Musar על ערכין 30:5

The third aspect concerning which man's conduct on earth will be examined is alluded to by the dual spelling of 25,30 in connection with the discussion of the applicability of the law on redemption of houses sold permanently. The Talmud Erchin 15 discusses the advantage enjoyed by a creature who has been given the power of speech. Basing themselves on Psalms 120,3 "What can you profit, what can you gain, O deceitful tongue," our sages portray G–d as saying to the tongue: "All the other limbs of man are upright; but you are in a prone position. All the other limbs of man are an appendix to his body, but you are within his body. Not only that but I have surrounded you with two חומות, walls, one of bone (the teeth) and one of flesh." The secret dimension of the letter מ is that it appears both as an open letter and as a closed letter (the final ם which in its form is like a closed wall). When a person is circumspect about the use of his tongue, the mouth acts like a dosed wall. When he is careless, his mouth is like a wall which has broken down. This is alluded to in the final, closed ם in the middle of the word in Isaiah 9,6: למרבה הפשרה ולשלום אין קץ, "In token of abundant authority and of peace without limit." In the verse just quoted, our Rabbis perceive that it is human nature to look for faults in others, i.e. "a broken wall" while considering themselves like a "closed wall," i.e. free from faults. This is the deeper meaning of what our sages (Baba Batra 2a) describe as היזק ראיה, damage caused to a person by invading his privacy by looking into his domain [a kind of peeping. Ed]. It is the moral background to the Mishnah in the above mentioned section of the Talmud in which the contribution a neighbor has to make to a wall dividing common property is discussed. The purpose of the wall is to prevent such damage. In other words, the so-called damage is due to the tendency of people to discover something discreditable about their neighbors. We see that even halachically the wall is perceived as something that prevents damage of an abstract nature such as היזק ראיה.