What does the Bible mean by the "heart" (lev)?

You came to this word with a modern ear, and that is no fault of yours. In English, "heart" means feelings, romance, sentiment, the place where emotion lives. So when Scripture says "heart," you naturally hear "how I feel." That is exactly what the word means in your language, and reading it that way is honest.

And there is something genuinely true the emotional reading protects. The inner life matters to HaShem. He is not after cold external compliance; He wants you, the real interior you, not a hollow performance. You are right to insist that faith reaches the inside of a person and not just the hands.

You may also have felt a small puzzle when the texts pile up. Pharaoh "hardens his heart," but that is not about his feelings; out of the heart "flow the springs of life," which sounds like more than mood. Something in the usage is bigger than emotion. Hold onto that puzzle; it is catching a real gap between the English and the Hebrew.

But the frame that makes the biblical heart mainly a feelings-organ is an import from our language, not a reading of theirs. The Hebrew word is lev, or levav, and it is the seat of will, thought, intention, and decision, the command center of the person. We would more naturally say "mind" or "will" than "heart." Put the feelings-only reading down.

Here is what the text shows. "Love HaShem your God with all your lev" (Deuteronomy 6:5) is not asking mainly for warm sentiment; it is asking for your decisions, your direction, your governing will turned toward Him. When Pharaoh hardens his lev (Exodus 8:15), his will is digging in, his resolve setting against HaShem; it is a decision, not a mood. "Guard your lev, for from it flow the springs of life" (Proverbs 4:23) is a charge to watch the place where you choose, because choices set the course of a whole life; "as a man thinks in his lev, so is he" (Proverbs 23:7). And when HaShem writes His Torah on the lev (Jeremiah 31:33), He is writing it where you actually decide, so that obedience flows from your own will rather than from pressure outside you. He looks on the lev (1 Samuel 16:7) because that is where the real person lives.

Do not take that from me. Take a concordance, find every "heart" in a passage, and try reading "will" or "mind" in its place. Watch how often the text snaps into sharper focus. Then ask it the question it keeps posing: when Scripture aims at your heart, is it after your feelings, or after the place where you choose?