Thomas Aquinas

“ The putting of enmities between him and the woman "means that we cannot be tempted by the devil, except through that part of the soul which bears or reflects the likeness of a woman. The seed of the devil is the temptation to evil, the seed of the woman is the fruit of good works, whereby the temptation to evil is resisted. Wherefore the serpent lies in wait for the woman's heel, that if at any time she fall away towards what is unlawful, pleasure may seize hold of her: and she watches his head that she may shut him out at the very outset of the evil temptation." ”

“ Now it is a condition attaching to human nature that one creature can be helped or impeded by another. Wherefore it was fitting that God should both allow man in the state of innocence to be tempted by evil angels, and should cause him to be helped by good angels. And by a special favor of grace, it was granted him that no creature outside himself could harm him against his own will, whereby he was able even to resist the temptation of the demon. ”

“ Now the devil is able to make suggestions to man without making use of an outward sensible creature. Since then our first parents were endowed with a spiritual mind, and adhered less to sensible than to intelligible things, it would have been more fitting for man to be tempted with a merely spiritual, instead of an outward, temptation.
Objection 3: Further, one cannot fittingly suggest an evil except through some apparent good. But many other animals have a greater appearance of good than the serpent has. Therefore man was unfittingly tempted by the devil through a serpent. ”