When Adam was created, good and evil were clearly defined. After he sinned by eating from the eitz hadaas, the evil entered his body. It became part of his spiritual makeup, thereby causing the ability to distinguish between good and evil to become much more difficult. Fortunately, as we will see, this confusion is a malady which is limited to the confines of the heart.
The Chovos Ha'Levovos tells us (Avodas Ha'Elokim chap. 5) that our intellect does not suffer from this difficulty. Moreover, it is clear from his words that the intellect is the tool that we were given to enable us to properly navigate our way through this world without crashing into the roadblocks of evil that were erected after Adam's sin. "One is to acknowledge Hashem by way of his intellect... What brings a person to this acknowledgment is one's clarity of the fact that Hashem implanted in the intellect the ability to recognize the praiseworthiness of truth and the deceit of falsehood, and the value to choose good and to refrain from evil." What people refer to as one's "conscience," should more correctly be labeled "the intellect granted to him by his Creator."
However, says Rav Wolbe (Daas Shlomo), there is a hitch in the intellect's ability to guide a person. This obstacle is spelled out in this week's parsha. "Do not accept a bribe (shochad), for a bribe blinds the eyes of the wise" (Shemos 23:8). The Gemara in Kesubos (105b) explains that the word "shochad" is actually a compound word - "she'hu chad" - "that he is one." A judge who accepts a bribe becomes one with the person who offered the bribe, and consequently does not have the ability to evaluate the situation objectively.
When one's hand accepts a bribe, his intellect becomes paralyzed. Additionally, a bribe does not have to come solely by way of the transfer of money from hand to hand. Our heart's desires are one of the biggest bribes that will ever be offered to us. These too have the ability to cause our hearts and minds to become one and cause the intellect to no longer be able to properly appraise life's circumstances. Our intellect can be compared to a compass. The needle of a compass always points to the north. However, put a small magnet next to the compass and it will throw off its sense of direction. Likewise, when we place a small desire next to our intellect, it throws off our sense of direction and thus our ability to navigate through the world.
So what are we supposed to do? How can we be guaranteed that what our intellect tells us is really true? The answer to this question can also be found in the Chovos Ha'Levovos (ibid. chap. 3). It was for this reason that we were given the Torah. The Torah is the ultimate compass. It was given to us from the hand of the Creator and therefore it is certainly not adulterated by human desires. He Who created the maze, also gave us the guide to find our way. Even if we ourselves have not succeeded in mastering the information, we always have our Torah leaders who are happy to show us the way.