When considering the conscience, I feel especially equipped as my conscience has been over active since the moment I could talk.
But what is the conscience? It is something that everyone has, or at least is aware of. To put it quite simply, the conscience is an aspect of the mind that helps to distinguish what is right from that which is wrong. Now, from this point on, discussions of the conscience will differ dramatically depending on your philosophical and religious perspectives. I will proceed from the traditional perspective of Christianity (although I will discuss other ideas on the conscience in a later post.)
Thomas Aquinas, a medieval theologian, argued that the conscience has two aspects. On one hand, the conscience is fueled by man's ability to reason. "I can determine that an action is good because I look at points x, y, and z as evidence." On the other hand, this reason is guided by a remnant of innate knowledge concerning absolute truth, Aquinas called this knowledge, "synderesis." So the conscience then, is a joint venture between man's ability to reason and his innate knowledge of absolute truth. But there is a problem...
According to Aquinas, the conscience has limited functionality. Our ability to know and properly perceive absolute truth was desperately handicapped after Adam and Eve's experience in Genesis 3, hence his use of the word "remnant." Man's desire to be like God, drove a wedge between us and God. This division, while mended through Christ, still affects us while we remain on earth. We still suffer from the physical effects of "the fall" and our loss of knowledge regarding absolute truth was a bitter result. Romans 1:21-23 gives us a picture of the situation, "For although they knew God, they did not honor him as God or give thanks to him, but they became futile in their thinking, and their foolish hearts were darkened. Claiming to be wise, they became fools, and exchanged the glory of the immortal God for images resembling mortal man and birds and animals and reptiles."
So our conscience is broken. That which is supposed to point us toward the right, deceives us and in fact, is deceived by us. In a perfect world, it would be a perfect indicator of what is right. However in an imperfect world, assuming that the conscience is always correct is more akin to assuming that your compass is correct when you have a magnet in your pocket.