6 This interior reality of each person is frequently concealed behind a great deal of “foliage”, which makes it difficult for us not only to understand ourselves, but even more to know others: “The heart is devious above all else; it is perverse, who can understand it?” ( Jer 17:9). We can understand, then, the advice of the Book of Proverbs: “Keep your heart with all vigilance, for from it flow the springs of life; put away from you crooked speech” (4:23-24). Mere appearances, dishonesty and deception harm and pervert the heart. Despite our every attempt to appear as something we are not, our heart is the ultimate judge, not of what we show or hide from others, but of who we truly are. It is the basis for any sound life project; nothing worthwhile can be undertaken apart from the heart. False appearances and untruths ultimately leave us empty-handed.
Source: Dilexit Nos (Vatican.va)