Every thought that you think is embedded at some level in the chemistry of your brain.  When you say that someone has a positive mind, you infer that their positive thoughts are embedded in the biochemistry of their brain.

Your brain consists of hundreds of different types of molecules (chemicals) arranged in a highly complex configuration.  Some of those chemicals are relatively stable structural molecules that create the anatomy of your brain.  Other chemicals are non-structural, and they create the physiology of the brain. It takes both types of chemicals to make your brain work.

Your brain contains trillions of cells, and each of those cells is a highly organized bag of chemicals.  Your brain cells talk to each other using neurotransmitters, which are a different class of chemicals.  Electrical impulses transmit messages throughout your central nervous system, and other types of chemicals called ions are responsible for creating those electrical potentials.  Your  brain is an infinitely complex living chemistry set.  It's the chemistry of life, and that is why it's called biochemistry.  The chemistry of your brain isn't static.  Furthermore, what you put into your mind actually influences the chemistry of your brain.

The distribution and balance of neurotransmitters in your mind vary with your mood and patterns of thought.  One type of brain chemistry is associated with depression and negative thoughts, and a different type is associated with a positive state of mind.

Positive self-talk pushes the chemistry of your brain in a positive direction.  When you use positive self-talk on a regular basis, you do more than create a positive mind, you actually create a positive brain.

Excerpt from :

Zero Tolerance To Negative Thinking : Good-by Depression - Hello Positive Mind

David J Abbott M.D.