Objection 1. It would seem that the manner and order of the first temptation was not fitting. For just as in the order of nature the angel was above man, so was the man above the woman. Now sin came upon man through an angel: therefore in like manner it should have come upon the woman through the man; in other words the woman should have been tempted by the man, and not the other way about.
Objection 2. Further, the temptation of our first parents was by suggestion. 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.
Objection 4. Further, the serpent is an irrational animal. Now wisdom, speech, and punishment are not befitting an irrational animal. Therefore the serpent is unfittingly described (Genesis 3:1) as "more subtle than any of the beasts of the earth," or as "the most prudent of all beasts" according to another version [the Septuagint]: and likewise is unfittingly stated to have spoken to the woman, and to have been punished by God.
On the contrary, That which is first in any genus should be proportionate to all that follow it in that genus. Now in every kind of sin we find the same order as in the first temptation. For, according to Augustine (De Trin. xii, 12), it begins with the concupiscence of sin in the sensuality, signified by the serpent; extends to the lower reason, by pleasure, signified by the woman; and reaches to the higher reason by consent in the sin, signified by the man. Therefore the order of the first temptation was fitting.
I answer that, Man is composed of a twofold nature, intellective and sensitive. Hence the devil, in tempting man, made use of a twofold incentive to sin: one on the part of the intellect, by promising the Divine likeness through the acquisition of knowledge which man naturally desires to have; the other on the part of sense. This he did by having recourse to those sensible things, which are most akin to man, partly by tempting the man through the woman who was akin to him in the same species; partly by tempting the woman through the serpent, who was akin to them in the same genus; partly by suggesting to them to eat of the forbidden fruit, which was akin to them in the proximate genus.
Reply to Objection 1. In the act of tempting the devil was by way of principal agent; whereas the woman was employed as an instrument of temptation in bringing about the downfall of the man, both because the woman was weaker than the man, and consequently more liable to be deceived, and because, on account of her union with man, the devil was able to deceive the man especially through her. Now there is no parity between principal agent and instrument, because the principal agent must exceed in power, which is not requisite in the instrumental agent.
Reply to Objection 2. A suggestion whereby the devil suggests something to man spiritually, shows the devil to have more power against man than outward suggestion has, since by an inward suggestion, at least, man's imagination is changed by the devil [Cf. I, 91, 3]; whereas by an outward suggestion, a change is wrought merely on an outward creature. Now the devil had a minimum of power against man before sin, wherefore he was unable to tempt him by inward suggestion, but only by outward suggestion.
Reply to Objection 3. According to Augustine (Gen. ad lit. xi, 3), "we are not to suppose that the devil chose the serpent as his means of temptation; but as he was possessed of the lust of deceit, he could only do so by the animal he was allowed to use for that purpose."
Reply to Objection 4. According to Augustine (Gen. ad lit. xi, 29), "the serpent is described as most prudent or subtle, on account of the cunning of the devil, who wrought his wiles in it: thus, we speak of a prudent or cunning tongue, because it is the instrument of a prudent or cunning man in advising something prudently or cunningly. Nor indeed (Gen. ad lit. xi, 28) did the serpent understand the sounds which were conveyed through it to the woman; nor again are we to believe that its soul was changed into a rational nature, since not even men, who are rational by nature, know what they say when a demon speaks in them. Accordingly (Gen. ad lit. xi, 29) the serpent spoke to man, even as the ass on which Balaam sat spoke to him, except that the former was the work of a devil, whereas the latter was the work of an angel. Hence (Gen. ad lit. xi, 36) the serpent was not asked why it had done this, because it had not done this in its own nature, but the devil in it, who was already condemned to everlasting fire on account of his sin: and the words addressed to the serpent were directed to him who wrought through the serpent."
Moreover, as again Augustine says (Super Gen. contra Manich. ii, 17,18), "his, that is, the devil's, punishment mentioned here is that for which we must be on our guard against him, not that which is reserved till the last judgment. For when it was said to him: 'Thou art cursed among all cattle and beasts of the earth,' the cattle are set above him, not in power, but in the preservation of their nature, since the cattle lost no heavenly bliss, seeing that they never had it, but they continue to live in the nature which they received." It is also said to him: "'Upon thy breast and belly shalt thou creep,'" according to another version [the Septuagint] "Here the breast signifies pride, because it is there that the impulse of the soul dominates, while the belly denotes carnal desire, because this part of the body is softest to the touch: and on these he creeps to those whom he wishes to deceive." The words, "'Earth shalt thou eat all the days of thy life' may be understood in two ways. Either 'Those shall belong to thee, whom thou shalt deceive by earthly lust,' namely sinners who are signified under the name of earth, or a third kind of temptation, namely curiosity, is signified by these words: for to eat earth is to look into things deep and dark." 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."
Source: Summa Theologica (New Advent)