the heart: the hidden inner center of a person, where one thinks, decides, feels, and stands answerable to God
This is the most common word the Hebrew Bible uses for the inner life, and it appears far more often than its near companions for the soul, the spirit, or the flesh. It is not mainly the seat of feeling that the English word "heart" suggests. By the count scholars have made of its roughly 850 occurrences, most refer to thinking, weighing, and deciding, with emotion as a real but smaller part of its range, and the literal organ in the chest rarer still. When the Old Testament wants to speak of who a person really is on the inside, this is the word it reaches for.
The senses below are not separate words but different angles on one inner center. The same heart that understands also chooses, the heart that chooses also feels, and the heart that feels also keeps a record of guilt or innocence. Hebrew runs these together far more than English does, so a single verse can speak of knowing in the heart, resolving in the heart, and being glad in the heart with no sense that the subject has shifted. Many occurrences could be filed under more than one heading at once, and the recent discussion has cautioned against pulling the thinking heart and the feeling heart too far apart, since the Bible itself rarely does. The divisions here mark where the weight of a passage falls, not hard walls between unrelated meanings.
From this inner center the word reaches in two further directions that share the same picture of a hidden core. It can name the literal organ in the chest, the body part that is struck through or that pounds and faints, and from there it stretches to the middle or depths of something with no inner life at all, the heart of the sea or the heart of the heavens. It is also said of God himself, who grieves in his heart over human evil and whose heart turns within him in compassion, and this is where the Bible's use is most its own: the divine heart it pictures is thoroughly moral.