The Ways to Wisdom
Aquinas Day by Day: Logic of Arguments: Demonstration

55

Aquinas’ Topic: Logic of Arguments: Demonstration

Scripture:

Aquinas’ text: Expositio libri Posteriorum , Bk. 1, lec. 4

Br. Thomas begins his explanation of demonstration with one of two definitions of the demonstrative syllogism.

After the Philosopher shows the necessity for the demonstrative syllogism, he begins his determination about the demonstrative syllogism itself. And his treatment is divided into two parts. In the first he determines about the demonstrative syllogism; while in the second he determines concerning the middle term by means of which the demonstrative syllogism proceeds (89b21) [Book II].

Concerning the first issue, one ought to know that in all things that exist for the sake of an end, the definition in terms of the final cause is both the explanation of the definition in terms of the material cause and is the middle term that proves it.  This is why a house must be made of stone and wood, because it is a structure protecting us from the cold and heat. Aristotle therefore gives two definitions of demonstration, one of which is taken from the end of demonstration, which is to know scientifically, and from this definition he concludes to the other definition, which is taken from the matter of demonstration.

Postquam ostendit philosophus necessitatem syllogismi demonstrativi, hic iam incipit de ipso syllogismo demonstrativo determinare. Et dividitur in duas partes: in prima, determinat de syllogismo demonstrativo; in secunda, de medio ex quo syllogismus demonstrativus procedit: et hoc in secundo libro; ibi: quaestiones sunt aequales numero et cetera. … Circa primum sciendum est quod in omnibus quae sunt propter finem, definitio quae est per causam finalem, est ratio definitionis, quae est per causam materialem, et medium probans ipsam: propter hoc enim oportet ut domus fiat ex lapidibus et lignis, quia est operimentum protegens nos a frigore et aestu. Sic igitur Aristoteles de demonstratione dat hic duas definitiones: quarum una sumitur a fine demonstrationis, qui est scire; et ex hac concluditur altera, quae sumitur a materia demonstrationis.

[Introductions and translations © R.E. Houser]

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