Gemara:


Ein Ayah: There are two ways to look at the cause of sin.
It is possible to look at a person as naturally straight, on his own. It is true that we see most people sinning seriously even on a daily basis. However, this stems from the impact of many external factors in life, which pull people toward sin. This includes problematic friendships, sad occurrences that prevent the light of happiness, and poverty, which affects a person’s thought process. When a person has weak resolve, such external factors can cause him to sin. Therefore, the solution to how to refrain from sin is connected to removing the external catalysts.
Flies are responsible for many of man’s illnesses, including very serious ones, as they transfer many infections from other places and cause them to enter a person’s body. The resulting conditions can be things that afflict the body and sometimes things that harm the spirit.
There are two sides to a person’s heart. One part inspires him to seek wisdom and justice, and one part causes him to seek wealth and animalistic desires. The external factors can bring disturbances in different areas. Sometimes they can incite one to go too far in seeking physical pleasures and bring him to sin. Sometimes a person can come to exaggerate the concern for a high spiritual level. This can also be dangerous by, for example, causing him to fast when he is not capable of doing so or to accept levels of apparent piety which negatively affect him or others.
In any case, the root of evil is not within the person’s heart but from without. It can be fought by avoiding bad friendships and by working on making a proper living so that he can remain in a happy state of mind and thus keep him walking on the straight path. This is Rav’s approach, as expressed in the metaphor of flies, which need to be kept away.
There is an opposing philosophy, which says that if man were totally straight by nature, then various external influences would not be able to sway him from his ways. After all, one who has the right approach can take any event and transform it into an opportunity to take out something positive. Rather, there is a set element with the propensity for evil that is within the essence of the person, and this is what causes him to sin in different ways. We call this element chet and it can be represented by chita, a grain of wheat. The grain is made of two parts that are connected to each other, but there is a crevice where the two sides are connected. The inclination can wreak havoc on either side. It can focus on the physical side, and make one act in an animalistic manner, and it can focus on the spiritual side and convince him to adopt dangerous practices and bad philosophies.
According to the latter approach, holding off the dangers has to be focused on moral education of the person. One needs study of proper things and inculcation in proper actions, with the goal being that his being will separate itself from the negative elements from within his soul. Only this and not avoiding negative external factors will help him.