St. Thomas Aquinas writes in Summa Theologica, "If all evil were prevented, much good would be absent from the universe. A lion would cease to live, if there were no slaying of animals; and there would be no patience of martyrs if there were no tyrannical persecution." What does it mean? Is good to bad what sun is to shadow, night is to day, soul is to body and heat is to cold, a kind of necessary condition for each other?

In other words, can we say that good is good for the same reason bad is bad? Can we say good is virtue in so much as bad is vice, both intrinsically involved in the expression of the same mystery that wavers in light and darkness? God is the ruler of the universe, the source of light that sustains truth and virtue. Satan is the Prince of Darkness, the Great Tempter, who is out there to seduce. Is it the conflict between God and his cosmic rival, the Satan, which is manifest in the conflict between good and evil?

W.H. Auden wouldn't agree. He believed in the normality of evil, which means that "evil is unspectacular and always human", which "shares our bed and eats at our own table." Good is also normal, its manifestation enmeshed in our daily life, in everything we speak, think or do. Yet evil is more convincing than good. Yet evil is more attractive than good.

Why? The French philosopher Andre Gide explained that evil acts could be as gratuitous as good acts. But how is it possible? Can evil happen as normally and naturally as good? Do people love as gratuitously as they hate? Do people destroy as gratuitously as they create? Does the impulse of virtue flow as instantaneously as the impulse of vice? Is evil ingrained in the human nature as intimately as good?

In fact, good and evil come in pairs: honest and dishonest, truthful and treacherous, piety and perfidy, kind and cruel, and so on. There is an evil for every good, and vice versa. And good and evil also coexist not only in the cosmos like sunshine and storm, but also in the conscience of man like a ruthless killer who loves his children.

Often good comes in the disguise of evil. Snakebite could save you from enemy bullet if it causes you to fall down and miss his aim. Again, the opposite also happens. The gunshot might scare the snake, while you are still standing tall in the line of fire. Hence, good and evil are correlated; one not only diminishes but also develops the other. Robin Hood robbed the rich to help the poor, an example of good being accomplished by evil means.

Now bad and evil are like urn and ash. The evil inhabits the bad and the bad holds the evil. You can say that bad is like a house, which has been occupied by evil by evicting the good. At times, it's not the whole house but a room or two in that house, where evil has displaced good. A dishonest man might give away some of his ill-gotten money in charity. A prostitute might be kind to the sick. A rogue man might save someone sinking in a river.

What is evil then? According to American car manufacturer Henry Ford, evil is ignorance bumping its head in the dark. If ignorance is bliss, then evil is when ignorance becomes adventurous and tries to go beyond its limit. In that sense, evil is ignorance two times over, when ignorance is ignorant of itself and has lost its head over that blind rage. But how does it explain why parents would kill their own children? How does it explain why a deranged son would kill his father who refuses to bankroll his drug addiction?

How is ignorance responsible for either of these evils? Are the perpetrators of these crimes ignorant of their dire consequences, or are they under the influence of a darkness, which overshadows their judgement? When an adult man contemplates to ravish an underage girl, what exactly is he ignorant of? Is he ignorant of the moral consequences of his act? Is he ignorant of its legal implications? Is he ignorant of how badly he was going to bruise the mind and body of his victim?

Evil is then of two types, impulsive and compulsive. Impulsive evil is like the bubbles, which grow and vanish on the spur of the moment. Frustrated by the arrogance of his former friend Thomas Becket, the Archbishop of Canterbury, King Henry had shouted in outrage: "Who will rid me of this meddlesome priest." Four knights hunted down Becket at the altar of a church and struck him down with their swords until his skull split open. Four years later, in an act of penance, the king donned a sackcloth walking barefoot through the streets of Canterbury while eighty monks flogged him with branches.

But compulsive evil is recurrent, it being manifest in the propensity of the serial killers to kill again and again. Criminals make a career out of compulsive evil, repeating their terrible acts under the influence of habit. These are people who cannot give up, because evil holds them in a straightjacket. Compulsive evil revolves like a vicious circle, crime perpetuated by crime into an endless repetition.

Which shows that evil has a range of its own, one that varies on the basis of how much it deviates from good and for how long. It is amazing how good and bad are standard bearers for each other, how one is the measure of another, how denial of good is the affirmation of evil, how each has the essence to enunciate other.

Why does evil happen? The answer is that it happens because good happens as well. There are days and nights in human hearts, where the sun rises and sets in the diurnal motion of virtues and vices. Destiny acts like gravitation that pulls together man's faith and fate, as his life goes through the force fields of right and wrong. Why are there saints and scoundrels amongst men? Why are there monster and messiah, righteous and rascal, angel and demon?

Good and evil set the parameters of human instincts, which come under moral judgement. Evil is misguided good, like a mad scientist is a misguided genius. Evil is reckless pursuit of good, when one is hell-bent to secure one's wellbeing without consideration for others. The rapist, thief, robber, usurper, liar, killer, the whole association of evil basically originates from selfish self-aggrandisement, when one is desperate for one's own good with utmost disrespect for others.

Turn the evil to yourself, and think of yourself, as your own victim, there will be no moral judgement. Molest yourself, steal from your own wealth, tell a lie to yourself and even take your own life, the world might take pity on you, but it will not call it evil. The thorn is not evil until it has pricked the hand, which plucked the flowers.

Mohammad Badrul Ahsan is a banker.