There are any number of people who speak in favor of positive thinking. I do not believe that this idea is correct.
Positive thinking leads to carelessness and short-sightedness. If you are thinking positive, you fail to anticipate problems, and you do all sorts of stupid things. Sometimes positive thinking really does work as advertised. Being positive is likely to win more friends than being a whiner. At other times, it gets one to act like a fool.
Beliefs have a role in shaping reality of people's lives – in fact a huge role. However they are in no way the only thing that does. Your thinking – positive or negative – did not create the solar system or America; and however positive you are thinking you will neither re-create nor destroy either one.
Positive thinking works when it is used to “win friends and influence people.” It fails completely in just about any other pursuit. Positive thinking will not create a workable computer. Positive thinking will not cure heart attack. Positive thinking will not win wars. Positive thinking will not allow you to know the character of the person who's wooing you. Positive thinking will not produce real solutions to social and political problems.
Is being negative the solution? Not at all. Negative thinking can be just as deluded as positive thinking. Depression is not better than blind optimism; in many ways it is worse. The real solution is not to think positive or think negative. It is to have an informed assayal of your own abilities as well as of whatever situation you have to face.
Optimism can in fact “win friends and influence people” and thus help one to get ahead. It is something you want to encourage in salesmen. It is not something you want to encourage in engineers, who have to make sure that the work that they are producing is completely free of error, and for whom positive thinking means producing equipment that will blow up on use.
Engineers are known for having bad personalities. That is most likely because their work demands that they be extra-cautious and intolerant of error, and they take that attitude into all sorts of places where this attitude does not apply. They become very critical and nit-picking. They keep picking at whatever flaws that they see in the next person, even if the next person is a much better person than they are themselves. Some would come at their wives with fists in case that they see a speck of dust on the floor. In relationships this attitude is disastrous. A speck of dust on the floor will not kill the family; abusive behavior however stands to destroy it. Nitpicking is the right thing to do while designing equipment; it is not the correct way to treat one's wife.
So if you are a salesman, by all means think positive. But do not force the same attitude on military or engineers. Positive thinking will get you places in sales; in any number of other pursuits it's disastrous. Apply the thinking that is appropriate to the pursuit. And never take either the attitude of the salesmen or the attitude of engineers into situations where they do not apply.