The Ways to Wisdom

The Ways to Wisdom

Aquinas Day By Day


Aquinas’s topic:  logic of argument: proper scientific principles


Aquinas’s text:  Expositio libri Posteriorum , Bk. 1, lec. 5

Br. Thomas here clarifies the nature of the two kinds of proper scientific principles, hypotheses, also called postulates, and definitions.

Then (72a19) he subdivides the second half of the original division, namely, “thesis,” and he says that there is one kind of thesis that takes one side of an enunciation, namely, either affirmation or negation. He refers to this kind when he says, “as when I say something is or is not.”  This kind of thesis is called a “hypothesis,” one hypothesizes it “as having truth.” A second kind of thesis is one which does not signify “is” or “is not,” such as a definition that is said to be a “thesis.”  For the arithmetician lays down the definition of unity as a principle, namely, that unity is what is “indivisible in quantity.”  Nevertheless a definition is not called a hypothesis, for that is hypothesized, strictly speaking, which signifies the true or the false. Therefore, he adds that “the quiddity of unity” which signifies neither the true nor the false, “is not the same as being one,” which does signify the true or the false.

Now here one might raise this question:  Since a definition is not a proposition signifying “to be or not to be,” how can it be set down as a subdivision of immediate propositions? Response:  In this subdivision he is not subdividing immediate propositions but immediate principles. Or a second response:  Although a definition in itself is not an actual proposition, nevertheless it is a proposition virtually, because when a definition is known, it is clear that the definition is truly predicated of the subject.

Deinde cum dicit: positionis autem quae est etc., subdividit alterum membrum primae divisionis, scilicet positionem: dicens quod quaedam positio est, quae accipit aliquam partem enunciationis, scilicet affirmationem vel negationem; quod significat cum dicit: ut dico aliquid esse aut non esse. Et haec positio suppositio dicitur, quia tanquam veritatem habens supponitur. Alia autem positio est, quae non significat esse vel non esse, sicut definitio, quae positio dicitur. Ponitur enim ab arithmetico definitio unitatis, tanquam quoddam principium, scilicet quod unitas est indivisibile secundum quantitatem. Sed tamen definitio non dicitur suppositio: illud enim proprie supponitur, quod verum vel falsum significat. Et ideo subdit quod non idem est, quod quid est unitas, quod neque verum, neque falsum significat, et esse unitatem, quod significat verum vel falsum.

Sed potest quaeri: cum definitio non sit propositio significans esse vel non esse, quomodo ponatur in subdivisione immediatae propositionis. Sed dicendum quod in subdivisione non resumit immediatam propositionem ad subdividendum, sed immediatum principium. Principium autem syllogismi dici potest non solum propositio, sed etiam definitio. Vel potest dici quod licet definitio in se non sit propositio in actu, est tamen in virtute propositio quia cognita definitione, apparet definitionem de subiecto vere praedicari.

[Introductions and translations © R.E. Houser]

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The Center for Thomistic Studies at the University of St. Thomas is the only graduate philosophy program uniquely focused on the thought of St. Thomas Aquinas in the United States. The Center is founded on the Church’s insistence of the perennial value of the thought of Aquinas as the new millennium proceeds.
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