What do we learn from the fact that man conquers the snake by crushing its head, and the snake conquering man by bruising his heel? (Gen. 3:15)
Sforno derives a lesson from this statement: "Man conquers the snake by crushing its head, and the snake conquers man by bruising his heel". Sforno interprets "head" and "heel" to mean "beginning" and "end" respectively. Using these interpretations, Sforno teaches that man conquers the instincts at their very outset - their initial onslaught against man's reason. When an instinctual urge arises in man, it is at this point that man has the highest probability of conquering such urges, as man is still in control of all his faculties. But if man allows the urge to take hold of him, and he does not fight it, the urge becomes greater, and man loses all chance of subduing the urge. This is how the psyche operates. Stating that man "crushes the head of the snake" means, according to Sforno, that man conquers the instincts at their "head", at their initial onslaught. The snake "biting man's heel" means that the instincts subdue man at the end, at the "heel" of the battle. Man is overcome at the end of the battle.
It makes sense that the Torah informs man of our psychological workings at the very commencement of this great work, the Five Books of Moses. The Torah instructs us in perfection. By definition, it must include an explanation of our definitive components; the mind and the instincts. Here, the Sforno understands the "snake's interaction with Eve" to parallel our very psychological design.
Additionally, Sforno teaches that man's perfection cannot be devoid of understanding. The gift of the Tzelem Elokim - the intellect - teaches us that God wills all our actions to be guided by reason. Therefore, God's Torah must enable man to understand how all our commandments aim towards our perfection. Such an understanding cannot exist if we are ignorant of how the commandments perfect us as psychological beings. Therefore, knowledge of our psychological workings is taught immediately in the opening sections of Genesis. King Solomon does the same in the opening of Ecclesiastes, Koheles. (It is quite interesting that in the opening verses of both works, we find the discussion of "rivers".)
Sforno's lead may also explain why we have two accounts of the creation of man: The first account is the creation of man as he is a Tzelem Elokim - an intelligent being. The second account omits any reference of the Tzelem Elokim, but refers to man as a "nefesh chaya", a living beast - the same description given to animals. Perhaps this subtle change intimates what each account addresses. This latter account, including the snake's deception, borrows the animal kingdoms' appellation of "living beast" and not "Tzelem Elokim" as it addresses the instinctual workings of man.