And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually. And it repented the LORD that He had made man on the earth, and it grieved Him at His heart (Genesis 6:5,6).
These two verses, describing the incurable wickedness of the antediluvian world which finally brought on the global flood, contain the first two of over a thousand occurrences of the word heart in the Bible. Note the contrast: mans heart was evil; Gods heart was grieved.
Both the Hebrew and Greek languages treated the heart as the center of a persons being, the seat of all feelings and thoughts, and we do the same in English. The writers knew that the heart was a physical organ, with its function of circulating the blood as basic to physical life. Leviticus 17:11, among other Scriptures, notes that the life of the flesh is in the blood, but only rarely was the word used thus in Scripture. Nearly always the word is used symbolically in reference to the deep essence of a persons being. It is also used occasionally to refer to the innermost part of physical objects (e.g., the heart of the earth, as in Matthew 12:40).
In this first occurrence it refers to the thoughts of the heart. Somehow, before one thinks with his mind, he thinks with his heart, and these deep, unspoken thoughts will determine the way he reasons with his brain. Jesus confirmed this in Mark 7:21: For from within, out of the heart of men, proceed evil thoughts.
How important it is, then, to maintain a heart that is pure. In fact, in sharp contrast to the first occurrence of heart in the Old Testament referring to mans evil thoughts, the first occurrence in the New Testament is in the gracious promise of Christ: Blessed are the pure in heart: for they shall see God (Matthew 5:8). HMM