Since the dawn of time, human beings have lived by the sun. Ancient cultures often worshiped it as a deity because they realized they were totally dependent on it: without light there would be no vegetation and no warmth and, thus, no life. Everything revolved around the sun, both figuratively and literally. The passage of time was marked by its rising and setting. The day the sun failed to rise was generally considered to be the mark of the end of time.
Light imagery was adopted by philosophers, ethicists, moralists, and theologians to describe the phenomena of death, sin, and ignorance. Saint Augustine wrestled with the problem of evil, and drawing on his background in neo-platonism, concluded that evil had no existence of its own; it is merely the absence of good, just as darkness is the absence of light. Philosophically, darkness represented a kind of intellectual deficiency. One who was "left in the dark" was one who was ignorant of what was going on. Darkness in this way represented a kind of shroud that presented the person from seeing reality in its fullness.
Saint Paul uses light imagry to paint a picture of the moral life in his letter to the Ephesians:
"For you were once darkness, but now you are light in the Lord. Live as children of light, for light produces every kind of goodness and righteousness and truth. Try to learn what is pleasing to the Lord. Take no part in the fruitless works of darkness; rather expose them, for it is shameful even to mention the things done by them in secret; but everything exposed by the light becomes visible, for everything that becomes visible is light." (5:8-14)
In the face of such suggestive language, it is tempting to want to equate light with "good" and darkness with "evil." But are such statements accurate? I would argue their value depends on one's presuppositions. Zen Buddhism has a strong tradition of debunking such reductive dualism. Value statements such as "good" and "not good" are intepretations of reality from the narrow perspective of the ego. Zen seeks to move beyond such a limited viewpoint into a realm where Reality is seen for what it is. In such an egoless realm, values like "good" and "evil" fall away because there is no subjective "I" to support them.
Mental illness in the West has depended on value judgments for self-definition, with the litmus being "normalcy." In such a paradigm, what is normal (that is, what is defined to be "normal") is assigned a value--i.e., normal is "good" or "desirable." Western psychiatry has both formally and informally set the parameters for what is and what is not "desirable" behavior. The Bible of psychiatry--The Diagnostic Statistical Manual of Mental Disorders--is the most concrete example of this. Behaviors which fall outside the parameters for normalcy are considered "disordered."
It is interesting that in Christian theology sin is also defined as a "disorder." It rests on the presupposition that there is a definitive order to Nature, an order prescribed by God, and that transgressions against such an order violate such a law; i.e., the Law of Nature or Eternal Law. St. Augustine defined sin as "something said, done, or desired contrary to the eternal law" (Reply to Faustus, XXII.27) The Cathechism of the Catholic Church draws on Thomas Aquinas as well and expands this definition to consider sin as "an offense against God and against reason" (1849-50).
It is difficult in a post-modern age to accept the idea of a definitive, objective order. Postmodernism (or more accurately, Poststructuralism) is more comfortable with the ambiguities that accompany a decentralization (or destabilization) of authority. A silly example might be employing the perspective of vampires as a method of destabilizing the commonly-held assumption that "light = good." Vampires would not consider light to be a good thing, and so the prescribed order that denotes "light = good" can no longer be considered universally applicable. Feminism might be another example: the assumption that all people are male is debunked, and the rulebook is rewritten so as to expand the parameters of the prescribed "order."
So if sin is a kind of disorder, and mental illnesses are designated in the DSM as "disorders," it is hard not to come to the conclusion that mental illness must be some kind of sin.
This is the conclusion affirmed by D. Tyler et al. in their book "When Sin is Called Sickness." They maintain that psychiatric disorders (depression, anorexia, etc.) are not diseases or sicknesses at all, but the manifestations of human sinfullness:
"The diagnosis is deceptive because it is not a real disease as disease is historically defined. There is no alteration in the anatomy (structure) or physiology (function) of the human body. Rather, the problem lies in feelings (depression, anxiety, guilt) and behavior (rebellion, anger, selfishness, immorality)."
I see where they are coming from, but I think this is very simplistic reasoning. It also feels very antiquated, like when people thousands of years ago thought they were blind or lame because they had sinned against God. To make such assertions today--that one is blind because of their personal sinfulness--is not only offensive but dangerous.