Cursus Philosophicus Thomisticus, Secunda Pars Artis Logicae

In the preceding question, article 1, we said that the question whether something exists concerns only existence or possibility—that is, whether some thing is given—yet not the cause or root on account of which it is given. The same procedure is to be followed in assigning the difference between demonstration quia and demonstration propter quid. For they differ chiefly in this: demonstration quia intends only to show evidently that something is true, that is, that it really obtains, without treating how it is true or the cause on account of which it obtains; demonstration propter quid investigates this cause.

The Philosopher distinguishes these demonstrations in Posterior Analytics, chapter 11 (otherwise 10)¹, both with respect to different sciences and with respect to one and the same science, for they can be found in either case. They occur in different sciences because many sciences do not know the cause and root of some thing, but know evidently only that such a thing exists, whether those sciences are disparate from one another or one is subordinate to another: knowing the why belongs to the subalternating science, and knowing the that to the subalternated science. We shall explain this more conveniently in the next question, when treating how subalternating and subalternated sciences differ. In one and the same science the Philosopher distinguishes these demonstrations by their different modes of proceeding; for, as St. Thomas observes in Posterior Analytics, lecture 23², a demonstration must proceed from true and immediate causes. Hence a demonstration can fall short of being propter quid either because it proceeds from effects rather than causes, or because it proceeds from remote rather than immediate and proximate causes, which are not the determinate root of this truth. For example, if someone proves that a stone does not breathe because it is not an animal, he gives a remote cause, since some animals do not breathe; hence this chiefly serves for negative demonstrations.

From this one may incidentally gather that some people, with insufficient care, confuse demonstration propter quid with demonstration a priori, and demonstration quia with demonstration a posteriori. This is not so: although every demonstration propter quid is a priori, the converse does not hold; and although every demonstration a posteriori is quia, the converse does not hold. For demonstration quia consists only in showing a truth, but not its proper and immediate root. That a truth obtains can sometimes be shown through something posterior, namely an effect; sometimes through something prior, namely a remote cause; and sometimes through something concomitant. Thus not every demonstration quia is only a posteriori, that is, from an effect; sometimes it proceeds through something prior, namely a remote cause. But every demonstration propter quid is always a priori, because it is always through a cause, and a proximate and immediate one.

Two things, then, remain to be explained here: First, whether this division is adequate; second, what sort it is, whether a division of a genus into species or of an analogue into its analogates.

As to the first, it should be said that this division is adequate; this is the common view, although the Commentator is commonly credited, at the beginning of the Physics, with adding another member of demonstration, which he called demonstration simpliciter, namely one that demonstrates not only the why but also that the thing exists. Others say that demonstration from a sign is neither propter quid nor quia.

NEVERTHELESS, OUR ASSERTION IS PROVED as follows: this division is given through members immediately opposed, one of which entails the contradictory negation of the other. For demonstration quia is that which presents some truth without showing its proper and particular root, whether this is done through a remote cause, an effect, or something else with which that truth has a per se connection, although it does not arise from it. Demonstration propter quid, however, is that which shows some truth according to its own proper cause and root, known as such. For an immediate, proximate cause is the contradictory opposite of a remote cause. Therefore these demonstrations differ by immediate opposition and thus adequately exhaust what is divided. This does not happen with demonstration a priori and a posteriori, because a demonstration can be made through something that is neither prior nor posterior, but concomitant, something simultaneous with another; for example, correlatives are simultaneous, and one is demonstrated through the other, as when, if there is a father, there is a son, and yet they do not stand as prior and posterior. Likewise, in mathematics demonstrations frequently occur that are founded on some proportion or correspondence between lines or other figures, without the notion of cause or effect intervening. To this also belong demonstrations drawn from acts of cognition or seeing, as when you say, “The stone is seen by me; therefore the stone exists”; and yet the stone’s being seen is neither the cause nor the effect of the stone’s existence. Whether demonstration propter quid and demonstration quia can be divided into other members, as the Commentator seems to hold in the cited place, we shall say in the solution of the last argument.

But you may ask what we mean by a remote cause and a proximate cause. The answer is that, as understood from Master Soto in Posterior Analytics I, question 5, and Cajetan on chapter 12 of Posterior Analytics I, a remote cause is one that is not convertible with its effect; for a remote cause influences its effect only remotely, and so exceeds it. When it is convertible with its effect, formally speaking and per se, it is called a proximate cause.

That a cause is not formally and proximately convertible with its effect can be considered in three ways. First, although it is convertible with the effect, it is not so immediately, but something must intervene between cause and effect, as essence considered with respect to the third and fourth properties, which it causes through the first or second. Second, by the opposite extreme, a cause is called remote when it is not convertible with its effect, nor does it entail the effect or follow from it, although it sometimes produces such an effect. For example, one might say that someone has a fever because he has an agitated pulse: sometimes an agitated pulse occurs without fever, as in someone running; sometimes, conversely, fever occurs without an agitated pulse, as in someone dying. Third, a cause is called remote when it is not adequately convertible with its effect, yet is a sufficient cause for inferring the effect’s negation, though not for positively positing the effect. Thus “It is not an animal; therefore it does not breathe” is valid, whereas “It is an animal; therefore it breathes” is not; and so the term “animal” is called a remote cause of breathing. To this belongs the example the Philosopher gives here of the man who, asked whether there were flute players in Scythia, replied that there were not even vines—a truly very remote cause.³

As for the character of this division, whether it is univocal or analogical, THE WHOLE DIFFICULTY comes down to whether both demonstrations cause knowledge simpliciter, or only demonstration propter quid. For the definition given above seems to belong to the latter alone: to know is to cognize the cause on account of which a thing is, and that this is its cause. Therefore the demonstration that does not cognize the cause on account of which a thing is does not simply produce knowledge, and so it agrees analogically with demonstration propter quid in the notion of scientific demonstration. On the other hand, demonstration quia and propter quid seem to agree univocally, since demonstration quia has certainty and evidence concerning its object; therefore it is opposed to opinion in the same way as knowledge propter quid. Opinion is uncertain and lacking evidence. Therefore it has the substance and essence of knowledge, since it has the substance and essence of evidence and certainty, which oppose and destroy opinion; thus it belongs to knowledge.

Nevertheless, one should say that demonstration or knowledge quia intrinsically and substantially has the nature of demonstration, insofar as it has certainty and evidence; thus it is absolutely a univocal demonstration according to the formal mode of proceeding certainly and evidently. But it participates imperfectly in the nature of knowledge, because it does not deal with a perfectly scientific object, which is quiddity and what follows upon quiddity; rather, it deals with the very question whether something exists, without attaining the root and cause of the truth on account of which it exists.

That it makes a true and proper demonstration on the side of certainty and evidence is deduced from St. Thomas, Summa theologiae I, q. 2, a. 2, where he proves that there can be a demonstration that God exists, although we proceed through effects, which are better known to us. The reason is clear. For demonstration quia rests on a necessary connection between effect and cause, or between a remote cause and an effect, as when through necessary connection it is proved that everything that comes to be comes to be from another, and that there is no infinite regress. Likewise, in negative demonstrations the negation of a remote cause rightly entails the negation of the effect, although the effect does not follow from the cause’s positive presence alone. Thus “It is not an animal; therefore it does not breathe” is valid, but not positively, “It is an animal; therefore it breathes.” Demonstration quia, then, is certain and evident.

But that on the side of the object it attains it is not perfect knowledge is proved because it does not attain the what, but only whether the thing exists. Yet the question whether it exists is presupposed to knowledge; hence no science can supply principles for proving whether its subject exists, but must presuppose these from elsewhere. Therefore demonstration quia necessarily proceeds from things external to the object, although not external to the connection with the conclusion demonstrated. Yet science that is properly and perfectly science does not proceed from things external, as the Philosopher proves in Posterior Analytics I, chapter 7⁴. Therefore demonstration quia deals only with preambles to science, not with the proper object of science, which is the what. Hence knowledge quia can belong and be reduced to the same knowledge propter quid. Although it proceeds from extraneous principles and in this respect seems to be of a different species, it is of a different species only imperfectly, like an embryo of knowledge serving the principal science in a ministerial way. For it deals with preambles and presuppositions to science, namely the question whether a thing exists, which will be explained better in the solution to the last argument. How it belongs to a subalternated science to know the that, and to a subalternating science to know the why, will be explained in the next question. This resolves the reasons for doubt raised at the beginning of this little question.

THE ARGUMENTS ARE SOLVED.

Against the first conclusion one argues, because on the one hand more demonstrations than propter quid and quia seem to be given, and on the other hand fewer. More seem to be given. First, there can be the demonstration that the Commentator in Physics I calls demonstration simpliciter, namely one that demonstrates both that the thing exists and why it exists, which is at once demonstration propter quid and quia. Again, demonstration through a sign demonstrates neither through a cause nor through an effect adequate to the thing; therefore it is a third kind of demonstration. Again, demonstration leading to the impossible, and likewise negative demonstration in which some negation is taken as the middle term, proceed neither through a cause nor through an effect, since neither negation nor impossibility is the cause or effect of the thing demonstrated. Finally, demonstration in which something is inferred to belong to an inferior from some superior predicate is neither quia nor propter quid, as in this syllogism: “Every animal is a substance; every human being is an animal; therefore every human being is a substance,” where the term “animal” is neither cause nor effect with respect to substance.

To the first example Cajetan replies in Posterior Analytics I, chapter 13, that the Commentator’s demonstration is not intermediate between demonstration quia and propter quid, but subdivides demonstration propter quid, as if to say: one demonstration is propter quid only, and another demonstrates both propter quid and at the same time that the thing exists. It seems truer, however, that demonstration propter quid always includes demonstration quia virtually and eminently. For one who demonstrates the cause of a truth, all the more demonstrates that such a truth obtains; but not conversely, for one who demonstrates that a truth obtains does not thereby also demonstrate the cause on account of which it obtains.

To the second example it is said that demonstration through a sign of itself proves only that the thing exists, and does not show its cause and root, whether the sign is an effect, a remote cause, or something concomitant; hence it belongs to demonstration quia.

To the third one replies that the demonstration of which the Philosopher treats here can be said to be only an ostensive demonstration; nevertheless, even demonstration by reduction to the impossible is reduced to demonstration propter quid or quia. For if it leads to the impossible through the causes of that impossibility, it is propter quid; but one that leads to the impossible from effects is demonstration quia. Thus, if you say, “If a horse reasons, it follows that it is rational,” the reasoning proceeds through effects. But if you say, “If a stone is alive, it follows that it moves itself,” it is propter quid. As for negative demonstration, we say that a negative can also in its own way be the cause of a negation, as “It is not an animal; therefore it is not rational” is valid. Thus, when the proper and immediate cause for denying something is brought forward, the demonstration is propter quid.

To the fourth it is said that this is demonstration quia: it proves only that such a truth obtains, without adducing its proximate and adequate cause, but rather an identical and remote one; for “animal” is not the proper and sole cause of something’s being a substance. However, when one proceeds from universal to singular, as in “Every human being is capable of laughter; therefore Peter is,” either this is not a proper demonstration but an exposition of the parts contained in the universal, or, if it is a demonstration, it is propter quid, for it presents the cause on account of which Peter is capable of laughter.

The second argument is: Demonstration quia, however certainly and evidently it proceeds, does not quiet the intellect, but leaves it still suspended until it knows the cause and root of the truth. But it is proper to knowledge to quiet the intellect; hence St. Thomas says in Posterior Analytics I, lecture 18⁵ that to know from proper principles alone is truly to know. And in lecture 23⁶ he says that demonstration propter quid is demonstration simpliciter. Therefore demonstration quia is not simply and univocally demonstration.

It is confirmed because demonstration quia is not composed of premises that are necessary without qualification; therefore it is not demonstration without qualification.

The antecedent is proved: the premises are either a remote cause or an effect. A remote cause does not have a necessary connection with its effect, as being an animal with being capable of breathing. Effects, too, are either supposed known through a cause or through experience. If through a cause, that demonstration will be propter quid; if through experience, its cognition is fallible and consequently does not ground a necessary demonstration.

The answer is that knowledge quia is imperfect knowledge and not knowledge simpliciter on the side of its object, because in that object it attains not the what but whether it exists. But with respect to what it does attain, it perfectly quiets the intellect, namely with respect to whether it exists, although it leaves the intellect unconvinced and unquiet with respect to what it is, since that does not pertain to it. In this respect that demonstration is imperfect, because it does not attain the notion of quiddity. Nor does St. Thomas mean more in those places where he attributes the nature of demonstration simpliciter only to demonstration propter quid: that is, if it is considered on the side of the perfectly scientific object, which is quiddity, for this it does not attain without qualification. But if it is considered according to the mode of attaining scientifically, that is, certainly and evidently, then demonstration quia too is demonstration simpliciter.

To the confirmation one replies that a remote cause serves chiefly for negative demonstration when it is not convertible with its effect, as “It is not an animal; therefore it does not breathe” is valid, but not positively, “It is an animal; therefore it breathes.” If, however, the remote cause is convertible with its effect, as when a mediated proper property is inferred from some essence, then one may also validly proceed positively from a mediated or remote cause. Nor can it be said that, in negative demonstration, the negation of a remote cause makes the demonstration propter quid, as when one demonstrates that a stone does not breathe because it is not an animal. This, I say, cannot be said, because although that negation sufficiently establishes the negation of breathing, since it does not deny the proper and immediate cause of breathing, it does not present the negation through its proper and immediate cause. For the proper reason for a negation is known through the immediate opposing form.

As for what is added about effects, they are per se known when they are known through certain and indubitable experience, or at least made evident inductively, because every science has its origin in induction; for example, when we show from effects that there is a first cause, it is indubitable that some effects exist.

The final argument is: Demonstration quia and propter quid proceed from different principles and under a different light, one from effects and the other from causes. Therefore they ought to generate habits different in species, because a scientific habit differs according to diversity of principles and light. Demonstration quia is therefore not reduced to knowledge propter quid, since it is not reduced to its principles.

THE ANSWER IS that if demonstration quia and propter quid proceed in different sciences, whether placed in a subalternating relation or disparate, then it is clear that they also belong to different habits. But if both demonstrations proceed in the same science—insofar as demonstration quia touches the question whether the object exists, and demonstration propter quid the question what it is, as, conversely, in theology God’s existence is proved by demonstration quia—then some say that, although demonstration quia and propter quid proceed from different principles and so are different in species, they nevertheless generate the same assent in species and consequently the same habit, just as heat produced by fire and by the sun is of the same species. So says Master Soto, Posterior Analytics I, question 5. Others, such as Master Cabero, say those assents are different in species.

But it seems to me that this can be resolved more clearly from Cajetan’s view in Summa theologiae I, q. 2, a. 3, where he says that, since no science proves from its own proper principles that its subject exists, but does so from extraneous principles appropriated to it, when those extraneous principles are better known to us, theology proves that God exists not from its own principles simply, but from principles proper to it ministerially, namely those known by natural light that serve theology. Hence we say that although demonstration quia, when it occurs in the same science in which one proceeds propter quid, proceeds from extrinsic principles and consequently principles distinct from those of demonstration propter quid, nevertheless, because they are appropriated to the object of that science and dispose one toward it—since they concern whether it exists and prepare one to understand its quiddity and demonstrate properties of it—demonstration quia is said to be reduced to demonstration propter quid. For although it proceeds from different principles, they are nevertheless appropriated to the object of demonstration propter quid, disposing and preparing for it through a demonstration of the thing with respect to whether it exists. And although the assents are different in species if these demonstrations are considered in themselves and separately, as it were in their entity, insofar as they are coordinated and one is appropriately ordered to the other, preparing for it, demonstration quia belongs reductively to the same species of knowledge as demonstration *propter quid—not absolutely, but appropriately and reductively, or dispositively—just as dispositions toward a substantial form, although entitatively, are of a distinct genus.