The story of the first sin is the story of all sins. The animal — in this case the serpent — applies its “logic” to overcome human conscience. The animal is not wrong is following animal logic — the animal instinct is the voice of G-d… to the animal. The serpent urges man to eat from the forbidden fruit, promising this will enlighten him so that he is “like G-d, knowing the difference between good and evil.” (Ber. 3:5). Indeed, animals are endowed with instinct — Divine guidance from within — and can act only in accordance with that extinct. They are “like G-d” and they can do only good and no evil. Animals have only one nature, whose call they must heed.
Not so man. Man’s distinction is his morality, born of the ability to choose good and shun evil. He has physical drives but must give them their due only out of a sense of duty, and always acting with moral freedom. His task is not to follow his animal instincts, but to overcome them. An animal acting in accordance with his instincts follows them, but a man who does the same — succumbs.
Man’s physical nature must, of necessity, be opposed to good and attracted to evil, for only in this way can he properly choose good, not because of his senses but in spite of them. For this reason, the voice of G-d speaks not from within him, butto him, telling him what is good and what is evil. But there is also a soft, innate voice of G-d within man. This is the conscience, whose messenger is the sense of shame. Shame allows us to rise above the animal instinct and choose good even when it is against our physical gratification. Shame is the faithful guardian of morality. It fortifies the ability to choose good. It allows us to repay debts, return lost objects, and refrain from illicit relationships. (See Duties of the Heart, Gate of Reflection, chapter 5.) Shame cautions man in general terms to do good and refrain from evil, but does not define what is good and what is evil. This, man learns only from the voice of G-d, which speaks to him from outside himself.
To the serpent, what is pleasing is good. To an animal, instinct is the Divine voice. The tree was appealing to man’s senses — it appeared good to the taste, tempting to the sight, and delightful to contemplate. (Ber. 3:6) Everything from within him said: “This is good.” But G-d’s word to him told him not to eat the fruit, and this was to be the guiding rule for man to distinguish between good and evil — G‑d’s word.
Man was placed in paradise not to enjoy its fruits, but to work and guard it. (Ber. 2:15) Only by following the rule of G-d, with the aid of his conscience, can he succeed.
Sources: Commentary, Bereishet 3:1