My son, give attention to My words; incline your ear to My sayings. Do not let them depart from your eyes; keep them in the midst of your heart; for they are life to those who find them, and health to all their flesh. keep your heart with all diligence, for out of it spring the issues of life. Proverbs 4:20–23

For those who live according to the flesh set their minds on the things of the flesh, but those who live according to the Spirit, the things of the Spirit. Romans 8:5

The heart is deceitful above all things, and desperately wicked; who can know it? I, the Lord, search the heart, I test the mind, even to give every man according to his ways, according to the fruit of his doings. Jeremiah 17:9–10