Your body knows what your mind thinks.
And it acts accordingly. It means that a belief is not just an idea, it is in your body and your mind.
Dr. Deepak Chopra is a famous author in USA who is invited all around the world for talks. According to him "To think is to practice brain chemistry."
When you are happy or you believe in achieving success in whatever you are doing, that thought is conveyed to your whole body. Your body is more energetic and less tired. When someone is unhappy and sad, his body is slow, dull, and tired.
In summary, your brain conveys your thoughts and beliefs to your body. If you think "I will succeed", your brain tells your body to get ready and expect success. If you think "I will fail", the brain tells the body "Do not even try, there is no chance of success."
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The human brain has about 15,00,00,00,000 (1500 crore) neurons or brain cells. Each single brain cell makes connection with 5,000 to 10,000 other brain cells. The network of brain cells is so complex that the brain can be imagined to be like a huge telephone network. When we learn something, new connections are created in the brain. Repetition strengthens these new connections. A different experience can modify the network. It means that you are changing continuously, though you do not know about it. It has been recently found that the connection or interrelation is much more stronger than what the doctors believed in the past. In the last ten years it has been discovered that there are about 50 to 60 chemicals taking messages from the brain to the body. The most well-known of these are called endorphins. In the past it was believed that endorphins were present only in the brain. But now they have been found in the body, carrying messages to practically every cell in the body. These chemicals carry messages in the body about everything including thoughts, happiness, feelings, beliefs, etc. Let us see an example. A person is walking in the park. He sees something like a snake and thinks in his mind "It is a snake." That message of fear goes to other parts of the brain, then to the spinal cord, and then to the rest of the body. In a very short period, the whole body knows about the danger and prepares itself to face the danger. The heart rate increases, blood pressure increases, breathing becomes faster. Blood is diverted from the brain to the muscles, the hands and the feet. As a result, the person is ready to "fight" or "to run away". This well known phenomenon is called "fight-or-flight" response. Just the thought "snake" creates this response. In the same way, there is a biological basis for every thought. Your Brain is a Neural Network
The mind and the body are connected.