Authors/Thomas Aquinas/posteriorum/L1/Lect23

Lecture 23 How demonstration “quia” and “propter quid” differ in a same science. Demonstration “quia” through an effect

Latin	English
	Lecture 23 (78a22-b13) HOW DEMONSTRATIONS “QUIA” AND “PROPTER QUID” DIFFER IN A SAME SCIENCE DEMONSTRATION “QUIA” THROUGH AN EFFECT
lib. 1 l. 23 n. 1 Postquam philosophus determinavit de demonstratione propter quid, hic ostendit differentiam inter demonstrationem quia, et demonstrationem propter quid. Et circa hoc duo facit: primo, ostendit differentiam utriusque in eadem scientia; secundo, in diversis; ibi: alio autem modo et cetera. Circa primum duo facit: primo, ponit duplicem differentiam utriusque demonstrationis in eadem scientia; secundo, manifestat per exempla; ibi: ut quod prope sint planetae et cetera.	After determining about demonstration propter quid, the Philosopher here shows the difference between demonstration quia and demonstration propter quid. And he does two things about this. First, he shows how they differ in the same science. Secondly, in diverse sciences (78b33) [L. 25]. Concerning the first he does two things. First, he states the twofold difference between the two in the same science. Secondly, he clarifies this with examples (78a30).
lib. 1 l. 23 n. 2 Dicit ergo primo: superius dictum est quod demonstratio est syllogismus faciens scire, et quod demonstratio ex causis rei procedit et primis et immediatis. Quod intelligendum est de demonstratione propter quid. Sed tamen differt scire quia ita est, et propter quid ita est. Et cum demonstratio sit syllogismus faciens scire, ut dictum est, oportet etiam quod demonstratio quae facit scire quia, differat a demonstratione quae facit scire propter quid. Et horum quidem differentia primo consideranda est in eadem scientia; postea consideranda est in diversis.	He says therefore first (78a22) that, as said above, demonstration is a syllogism causing scientific knowledge and proceeds from the causes both first and immediate of a thing. Now this is to be understood as referring to demonstration propter quid. But there is a difference between knowing that a thing is so and why it is so. Therefore, since demonstration is a syllogism causing scientific knowledge, as has been said, it is necessary that a demonstration quia which makes one know that a thing is so should differ from the demonstration propter quid which makes one know why. Consequently, this difference must be considered first in the same science and later in sciences that are diverse.
lib. 1 l. 23 n. 3 In una autem scientia dupliciter differt utrumque praedictorum, secundum duo quae requirebantur ad demonstrationem simpliciter, quae facit scire propter quid; scilicet quod sit ex causis, et quod sit ex immediatis. Uno igitur modo differt scire quia ab hoc quod est scire propter quid; quia scire quia est si non fiat syllogismus demonstrativus per non medium, idest per immediatum, sed fiat per mediata. Sic enim non accipietur prima causa, cum tamen scientia, quae est propter quid, sit secundum primam causam. Et ita non erit scientia propter quid.	In one and the same science each of the above is said to differ in regard to the two things required for demonstration in the strict sense—which causes knowledge of the why—namely, that it be from causes and from immediate causes. Hence one way that scientific knowledge quia differn from propter quid is that it is the former if the syllogism is not through, immediate principles but through mediate ones. For in that case the first cause will not be employed, whereas science propter quid is according to the first cause; consequently, the former will not be science propter quid.
lib. 1 l. 23 n. 4 Alio modo differunt, quia scire quia est quando fit syllogismus non quidem per media, idest per mediata, sed per immediata, sed non fit per causam: sed fit per convertentiam, idest per effectus convertibiles et immediatos. Et tamen talis demonstratio fit per notius, scilicet nobis: alias non faceret scire. Non enim pervenimus ad cognitionem ignoti, nisi per aliquid magis notum. Nihil enim prohibet duorum aeque praedicantium, idest convertibilium, quorum unum sit causa, et aliud effectus, notius esse aliquando non causam, sed magis effectum. Nam effectus aliquando est notior causa quoad nos et secundum sensum, licet causa sit semper notior simpliciter, et secundum naturam. Et ita per effectum notiorem causa potest fieri demonstratio non faciens scire propter quid, sed tantum quia.	It differs in another way, because it is science quia when the syllogism, although not through middles, i.e., mediate, but through immediate things, is not through the cause but through “convertence,” i.e., through effects convertible and immediate. Hence a demonstration of this kind is through the better known, namely, to us; otherwise it would not effect scientific knowledge. For we do not reach a knowledge of the unknown except through something better known. However in the case of two things equally predicable, i.e., convertible, one of which is the cause and the other the effect, there is nothing to preclude that now and then the better known will not be the cause but the effect. For sometimes the effect is better known than the cause both in respect to us and according to sense-perception, although absolutely and according to nature the cause is the better known. Consequently, through an effect better known than the cause there can be demonstration which does not engender propter quid knowledge but only quia.
lib. 1 l. 23 n. 5 Deinde cum dicit: ut quod prope etc., manifestat praedictam differentiam per exempla. Et dividitur in duas partes: in prima, ponit exempla de demonstratione quia, quae est per effectum; in secunda, de demonstratione quia, quae est per causam mediatam; ibi: amplius in quibus medium et cetera. Prima in duas: in prima, ponit exempla de syllogismo qui fit per effectum convertibilem; in secunda, de syllogismo qui fit per effectum non convertibilem; ibi: in quibus autem media et cetera. Prima dividitur in duas partes secundum duo exempla quae ponit; secunda pars incipit ibi: item sic lunam et cetera. Circa primum duo facit: primo, ponit exemplum de demonstratione quia, quae est per effectum; secundo, docet quomodo posset converti in demonstrationem propter quid; ibi: contingit autem et cetera.	Then (78a30) he clarifies these differences by examples. And this is divided into two parts. In the first he develops an example of the demonstration quia which is through an effect. In the second of demonstration quia which is through a mediate cause (78b13) [L. 24]. The first is divided into two parts. In the first he gives an example of a syllogism which is through a convertible effect. In the second of a syllogism through a nonconvertible effect (78b10). The first is divided into two parts according to the two examples he gives, the second of which begins at (78b3). Concerning the first he does two things. First, he gives an example of demonstration quia which is through an effect. Secondly, he states when it can be converted into a demonstration propter quid (78a39).
lib. 1 l. 23 n. 6 Dicit ergo primo quod demonstratio quia per effectum est, si quis concludat quod planetae sunt prope propter hoc quod non scintillant. Non enim non scintillare est causa quod planetae sint prope, sed e converso. Propter hoc enim non scintillant planetae, quia sunt prope. Stellae enim fixae scintillant, quia visus in comprehensione earum caligat propter earum distantiam. Formetur ergo syllogismus sic: omne non scintillans est prope; planetae sunt non scintillantes; ergo sunt prope. Sit in quo c planetae, idest accipiatur planetae quasi minor extremitas. In quo autem b sit non scintillare, idest non scintillare accipiatur medius terminus. In quo autem a sit prope esse, idest prope esse accipiatur ut maior extremitas. Vera igitur est haec propositio: omne c est b, quia planetae non scintillant. Et iterum verum est quod omne b est a, quia omnis stella non scintillans prope est. Huiusmodi autem propositionis veritas oportet quod accipiatur per inductionem, aut per sensum, quia effectus hic est notior causa quantum ad sensum. Et sic sequitur conclusio quod omne c sit a. Et sic demonstratum est quod planetae sive stellae erraticae sunt prope. Hic igitur syllogismus non est propter quid, sed est quia. Non enim propter hoc quod non scintillant, planetae sunt prope, sed propter id quod prope sunt, non scintillant.	He says therefore first (78a30) that demonstration quia is through an effect if one concludes for example that the planets are near because they do not twinkle. For non-twinkling is not the cause why the planets are near, but vice versa: for the planets do not twinkle because they are near. For the fixed stars twinkle because in gazing at them the sight is beclouded on account of the distance. Therefore, the syllogism might be formed in the following way: “Whatever does not twinkle is near; but the planets do not twinkle: therefore, they are near.” Here we let C be the planets, i.e., let “planets” be the minor extreme, and let B consist in not twinkling, and A “to be near” be the major extreme. Then the proposition, “Every C is B,” is true, namely, the planets do not twinkle. Also it is true that “Every B is A,” i.e., every star that does not twinkle is near. Rowever, the truth of such a proposition must be obtained through induction or through sense perception, because the effect here is better known than the cause. And so, the conclusion, “Every C is A,” follows. In this way, then, it has been demonstrated that the planets, i.e., the wandering stars, are near. Consequently, this syllogism is not propter quid but quia. For it is not because they do not twinkle that planets are iiear but rather, because they are near, they do not twinkle.
lib. 1 l. 23 n. 7 Deinde cum dicit: contingit autem et per alterum etc., docet quomodo demonstratio quia convertatur in demonstrationem propter quid, dicens quod contingit et per alterum demonstrare alterum, idest per hoc quod est prope esse, demonstrare quod non scintillant; et sic erit demonstratio propter quid. Ut sit c erraticae, idest accipiatur stella erratica minor extremitas; in quo b sit prope esse, idest prope esse accipiatur ut medius terminus, quod supra erat maior extremitas; a sit non scintillare, idest accipiatur non scintillare maior extremitas, quod supra erat medius terminus. Est igitur et b in c, quia omnis planeta prope est; et a est in b, quia omnis planeta, qui prope est, non scintillat; quare sequitur quod et a sit in c, scilicet, quod omnis planeta non scintillet. Et sic erit syllogismus propter quid, cum accepta sit prima et immediata causa.	Then (78a39) he teaches how a demonstration quia is changed to a demonstration propter quid. And he says that “it is possible to demonstrate the one through the other,” i.e., to demonstrate that they do not twinkle, because they are near. Then the demonstration will be propter quid. Thus let C be the wanderers, i.e., let “wandering star” be the minor extreme; let B consist in being near, i.e., let “to be near,” which was the major extreme above, be the middle term; and let A consist in not twinkling, i.e., let “not to twinkle,” which above was the middle term, now be the major term. Therefore, B is in C, i.e., “Every planet is near”; and A is in B, i.e., “Any planet which is near does not twinkle.” Wherefore, it follows that A is in C, i.e., “A planet does not twinkle.” In this way we have a syllogism propter quid, since it rests on the first and immediate cause.
lib. 1 l. 23 n. 8 Deinde cum dicit: item sic lunam demonstrant etc., ponit aliud exemplum ad idem, dicens quod sic (idest demonstratione faciente scire quia), demonstrant quod luna sit circularis per incrementa, quibus scilicet omni mense augetur et minuitur, sic argumentantes: omne quod sic augetur quasi circulariter, circulare est; augetur autem sic luna; ergo est circularis. Sic igitur factus est syllogismus demonstrans quia. Sed e converso, posito medio ipsius, fit syllogismus propter quid, scilicet si ponatur circulare ut medius terminus, et augmentum ut maior extremitas. Non enim ideo circularis est luna, quia sic augetur, sed quia circularis est, ideo talia augmenta recipit. Sit ergo luna in quo c, idest minor extremitas; augmentum in quo b, idest medius terminus; circularis autem in quo a, idest maior extremitas. Et hoc intelligendum est in syllogismo quia. E converso autem in syllogismo propter quid.	Then (78b3) he presents another example of this, saying that “in this way” (i.e., by means of a demonstration quia), “one demonstrates that the moon is round because of its phases,” according to which it waxes and wanes every month. They argue thus: “Everything which waxes thus circularly is circular; but the moon waxes thus: therefore, it is circular.” “Put in this form it is a syllogism demonstrating quia. But if the middle be interchanged,” i.e., if “circular” be made the middle term and “waxes” the major term, “it becomes a demonstration propter quid.” For the moon is not circular because it waxes in that way, but because it is circular it undergoes such phases. Therefore, let C be “the moon,” i.e., the minor extreme; let “waxing” be A, i.e., the major extreme, and let “circular” be B, the middle term. This will be the situation in the syllogism propter quid.
lib. 1 l. 23 n. 9 Deinde cum dicit: in quibus autem media etc., ostendit quod sit demonstratio quia per effectum non convertibilem, dicens quod in illis etiam syllogismis in quibus media non convertuntur cum extremis, et accipitur ut notius quoad nos, scilicet loco medii, quod non est causa, sed magis effectus, demonstratur quidem quia, sed non propter quid. Et quidem si tale medium convertatur cum maiori extremitate, et excedat minorem, manifestum est quod conveniens fit syllogismus. Sicut si probetur de Venere quod sit prope, quia non scintillat. Si autem e converso minor terminus esset in plus quam medium assumptum; non esset conveniens syllogismus. Non enim potest de stella universaliter concludi quod sit prope, propter hoc quod non scintillat. In comparatione autem ad maiorem terminum est e converso. Nam si medium sit in minus quam maior terminus conveniens fit syllogismus. Sicut si per hoc, quod est moveri motu progressivo, probetur de aliquo quod habeat animam sensibilem. Si autem sit in plus, non fit conveniens syllogismus. Nam ab effectu, qui a pluribus causis procedere potest, non potest una illarum concludi. Sicut non potest concludi, quod aliquis habeat febrem, ex excitatione pulsus.	Then (78b10) he shows that a demonstration through a non-convertible effect is quid. He says, therefore, that even in those syllogisms in which the middles are not converted with the extremes, and in which an effect rather than a cause is taken as the middle better known in reference to us, even in those cases the demonstration is quia and not propter quid. If the middle be such that it can be converted with the major extreme and it exceeds the minor, then obviously it is a fitting syllogism; for example, if one proves that Venus is near because it does not twinkle. On the other hand, if the minor exceeded the middle, it would not be a fitting syllogism: for one cannot conclude universally of stars that they are near because they do not twinkle. Quite the contrary is true in comparison to the major term: for if the middle is in less things than is the major term, the syllogism is fitting, as when it is proved that someone has a sensible soul on the ground that he is capable of progressive local motion. But if it is in more, than the syllogism is not fitting, for from an effect which can proceed from several causes, one of them cannot be concluded. Thus, one cannot conclude from a rapid pulse that he has a fever.