Incidentally, in an aside at the end, Bates brings up the old claim that Aquinas holds that the female is a defective male. In fact, it is Aristotle as received in the Latin who holds that the female is a defective male; Aquinas on the contrary argues that the only thing this can reasonably mean is the Aristotelian view that males result from the semen, as the 'male' principle, overpowering the female principle and that females result from a defect in such power, as a result of which the female principle wins. He denies it any more significance than this. There definitely are important points at which one can say that Aquinas's reasoning is incomplete because he uncritically takes a male perspective; but this is not one of them.
(It should be noted, incidentally, that this is not quite right; I shouldn't have talked about 'female principles' here, since, while Aquinas holds there is such a thing, it doesn't play much of a role in this type of discussion. But this doesn't affect what follows.) One of the puzzles with which Aquinas had to deal in handling Aristotle was Aristotle's claim that woman was (in the Latin translation) occasionatus and imperfectus, i.e., by chance and incomplete. The place where he most clearly discusses the text is ST 1. 92.1, where he is faced with the objection that, because woman is occasionatus and imperfectus, it was inappropriate for her to be made prior to man's sin. To which Aquinas replies by confining the Aristotelian claim entirely to the field of biology. That is, it's not a normative claim about women, but a claim about how women are generated:
"To the first it must be said that with respect to the particular nature, the female is something deficiens et occasionatum, because the active virtue which is in the seed of the male, is disposed to produce (intendit producere) its perfect likeness, according to the masculine sex, but for the female to be generated is due to a weakness in the active force, or to some indisposition in the matter, or even to some transmutation from something extrinsic, like the south wind, which is humid, as is said in the book on the Generation of Animals. But with regard to the universal nature, the female is not something occasionatum but is according to the disposition of nature to the orderly work of generation. But the disposition of the universal nature depends on God, who is the universal author of nature. And thus in instituting the nature He produced not only male, but also female."
The "particular nature" here, as is clear from elsewhere, is the particular nature of the semen, which in Aristotelian fashion he treats as the virile principle disposed to produce men. Deficiens applied to products of processes also clearly means not 'defective' but 'unfinished, incomplete'. He also considers in ST 1.99.2 the claim that because the female is occasionatus she is unnatural; which Aquinas denies on the same grounds.
Earlier in the Commentary on the Sentences he had dealt with a similar use of the Aristotelian claim in which it was argued that it was inappropriate for women to be resurrected; he had responded with exactly the same point, and, moreover, with regard to another argument to the same conclusion, based on the idea that nothing defectus will be resurrected, he goes on to say that in the resurrection no one will be defectus because of their sex.
So when Aquinas addresses the claim that the female is a defective male, he confines it to a purely biological claim about generation, and makes clear that there is nothing inherently defective about the female sex and that women are essential to the completion of human nature. Aquinas raises the claim in each case to deny that it has significance outside of the subject of human generation.
It is noteworthy, in fact, that Aquinas always confines the claim to the biological claim of generation. He does not regard woman as a defective male insofar as she is woman, but insofar as she is thought to be in Aristotelian biology the incidental product of a biological process of generation disposed to produce males.
Aquinas does, of course, think that men are perfectior, more complete. And he does not confine this to biology, because he thinks men are perfectior not simply relative to this biological process of generation, but also relative to 'vigor of the soul'. He also thinks the male sex the naturally supereminent sex. There is no question that Aquinas is sexist in this way. But even where he is very clear about this (e.g., Super I Cor., cap. 11), he treats the claim that the female is occasionatus as having to do only with how the woman is produced in a generation process assumed to be disposed toward the producing of males.
So, again, in all charity and justice, the claim needs to die. In fact, the claim, which is simply false of Aquinas, and utter nonsense in light of the actual texts, simply obscures the real issues of sexism here. More on that later. First I want to address a passage (De Veritate 5.9 ad 9) that Bates, rather mysteriously, thinks supports his case:
Nisi ergo esset aliqua virtus quae intenderet femineum sexum, generation feminae esset omnino a casu, sicut et aliorum monstrorum.
Which Bates translates (again, mysteriously) as:
If it were not for some [divine] power that wanted the feminine sex to exist, the birth of a woman would be just another accident, such as that of other monsters.
But this is assuredly not right. 'Intenderet', for instance, does not mean 'wanted'; something that 'intends' in the scholastic sense disposes something to some end. Further, it's pretty clear that it's not divine power that's in view in this particular context, but the power of the celestial bodies as universal causes. So the real meaning here is something like: Were it not that some power aimed at the feminine sex, the generation of woman would be wholly by chance as with other monsters. But this is easily understood. If nothing disposed things so as to produce females, females would be wholly accidental, like other things that are produced contrary to natural dispositions (monstrosities). But Aquinas, in fact, believes, and insists very clearly, that something does intend females; it is, in fact, very important for him to deny that women are omnino a casu like monsters. I don't see how this can be misunderstood. It's right there in the text when read as a whole.
If you read what I wrote in my original response (which I quote above) I did not in fact deny that there were sexist aspects to Thomas Aquinas, although perhaps I could have been a little more clear about that. But proper understanding of them requires that we separate the blatantly false accusations from the more subtle accusations. To reflect seriously on sexism in Aquinas there is no need to libel him.
The issue of sexism in Aquinas is actually very complicated. It's clearly there, but the precise character of it is difficult to pin down. It goes roughly like this:
(1) Aquinas clearly regards women as subject to men (when he discusses it explicitly he also always confines it to domestic life). He insists that this is not a servile subjection but a wardship. He's clearly sexist in this regard; but it's a carefully limited sexism.
(2) He does indeed, as Bates notes, say that the reason women are subject to men is that "naturally in man a greater discretion of reason abounds." Since Aquinas holds elsewhere that it is the passions that affect discretion of reason (distinctio rationis, in fact, probably being simply the distinctness of reason from the senses), this is very likely the age-old claim that women are more 'emotional' or more 'ruled by passion' or more 'affected by sensibility' than men. However, he elsewhere also says men and women are both in the image of God, inasmuch as in the mind they both possess no sexual distinction.
(3) He holds that man is the 'nobler sex', which is, he says, why Christ became a man; but also says that Christ was born of woman in order to show that women should not be despised.
(4) Aquinas holds that women cannot become priests precisely because they are naturally subject to men, due to the kind of spiritual authority priests must have; but in arguing this he also insists that they can have higher spiritual authority, as prophets, if God grants it to them; delegated spiritual authority (as is the case with abbesses); and temporal authority to judge and rule in the world.
(5) He further denies that woman will be subject in the world to come, since the world to come will be based not on biology but on merit, and if women burn with greater charity than men, they will receive greater glory than men.
So it's a complex matter requiring careful discernment of distinctions. We must beware of a dismissal of Aquinas that throws the baby out with the bathwater, and ignores the things he gets right in order to make exaggerated claims about what he gets wrong. That he does get some things very wrong is not an excuse. One of the very crucial functions of history of philosophy as a discipline is justice-based critique; and we must be extraordinarily careful not to damage the credibility of this type of task by making accusations that don't stand up in close examination of the evidence. Claims like those above, that Aquinas holds that women are defective monstrosities, are exactly the sort of thing that give justice-based critique a bad name. And doing that is a great disservice to everyone.