Cursus Philosophicus Thomisticus, Secunda Pars Artis Logicae

THREE COMPARISONS can be made between opinion and science. The first compares act with act, namely, certain and evident assent, which is science, with uncertain and nonevident assent, which belongs to opinion or faith. The second compares habit with habit: the habit of science is an inclination to evident assent, contrary to the inclination that inclines toward nonevident assent. The third is between habit and act, as when someone has the habit of opinion and elicits an act of science, or the reverse.

As for the first comparison, some absolutely judge that an act of science and an act of opinion can coexist in the same intellect concerning the same object, because they think they are merely disparate, not privatively opposed as evident and obscure, insofar as they proceed from different motives. So Master Cabero here, tractate 7, disputation 4, doubt 6. Others think that opinion, which is joined to actual hesitation and fear, cannot coexist with an act of science, but that it can if it lays aside that fear in the manner explained in the preceding article. So the Conimbricenses, Posterior Analytics, book 1, ch. 26, q. 2, a. 2. They also hold, in a. 3, ad 4, that divine faith, which has certainty, does not conflict with science; thus they seem to posit no opposition between evidence and nonevidence once uncertainty is removed. Finally, some recent authors, commenting on St. Thomas, Summa theologiae, II-II, q. 1, aa. 4 and 5, distinguish abstractive evidence from intuitive evidence, which is vision. They say that vision is wholly incompatible with faith or opinion because it reaches the thing immediately in itself, but that abstractive evidence is not incompatible, because it does not immediately reach the thing itself and thus does not directly conflict with the obscurity of faith.

NEVERTHELESS, I SAY FIRST: An obscure and an evident act, whether of opinion or faith, and whether of intuitive or abstractive evidence, are opposed to one another in the same intellect by privation on the side of the subject; the same holds for a certain and an uncertain act.

Many authors agree with this conclusion; you can consult them in the Carmelite Course, disputation 20 of Logic, question 4. It is also the express position of St. Thomas. For in On Truth, q. 14, a. 9, ad 6,¹ he says “that it does not seem possible for someone to have both science and opinion of the same thing, because opinion involves fear of the opposite part, and science excludes this fear.” And in Posterior Analytics, lecture 44,² he says: “It is manifest that the same thing cannot be wholly known and opined at the same time, because a person would at once judge that it could be otherwise and that it could not be otherwise.” This agrees with what he resolves in Summa theologiae, II-II, q. 1, a. 5, ad 4, where he teaches “that in one person science cannot coexist with opinion or faith at the same time concerning the same thing in the same respect, though for different reasons. Science and opinion cannot coexist simply concerning the same thing, because it belongs to the nature of science that what is known be judged unable to be otherwise, whereas it belongs to the nature of opinion that what is opined be judged able to be otherwise.” Of faith and science he adds “that they conflict only on the side of what is seen and what is not seen.”

It is no objection that St. Thomas, in Summa theologiae, I-II, q. 67, a. 3, teaches absolutely that nothing prevents perfect and imperfect cognition, on the side of the medium, from applying to one object in one subject: one person can know the same conclusion through both a probable and a demonstrative medium. Therefore, he plainly judges that an act of opinion and an act of science can coexist in the same subject, since they proceed through different media; for a probable medium and an opinion-forming medium are the same. He teaches something similar in 3 Sent., d. 31, q. 2, a. 1, question 1, ad 4³, and Summa theologiae, III, q. 9, a. 3, ad 2. These passages, I say, are no objection, although the cited texts seem difficult in particular, so that Father Suarez, in his commentary on that question 9 of the third part, wanted St. Thomas to have held that opinion and science can coexist. Yet the solution is derived from this very passage, where St. Thomas says “that opinion caused by a dialectical syllogism is a way to science, which is acquired through demonstration; once science has been acquired, however, cognition acquired through a dialectical syllogism can remain, as it were, consequent upon demonstrative science, which is through a cause.” Cajetan astutely weighs this passage: St. Thomas's words must be distinguished, for he first names a dialectical syllogism as opinion, and in the second place does not call it opinion but cognition. This shows that opinion is not simply the same as probable cognition taken in any way whatever, but rather as it is exercised as a reason and motive for assent with fear of the opposite part. But when St. Thomas speaks simply of cognition through a probable medium, with respect to the force of its probability concerning a given truth—a cognition that can arise through reflection consequent upon demonstrative science—then it is not opinion. St. Thomas seems to have spoken with the same care and qualification in the cited passage from Summa theologiae, I-II. For he says that cognition of a conclusion through a probable medium and through a demonstrative medium differs on the side of the medium according to perfection and imperfection. On the side of the subject, however, opinion, faith, and science differ according to perfection and imperfection; for it belongs to the nature of opinion that one part be accepted with fear of the other opposite. Thus, in the first place he does not name opinion, but in the second he does.

From this it appears that, according to St. Thomas, opinion in its formal character and exercise cannot remain with science, since they conflict on the side of the subject. A probable medium can nevertheless remain along with a demonstrative medium—not insofar as it constitutes and exercises an act of opinion in the intellect, but because cognition of the probable medium remains, as it were, reflectively. A person with science better knows the probative force of probable signs concerning a conclusion, but is not led by those signs, in their actual exercise, to assent. This is what St. Thomas expresses in the cited passage from the third part, saying “that dialectical cognition remains consequent upon demonstrative science, because one who knows the cause can also better know the probable signs from which a dialectical syllogism proceeds.” Therefore, according to St. Thomas, a probable medium remains with a demonstrative one only insofar as the probable signs are better known through scientific cognition. This happens only because the probability of those signs is more deeply understood, which is a kind of reflective cognition of the probable medium itself. Thus it does not constitute an act of opinion in its exercise concerning the truth and object of scientific assent⁴, but rather, in a signified act, the probability of the signs is demonstrated.

It is no objection that, on this account, one could also say that faith and science can coexist in the same subject, which St. Thomas denies. The inference is clear, because through science someone can better know, in a signified act, the authority of a speaker as a reason for believing. Therefore that interpretation is insufficient. I reply: I deny the antecedent. From something demonstrated through a probative medium it does not follow that someone thereby better knows the speaker's authority as a motive, since authority is not manifested through a probative medium; it lacks proof and reasoning, and is reached by simple intuition, while the motive of authority can increase through an act of the will. Opinion, however, or a probable medium, also proceeds in a probative way; therefore its probability becomes better known when aided by the higher probative light of science.

THE BASIS OF THIS CONCLUSION is taken from the very opposition between evidence and nonevidence, certainty and uncertainty, concerning the same truth. To discern and understand this opposition, consider that there is no difficulty in evidence and nonevidence producing incompatible effects in the intellect itself if they affect it absolutely, because evidence and nonevidence in themselves express a privative opposition.

The difficulty is how this opposition is found when the effects arise through different media. It is not incompatible for the intellect to see through one medium and not see through another; thus, when using different media, seeing and not seeing are not contradictory, just as knowing more or less perfectly through different sciences of greater or lesser perfection is not contradictory.

There is, however, a great equivocation here, because different media can coexist in the same subject with respect to the same truth only when the media are not opposed to one another and do not produce opposed effects, or when they are referred back to the objects themselves so as to constitute a different truth or object. But if different media are opposed in such a way that they produce opposed effects—namely, assents, one evident and the other nonevident—and thus affect the intellect concerning the same truth, then they are incompatible even if they proceed from different media. It is like darkness and light: although they are caused by different causes, if they are placed in the same subject they are privatively opposed.

Therefore, when an evident and an nonevident medium, a certain and an uncertain one, concur—and they are such that one lacks in itself a perfection that the other includes—to see whether they also exercise opposition on the side of the subject, we must attend to the effects they leave in the subject. They can be compared with the subject in only two ways. In one way, they affect the intellect absolutely and in their exercise, leaving it at once seeing and not seeing, or failing to reach vision of, one truth; at rest and convinced of the truth and not at rest or fully convinced of that same truth, even though these different motions and affections arise from different principles or motives. In the other way, they do not affect it absolutely and in their exercise in this manner, but with a certain reflection and connotation: for example, judging that something is nonevident and uncertain insofar as it is subject to such nonevident and uncertain premises, but certain and evident insofar as it is subject to other premises. Here being subject to such premises is not only an operative and effective motive leading the intellect to see or not see; it is also an object about which judgment is made as to whether it entails vision or does not entail vision.

In the first way, there must be opposition and conflict between these assents and between the media themselves. Thus St. Thomas, in Posterior Analytics, lecture 44,⁵ teaches from Aristotle that science and opinion conflict in the same way as true and false opinion, which are certainly opposed concerning the same object. The reason is that they produce wholly opposed effects and contrary motions in the same intellect. Through one motion the intellect comes to see clearly; through the other it remains without evidence and does not come to see one and the same truth. Through one it fully rests and its assent is brought to an end; through the other it is not convinced, does not rest, and its indifference is not overcome. Thus to affect a power in this way is to posit incompatible effects in it on the part of the power itself: being moved and not moved, or resting, seeing and not seeing the same thing in act. This is like leading the same eye to see a thing clearly and not see that same thing clearly. Although this occurs through different causes and media, the effects themselves are incompatible and therefore cannot be posited together in the same subject. Generally speaking, all incompatible effects tend to arise from different causes and under different motives, yet they cannot be united in the same subject, because the efficacy of different causes does not remove the formal incompatibility of their effects. So here, if different motives and media function as causes that effectively produce incompatible effects—seeing a thing and remaining obscure without evidence—their incompatibility is not removed by their being introduced through different media or causes.

If it happens in the second way, it is clear that opinion and science are not incompatible, because then it is not opinion but certainty, and the judgment concerns not the same truth but a different one. For the judgment that some conclusion, insofar as inferred from probable premises, is nonevident and uncertain is itself entirely certain: it is not made about the truth inferred, but about its inference and dependence on the premises, and it is certain that this inference is uncertain. In this way, as we said above from St. Thomas, a probable medium can coexist with science as its consequence, because from the cause that demonstrates, one can know the probable signs themselves more perfectly. But we are not speaking here in this sense; we speak of opinion and science insofar as they exercise their effects on the intellect concerning the same truth of the conclusion, not insofar as one is judged in a signified act to be uncertain and the other certain.

That there is opposition between opinion or faith and the evidence of vision, and also the evidence of science, even abstractive science, as St. Thomas teaches in Summa theologiae, II-II, q. 1, a. 5, is clear from this: all scientific evidence, even abstractive evidence, originates in some intuitive vision and is necessarily connected to it, as we saw in the preceding article when St. Thomas enumerated all the modes of evidence. Since scientific evidence is intrinsically connected with and originates from the vision of principles, obscurity, which is immediately opposed to science, is mediately and through a necessary connection opposed to vision. The opposition by which one thing opposes and destroys another also destroys what is necessarily joined to it, just as what is opposed to risible is opposed to rational.

As for the second comparison, between the habit of opinion and the habit of science, many think that these habits are not opposed to one another, but can coexist in the same subject; for they do not involve actual obscurity and evidence, but only an inclination toward formally obscure or evident acts.

NEVERTHELESS, I MUST SAY SECOND: These habits are contrarily opposed to one another as opposed inclinations toward opposed acts, just as in natural things heaviness and lightness are opposed, inclining toward opposed motions, and as virtues and vices are opposed in moral matters.

Thus, when St. Thomas, in the place cited above from Posterior Analytics, lecture 44,⁶ had said, following Aristotle, that opinion and science are opposed like true and false opinion, he speaks in the same terms of opinion as a habit and of the other habits that are always directed toward truth, with which he compares science, as is clear there in the paragraph “He compares science with certain habits.”⁷ Therefore, when he posits contrariety between them, he is speaking not only of opinion as an act, but also as a habit. He speaks in the same way of the habit of opinion and of science in Posterior Analytics, book 2, lecture 20, at the end.⁸

The REASON is taken from what was said: it is impossible for some acts to be opposed to one another while the habits inclining toward those acts are not opposed, since inclinations are called opposed precisely insofar as they incline toward opposed motions. Therefore, wherever the opposed motions or acts themselves are found, the inclinations must also be opposed. Opposed inclinations are contrary to one another, as is clear in the examples given. Therefore they cannot coexist in the same subject in the way contrary forms cannot coexist.

It should be noted, however, that contrary forms expel one another from a subject when they are perfect, not when they are in an imperfect and weakened state. A contrary form, if in a perfect state, perfectly subjects the subject's indifference to itself and reduces it to uniformity with itself, and so does not allow indifference toward the contrary opposite to remain. Therefore the evident habit must inform the subject perfectly in order to expel the obscure one. This is why the light of glory, unless given in a perfect state, does not expel the habit of faith, since it is not given in the mode of a habit, but in a transient manner. Thus St. Thomas teaches in Summa theologiae, II-II, q. 175, a. 3, ad 3, that the light of glory given to Paul when he saw the divine essence in rapture did not remove the habit of faith, because it was not given in the mode of a habit. Likewise, if some demonstration were given in so imperfect a way that it did not generate a habit, as with that vision of glory, or if it were not given permanently but transiently, it would not remove the habit of opinion. But if the demonstration is true and perfect, so that it is clearly and manifestly connected with a self-evident proposition, a single act fully convinces the intellect and establishes it in the inferred truth, as St. Thomas says in Summa theologiae, I-II, q. 51, a. 3. Therefore the habit of science by its nature always expels the habit of opinion, because it is always a perfect habit and thus is like an intense contrary form that always expels its opposite.

As for the third comparison, between a habit and an evident or nonevident act, I SAY FINALLY: By their nature they are not formally opposed, but can coexist in the same subject; nevertheless, they can expel one another when an act generates a perfect habit, which, precisely because it is perfect, expels the opposite from the subject.

THE FIRST PART of the conclusion is proved because there is no formal opposition between a habit and an act, but an effective or dispositional one, insofar as an act disposes toward the opposite habit, or insofar as a habit produces an act formally opposed to another act. Act and habit are not opposed to each other: a habit signifies only an inclination toward something, whereas an act signifies an actual tendency. A tendency toward the opposite can nevertheless coexist with an inclination toward one thing. Thus a stone can tend upward through a violent impulse while its heaviness, which inclines it toward the center, remains; a person with the habit of virtue can actually sin; and a person with the habit of faith can actually see God in a transient manner, as St. Thomas expressly teaches in Summa theologiae, II-II, q. 175, a. 3, ad 3. Therefore, in the same way, someone with the habit of science can elicit an act of opinion and yet not lose his science.

ON THE CONTRARY, someone who has the habit of opinion, if he elicits a demonstrative act, by that single act generates a perfect habit of science, because it perfectly convinces the intellect and overcomes all its resistance. Such a habit cannot coexist with the habit of opinion because it fully convinces, just as one perfect contrary form expels another. But this occurs through the perfect habit generated by that act, not merely through the act's formal character in relation to the opposite habit.

If infused habits, such as grace and charity, are expelled by a single act opposed to sin, as St. Thomas teaches in Summa theologiae, II-II, q. 24, a. 12, this is not because the act formally destroys the opposite habit, but because it impedes the influx of the preserving cause. By sin a person turns away from God and thus places an obstacle to receiving from God the light of grace, by which a person is turned toward God as an end. It is like someone who closes a window and thereby prevents the light of the sun from illuminating the air. Therefore those habits are not destroyed through the formal opposition of an act, but through the opposition of an impediment. This does not apply here, where the habits of science and opinion do not depend for their coming-to-be and preservation on an extrinsic cause whose influx could be impeded by an opposed act.

ARGUMENTS ARE ANSWERED.

First it is argued, because the whole basis of the opposition between an act of science and an act of opinion is either the plurality of acts or contrariety. It is not merely their plurality; otherwise they would be no more incompatible than two acts of science or two acts of opinion, or any other disparate acts. Nor is it contrariety, because this whole contrariety is due to clarity and obscurity, and to certainty and uncertainty. But these are not contrary-incompatible insofar as they are had through different media. The same thing can have different causes, some necessary and some contingent; according to its necessary causes we judge it unable to be otherwise, and according to its contingent causes we judge it able to be otherwise. We affirm without contradiction the proposition “The dead rise” according to supernatural causes and deny it according to natural causes. Therefore, from different causes and motives, we will likewise judge the same conclusion certainly and uncertainly, evidently and nonevidently. Hence St. Thomas, in Summa theologiae, I-II, q. 67, a. 3, posited opposition between opinion and science, or between a probable and demonstrative medium, only with respect to the same act or medium, not with respect to the same subject through different media.

It is confirmed that contrary acts concerning the same object can coexist in the will; therefore acts of opinion and science can also coexist in the intellect.

The consequence is clear, because these acts are not privatively or contradictorily opposed to one another, since each is a positive act; nor are they contradictory on the side of the object, since each affirms the same truth.

The antecedent is proved as follows: Christ's will was simultaneously affected by joy and sadness concerning the same object, namely his own passion, which grieved him as a natural evil and gladdened him as the redemption of humankind.

I ANSWER that the opposition between an act of opinion and an act of science is due to contrariety, not just any contrariety, but one that entails privatively opposed effects: seeing and not clearly seeing the same object, and being certain of it by judging that it cannot be otherwise, or not being certain by judging the opposite. St. Thomas, in the passage cited above, equates this opposition with the opposition between true and false.

As for what is said about different media, we reply that assents given through different causes and media, necessary and contingent, as the formal reasons of opposed judgments, are opposed in themselves. They are not opposed if the causes and media themselves are considered reflectively and in a signified act as objects about which a judgment of probable opinion or necessity is made; for then the judgment is not about the same object. Likewise, in the proposition “The dead rise” and “The dead do not rise,” if both are taken absolutely, they contradict and conflict in the intellect. But if they are not taken absolutely and one explains something the other does not—that is, “The dead rise according to God's power, but do not rise according to natural causes”—then they do not concern the same thing and consequently do not contradict one another. Opinion and science likewise, when exercised absolutely, produce opposed effects. But if they are considered in a signified act and a judgment is formed not merely about the things themselves, but about the certainty or uncertainty they have insofar as they depend on such causes and media, then they do not contradict one another, and the judgment is not made in an act of opinion but with certainty. In sum, opinion and science are opposed like true and false opinion, as Aristotle says in ch. 26 of Posterior Analytics, book 1⁹, and St. Thomas in lecture 44.¹⁰ Therefore, just as we speak of judgments opposed in truth and falsity, so we must speak of an opining and a scientific judgment. In Summa theologiae, I-II, q. 67, a. 3, St. Thomas posited opposition between science and opinion not only on the side of the medium or act, but also on the side of the subject, as he expressly says there and as we explained in the preceding article.

In reply to the confirmation, we say that contrary motions of joy and sadness in the will do not concern the same object according to the same account of good and evil, but according to different accounts. Christ the Lord rejoiced in his passion insofar as it was for the salvation of humankind, but grieved over it insofar as it was a natural evil. These are formally different objects, even though materially they are the same.

If you press the point: Why could opinion and science likewise not concern the same material object, if they concern it under different considerations? I reply that if the formal aspects of opinion and science produced different truths by constituting different objects, the same account would apply as in the case of the motions of the will.

In the intellect, however, the fact that one truth is inferred with respect to an evident medium or with respect to a nonevident one does not change its character as an objective truth. If the same truth remains, but its illumination comes from different media, one of which illuminates it fully and the other obscurely, they thereby produce opposed effects in the intellect concerning the same truth, as the matter illuminated by these two opposed lights, even though the lights belong to different habits; they always produce opposed effects.

But the object of the will, which is the good, changes in its very character as good through a different relation and reference alone. The same thing, compared with one thing, is good and suitable, and compared with another is unsuitable, because good and evil are taken relationally. Thus a different object is constituted in the character of good and suitable. Different motions therefore become nonopposed precisely because their objects change. Yet even in the sensitive appetite, different motions of joy and sadness cannot occur concerning different objects, because they involve bodily motions of expansion and contraction around the heart, which are absolutely incompatible at once because they occur in different positions.

Second it is argued: Divine faith is obscure and does not conflict with natural science, which is evident; therefore not every obscurity conflicts with evidence.

The antecedent is proved first: faith and vision conflict insofar as vision is related to the intellect's rest, whereas believing is like motion. But divine faith is only a motion tending toward the vision of glory, not toward natural science; therefore it does not conflict with natural science, but with the vision of glory. Second, faith remains in a Christian philosopher concerning the proposition “God exists”; otherwise he would hold it with less certainty than a faithful layman who believes it, and so would be worse off. Third, Paul had faith together with the vision of glory in his rapture, as St. Thomas says in Summa theologiae, II-II, q. 175, a. 3, ad 3; therefore faith does not conflict with an act of vision or science. Finally, according to the opinion that one body can be placed in two locations, if a person is placed in Rome and in Complutum, he will see the Pope in Rome and believe what is said about him in Complutum. Therefore clear and obscure vision will not be incompatible in the same subject concerning the same thing.

I ANSWER by denying the antecedent, which is the express position of St. Thomas in Summa theologiae, II-II, q. 1, a. 5. The opposition between obscurity and the evidence of vision is the same as that between obscurity and the evidence of science, which is intrinsically connected with and necessarily derived from the vision of principles.

To the first proof, we reply that divine faith is related to vision as motion to rest in two ways. In one way, with respect to the character of an end and perfect enjoyment; in this way its end and rest is only the vision of glory. Second, with respect to the manner of proceeding, though not with respect to the fullness of enjoyment; in this way the evident manner of proceeding in natural science also terminates the manner of proceeding in faith, because through faith the intellect is taken captive by obscurity and, as it were, placed under another's terms, whereas through science it is evidently convinced by its proper object.

To the second proof: theologians have a long dispute about how faith remains in a Christian philosopher, which we cannot resolve here. Briefly, however, we say first that faith can remain in a Christian philosopher concerning the proposition “God exists” with respect to readiness of mind. Thus he is not worse off with respect to merit and habitual certainty, even if he is so with respect to actual certainty.

Second, faith remains concerning the proposition “God exists” by believing that God exists as a supernatural author, in which way he must be known by one who is justified. For in the article “God exists and is a rewarder” all the other supernatural articles are implicitly contained, as St. Thomas teaches in Summa theologiae, II-II, q. 1, a. 8. Supernatural truths cannot be contained in the truth “God exists” insofar as it is precisely a natural truth. Hence St. Thomas, On Truth, q. 14, a. 9, ad 8,¹¹ says “that God's existence, insofar as it is demonstrated, is not an article of faith, but the unity of the divine essence, as it is held by the faithful together with omnipotence, providence over all things, and other such matters that cannot be proved.” He teaches the same in Summa theologiae, II-II, q. 1, a. 8, ad 1. But science, not faith, remains concerning the proposition “God exists as a natural author and as knowable evidently from natural effects.” Taken in this way, however, it is a formally distinct truth and a distinct proposition from the first, which reaches God's existence according to truths supernaturally revealed.

Or, finally, third, we say that concerning the proposition “God exists,” even insofar as God is a natural author, a Christian philosopher has greater actual certainty than a faithful layman, which is shared from faith through its connection with that proposition. Faith leaves behind a certain judgment that the things of faith are more certain than every natural cognition; in this judgment the intellect apprehends certainty, not the obscurity of faith. This judgment strengthens the intellect as it demonstrates, so that it is certain of its demonstration and of the truth “God exists,” and does not suppose that it is false or that the demonstration is fallacious. In other natural demonstrations, however, because of the weakness of our reason, one can fear this, since errors are often mixed into them; faith serves to correct these, as St. Thomas teaches in Against the Gentiles, book 1, ch. 4, and Summa theologiae, II-II, q. 2, a. 4. Thus the demonstration itself is strengthened in this way and shares certainty from faith, by forming that judgment without receiving obscurity from faith in that judgment. In apprehension, however, we can receive a judgment about certainty without apprehending what concerns obscurity; thus, through such a distinction, faith influences the demonstration with certainty through that judgment, not with obscurity.

To the third proof, we reply that the act of vision in Paul did not exclude the habit of faith, because that act of vision was without a habit, since it did not have a permanent state. Rather, it was elicited by a help—that is, by a transient quality supplying the place of a habit—and so the habit of faith was not excluded.

In reply to the confirmation, we say that, granting the case of a body being placed in two locations (which we shall nevertheless reject in Physics, book 4¹²), one must say either that the vision and nonvision do not conflict because they concern different locations, in which the person functions, as it were, as two persons and subjects; or that he would see in one location everything he saw in the other, because the seeing eye is identical, and he would be in each location as if, while existing in one, he saw so perceptively as to reach things in the distant location, since he reaches both locations while existing in either.

Finally it is argued: Someone who has the habit of science can use a probable medium to prove a conclusion already demonstrated; for it often happens that probable media and arguments are accumulated to prove the same thing, some probable and others demonstrative. Then these probable arguments generate some habit, since repeated acts generate a habit, and it is not a scientific habit, as is clear in itself; therefore it is an opinion-forming habit. Therefore these habits can coexist.

I ANSWER that if someone with the habit of science elicits an act of opinion by exercising the very fear and assent of opinion, and not merely by knowing the force of probable signs more perfectly (as St. Thomas says can happen in someone who uses a probable medium after a demonstration), that opinion-forming assent will not generate a habit because of an impediment in the subject, in which the habit of science is present. This habit actualizes the subject firmly and perfectly and does not allow an opposite habit to coexist. Here it differs from the habit of virtue and vice: virtue is generated not by a single act but by habit and practice, because a single act does not overcome the whole indifference of reason toward the good, owing to contingency and freedom toward the good and the many circumstances it requires, as St. Thomas teaches in Summa theologiae, I-II, q. 51, a. 3. Thus it is also not expelled by a single act; rather, several vicious acts can occur, through which the mind becomes accustomed to the opposite of virtue and thereby adheres less firmly to virtue. This leaves room for the opposite habit to be generated gradually. This does not apply to the habit of science in relation to opinion, because it is generated perfectly by a single act and makes the intellect adhere with certainty by the force of evidence, not by the force of habit.