Proverbs 27:19 (ESV)

19 As in water face reflects face, so the heart of man reflects the man.

Mirrors and other reflective surfaces, like a pool of water, do not lie. They reveal the true image of our faces but cannot reveal our character. To see our true character, we need to reflect upon our hearts, honestly considering our thoughts and intentions, to see whether we are pure, holy, and loving or defiled, unholy, and selfish. Naturally, all hearts are bad, despite their attempts to deceive us into thinking that we are good. But the Word and Spirit of the Lord compel us to honestly examine ourselves, and by saving grace to receive and nurture new, true hearts. With renewed minds we can judge ourselves more accurately, not by outward appearance or action, but by inner motivation, which weighs most with God (1 Samuel 16:7).