved back somewhat toward reality.
At the beginning of Jesus' ministry he faced the full weight of the powers of darkness. The temptations Jesus faced went to the heart of his capacity to succeed in bringing in God's promised new age. Even as he faced the horror of the cross he was tempted to run but stood firm - "Not my will, but thine." Thankfully Jesus didn't say "it can't be done". He was willing to give it a go. And above all he did so trusting God's promises.
For us then, life is full of possibilities, especially the impossible possibilities promised by God.