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From St. Thomas' Summa theologiae I.2.2c:
I answer that it must be said that demonstration is twofold: One which is through the cause, and is called demonstration "propter quid" [lit., 'on account of which'] and this is [to argue] from what is prior simply speaking (simpliciter). The other is through the effect, and is called a demonstration "quia" [lit., 'that']; this is [to argue] from what is prior relatively only to us (quoad nos). When an effect is better known to us than its cause, from the effect we proceed to the knowledge of the cause. And from every effect the existence of its proper cause can be demonstrated, so long as its effects are better known to us (quoad nos); because since every effect depends upon its cause, if the effect exists, the cause must pre-exist.
From Aristotle's Posterior Analytics I.13:
"Knowledge of the fact (quia demonstration) differs from knowledge of the reasoned fact (propter quid demonstrations). [...] You might prove as follows that the planets are near because they do not twinkle: let C be the planets, B not twinkling, A proximity. Then B is predicable of C; for the planets do not twinkle. But A is also predicable of B, since that which does not twinkle is near--we must take this truth as having been reached by induction or sense-perception. Therefore A is a necessary predicate of C; so that we have demonstrated that the planets are near. This syllogism, then, proves not the reasoned fact (propter quid) but only the fact (quia); since they are not near because they do not twinkle, but, because they are near, do not twinkle...."
A (major term) = close heavenly body
B (middle term) = non-twinkling heavenly body
C (minor term) = planet
Major Premise: B is A
Minor Premise: C is B
Conclusion: C is A
=
Major Premise: Non-Twinkling heavenly bodies are close heavenly bodies.
Minor Premise: Planets are non-twinkling heavenly bodies (effect).
Conclusion: Planets are close heavenly bodies (cause).
From Aristotle's Posterior Analytics I.13 (cont'd):
"The major and middle of the proof, however, may be reversed, and then the demonstration will be of the reasoned fact (propter quid). Thus: let C be the planets, B proximity, A not twinkling. Then B is an attribute of C, and A-not twinkling-of B. Consequently A is predicable of C, and the syllogism proves the reasoned fact (propter quid), since its middle term is the proximate cause...."
A (major term) = non-twinkling heavenly body
B (middle term) = close heavenly body
C (minor term) = planet
Major Premise: B is A
Minor Premise: C is B
Conclusion: C is A
=
Major Premise: Close heavenly bodies are non-twinkling heavenly bodies.
Minor Premise: Planets are close heavenly bodies (cause).
Conclusion: Planets are non-twinkling heavenly bodies (effect).
3 comments:
Why is this important? Well, for many reasons.
For example, Aquinas teaches that God's existence can be demonstrated with a quia demonstration, but not with a propter quid demonstration.
Also, the most scientific kind of demonstration is the propter quid demonstration, because it proceeds from a knowledge of the essence or of the cause of something. Only propter quid demonstrations can completely satisfy the intellect's desire to know something. As lovers of wisdom, therefore, we must seek propter quid demonstrations when they are available, and not be content merely with quia demonstrations, since quia demonstrations do not completely satisfy the intellect.
A demonstration propter quid = a demonstration a priori, and a demonstration quia = a demonstration a posteriori, or is there a subtle difference? Thanks
Well, a 'a priori' and 'a posteriori' are not Thomistic concepts. They are modern concepts, typically found in Kant, Descartes, and their ilk. In the Kantian sense, 'a priori' knowledge is knowledge obtained independently of sense experience; whereas 'a posteriori' knowledge is knowledge that is obtained from sense experience (for Kant, only 'a priori' knowledge is worthwhile, because he thinks the senses are not reliable).
At least given this Kantian meaning, 'a priori' (a la Kant) is obviously not the same as propter quid (in the Thomistic sense), since in Thomistic epistemology 'there is nothing in the intellect that does not first come through the senses.'
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