From "The Success Principles" by Jack Canfield:
[T]he mind is such a powerful instrument, it can deliver to you literally anything you want. But you have to believe that what you want is possible.
Scientists used to believe that humans responded to information flowing into the brain from the outside world. But today, they're learning instead that we respond to what the brain, on the basis of previous experience, expects to happen next. Doctors in Texas, for example-studying the effects of arthroscopic knee surgery-assigned patients with sore, worn-out knees to one of three surgical procedures: scraping out the knee joint, washing out the joint, or doing nothing.
During the "nothing" operation, doctors anesthetized the patient, made three incisions in the knee as if to insert their surgical instruments, and then pretended to operate. Two years after surgery, patients who underwent the pretend surgery reported the same amount of relief from pain and swelling as those who had received the actual treatment. The brain expected the "surgery" to improve the knee, and it did.
Why does the brain work this way? Neuropyschologist who study expectancy theory say it's because we spend our whole lives becoming conditioned. Through a lifetime's worth of events, our brain actually learns what to expect next-whether it actually happens that way or not. And because our brain expects something will happen a certain way, we often achieve exactly what we anticipate.
This is why it's so important to hold positive expectations in your mind. When you replace your old negative expectations with more positive ones-when you begin to believe that what you want is possible-your brain will actually take over the job of accomplishing that possibility for you. Better than that, your brain will actually expect to achieve that outcome.