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Many of Aquinas' proofs are based on Aristotle's writings. But, why did Aquinas rely so heavily on Aristotle? Science proves that Aristotle sometimes erred in his own writings. For example, Aristotle wrote (On the History of Animals, Book I, Sec. 15) that humans had eight ribs on each side,

Common to the upper and lower part of the trunk are the ribs, eight on either side...

Κοινὸν δὲ τοῦ ἄνω καὶ κάτω τοῦ θώρακος πλευραί, ἑκατέρωθεν ὀκτώ...

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It depends on how you count. If you define a rib as a bone that is connect to the sternum, then yes - human has eight ribs. This is of course different from today's definition, but this must be how Aristotle and Aquinas counted them. Thinking that Aristotle and Aquinas never saw a human skeleton is ridiculous. This is similar to the myth that Aristotle believed fly has 4 legs. Aristotle wrote about a species of mayfly that uses only four legs for walking. –  zefciu Feb 19 '13 at 9:14
I am reminded that Socrates had the habit of calling individuals gadflies. –  Juann Strauss Aug 14 '13 at 15:14

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Thomas Aquinas definitely took advantage of Aristotle (following the lead of Albertus Magnus) but he was not uncritical in his reception. Aristotle's major writings were only recently rediscovered in the West and were the topic of intense scholarly effort. But at the same time, there was a lot of doubt about whether Aristotle was a suitable source of knowledge for Christians - he was not only a pagan, but his work was also seen as potentially 'contaminated' by its transmission through the Muslim world. As a scholar, then, the aim of Aquinas could not have been to simply parrot the teachings of Aristotle, but to reconcile or absorb them into Christian thought. For example, Aristotle argues in the Physics that the world has always existed, but Aquinas says "that conflicts with our faith" (Sententia super Physicam 8.2).

In any case, for Aquinas and Albertus, the main attraction of Aristotle wasn't just that he was a fertile source of good philosophical material in general. They were really interested in his metaphysics, because of the promise it had for establishing a coherent system of thought covering theology, philosophy, science, ethics, and politics. Thomism includes an understanding of 'natural law' and 'natural theology' that is the bridge between revealed truth on the one hand, and observation and reason on the other. In this way, Scripture and tradition can be brought to bear on philosophical questions that would otherwise be inaccessible, since the Bible is not a book of philosophical propositions. It also allows better engagement with non-Christians, since "Muslims and pagans do not agree with us on the authority of any scripture" and so "one must fall back to natural reason" (Summa contra Gentiles 1.2). Aristotle's ethics were another part of this - Thomas wrote a commentary on the Nicomachean Ethics, and his natural law approach to ethics is very much due to Aristotle's teleological ideas, again resting on his metaphysics. If you look at modern work in the tradition of Thomas, it's all about this kind of thing: the twenty-four theses of Pius X in Postquam sanctissimus (1914) are primarily metaphysical (in re praesertim metaphysica).

It is true that there are problems with Aristotle's science which Thomas did not correct. And the same goes for other philosophers he used. The rib issue is pretty harmless but there are other points, like the nature of women, which are not. He follows Aristotle in assuming that women are imperfect men, suffering from a birth defect caused by excessive moisture in the womb (On the generation of animals; Summa Theologiae 1.92.11) and therefore in a state of subjection (Summa Supplement 39.1). Of course we now know that this is wrong, but it's not reasonable to expect a thirteenth-century man to know what we do. These flaws in Aristotle are evident to us but would not have been so glaring to Aquinas. In any case, Thomas is more interested in the methodology of Aristotle (including his logic and metaphysics) than in his specific data (e.g. that men have more teeth than women, History of Animals 2.3). Again, the modern iteration of Thomism would be able to reject "anything that ill agrees with the discoveries of a later age" (Leo XIII, Aeterni Patris, 1879) while still accepting the basic approach.

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