This is an extract of a verse in the Bible from the Book of Proverbs, chapter 23, verse 7 by King James Version (KJV). For as he thinketh in his heart, so is he: Eat and drink, saith he to thee; but his heart is not with thee.
The sentence “as a man thinketh so is he” has been translated differently on NASB: “As he thinks within himself, so he is.” This translation is easy to understand.
“as a man thinketh so is he” means to say that you are what you think or your thought represents who you are.
Well, “As he thinks within himself, so he is” can have different meaning according to the context. But it literally means that you are whatever you think.
We can also make other sense out of it. You will be happy if you think positive and you will be sad if you think negative.