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Keep thy heart with all diligence in Proverbs 4:23 means be very very careful about what you listen to and meditate on because those things will go into your heart, whether good or bad . Do what Philippians 4:8 says: meditate on all good things to keep your heart right. Do not lend your ears to things that are unholy, like someone talking bad about someone, always walk away do not be a part of that. This is keeping your heart.