If you change your mind - from pessimism to optimism - you can change your life. Do you see the glass as half-full rather than half empty? Do you keep your eye upon the doughnut,not upon the hole? Suddenly these cliches are scientific questions, as researchers scrutinize the power of positive thinking. Research is proving that optimism can help you to be happier, healthier and more successful. Pessimism leads, by contrast, to hoplessness, sickness and failure, and is linked to depression, loneliness and painful shyness. If we could teach people to think more positively, it would be like inoculating them against these mental ills. Your habits count but the belief that you can succeed affects whether or not you will. In part, that's because optimists and pessimists deal with the same challenges and disappointments in very different ways.