“… speaks truth in his heart.” (15:2)
To lie convincingly, most people, unless they suffer from a personality disorder, need to believe the lie. If you fully acknowledge the truth in your heart and mind, it is very difficult to lie. Your body will most likely give you away. Your eyes will shift, your tongue will stutter, or your voice will drop. Your body physically resists telling what it knows to be a lie. It is possible to override your body’s impulse and teach it to lie more effectively, but it is so much easier to teach your yetzer hara (selfish inclination) to tell the truth, inside and out.