Via The Napoleon Hill Foundation:

It is the nature of opportunity that it simply refuses to attach itself to negative thinkers. Negative minds cannot conceive exciting new business opportunities, invent innovative new products, solve difficult problems, or create beautiful music or works of art. All of these activities require a positive belief in yourself and your abilities. When you approach every challenge with a Positive Mental Attitude, you will always discover opportunities that others have overlooked. Relish your achievements and recall them when the going gets tough. Take comfort in the knowledge that you have succeeded in the past and you can do it again. You can do it if you think you can!