John Buridan (1300 - ca. 1360) taught at the University of Paris for decades. Unusually, he did not become a theologian but remained on the philosophy faculty. Based in part on the translation by Gyula Klima in Medieval Philosophy: Essential Readings (Blackwell, 2007). The text (in Latin) comes from a 1518 edition, which has a fair number of errors. As of yet, there is no modern edition. John Buridan, Questions on Metaphysics II.1 Is it possible for us to comprehend to truth about things? It is argued first that it is not, with respect to the senses. 1. Donkeys judge some things, such as thistles, to taste good, which we judge not to taste good. Yet it should not be said that donkeys are deceived in this, because they judge this naturally, across their whole species, and nature never errs across a whole species. Therefore donkeys judge well and thus we judge badly to the contrary. 2. Dogs have a better sense of smell than we do, as is apparent from their perceiving slighter differences in odor and from a greater distance. Yet dogs judge a cadaver to have quite a good odor, whereas we judge not just that it does not, but even that it is foul and stinking. Therefore we are deceived about such odors. Thus our senses are deceived with respect to their proper objects namely, taste with respect to flavor and smell with respect to odors. And if they are deceived with respect to their proper objects it seems likely that they are more deceived with respect to other objects. Accordingly, through our senses we have certainty about nothing. Each sense has its own proper object. The proper object of small, for instance, is odor. This idea is very important here, because the standard Aristotelian view, which Aquinas for instance accepts, is that the senses never err with regard to their proper objects. Buridan thinks this is true only in a very qualified way. 3. Someone healthy judges a thing to be sweet while a sick person judges that same thing to be bitter. Now you will say that we ought to believe the judgment of the healthy person, not the sick person. But, on the contrary, if we ought to believe the healthy person more than the sick person, this would be only because his organ is better disposed whereas the sick person s is worse. But then it would further follow that we never judge entirely perfectly except when we have an organ of sense that is absolutely perfectly disposed. Yet no one, or hardly anyone, has this, and so no one or hardly anyone will 1
judge well through the senses. 4. An agent will never generate in a patient a species that is contrary to it, no matter how badly disposed that patient is. For it is the nature (de ratione) of an agent to assimilate the patient to it, as is stated in De generatione I. Therefore sweetness will never generate bitterness, or the species of bitterness, on the tongue of someone sick. Therefore it will never be the case that what the sick person judges to be bitter will be sweet. Therefore the healthy person judges wrongly in judging it to be sweet. Furthermore, children, the elderly, men, and women, even if they are all healthy, judge flavors in various ways. For sweet things appear more flavorful to children and women, [whereas] salty and acidic things [appear more flavorful] to others. Therefore it is uncertain to which of these groups the flavor appears as it truly is. 5. If a person in a boat is moved with the boat s motion, it sometimes appears to him through sight that a tree on the shore is moving, whereas to another who is in the field next to the tree it appears that the tree is at rest. Now you will say that we should believe the one who is in the field next to the tree. I prove the contrary, because I stipulate that the person in the boat does not see the boat but only the tree. He, through sight, will perceive motion, since otherwise he would make no judgment about motion. But he will not perceive the motion of the boat, nor his own motion, because it has been stipulated that he sees neither himself nor the boat. Therefore he sees the motion of the tree, and so it is that he judges the tree to be moving, and thus it is the other person who judges wrongly. A species is a likeness of the sensible quality (here, the sweetness of some food). In every case of perception, the species travels from the object to the sense organ, conveying information about the object. Here and in the following two objections there s a quick back and forth within the objection: a quick objection, followed by a quick reply ( you will say ), followed by the objector s reply ( I prove the contrary ). Neither the you nor the I here is necessarily speaking for Buridan. 6. Through mediums that are different, a color appears to be different, and likewise with size and so on. This is clear, because often at dawn or dusk the sun appears red, whereas at midday it appears white, and it also appears larger at dawn or dusk. Now you will say that we should not believe the appearance 2
that is at dawn or dusk because the medium is impure. For the air that is close to the earth through which we see the sun at dawn or dusk is mixed with vapors and exhalations, on which account the sun does not appear to be as it is. But then it is objected that if we judge wrongly because of the medium s impurity, it follows that we never judge perfectly rightly through sight, unless we have a pure medium. But this is something we never have, because the lower air in which we dwell is always mixed with vapors and exhalations given off by bodies on water and land. Therefore we never judge without error. 7. With regard to colors, sizes, motions, etc., we judge differently up close and at a distance. Now you say that we should believe our judgments up close. On the contrary, it would follow that we ought to deny the existence of rainbows and clouds, which never appear to us up close. Indeed, if we go to the place where the rainbow appeared, it will no longer appear. Hence we cannot determinately know which appearance is true. 8. All sensibles are continuously in motion and change, and lower things in particular are continuously generated and corrupted. And even if this does not happen all at once as a whole, still it happens to parts of them. For [e.g.] some part of the ocean continuously evaporates and some other new part is continuously added to it, and so the whole ocean is never the same, before and after, and likewise neither is the whole air. So too neither are human beings nor horses, since through nutrition something new is continuously added and something else taken away through exhalation and evaporation due to heat. Therefore no one can make a claim with certainty about such things, because before the claim will have been completed, the thing is already different from before. Lower things are the things of the sublunary world (i.e., earth and its atmosphere), as opposed to the various celestial bodies, which the standard science of the day (until Galileo) took to be unchanging. 9. To the sense of sight up close, through air that is rightly 3
disposed, the color green appears where there is no greenness. For instance, if you place a yellow glass on top of a glass that is azure or blue, and if you look at those glasses together, the whole will appear green to you. Thus a sense is entirely deceived with regard to its proper object, and therefore we have no certainty about things through the senses. 10. We cannot see a color without a light falling upon it, and yet with light the color appears otherwise than it is. For it is clear from this that the more light falls on a color the more the color appears whiter or less black, and the more it is removed the more the whole appears blacker or less white. Therefore through sight you will never have perfect certainty about color. 11. The senses can be deceived, as is commonly said, and it is certain that a species of sensible things can be preserved in the sense organs in the absence of these things, as Aristotle says in On Sleep and Waking. When that happens, we judge what is not there as if it were there, and that is why we err through the senses. And the difficulty is greatly increased by the things we believe on faith. For God can form in our senses the species of sensible things without these sensible things, and can preserve them for a long time. In that case we judge as if those sensible things were present. Moreover, since God can do this and even greater things, you do not know whether God wills to do this. Hence, you do not have certainty and evidentness about whether you are awake and there are people in front of you, or whether you are asleep, for in your sleep God could make sensible species just as clear as, or even a hundred times clearer than, those that sensible objects can produce. So you would formally judge that there are sensible things in front of you, just as you do now. Therefore, since you know nothing about the will of God, you cannot be certain about anything. Here the proper object in question is color, the proper object of sight, and the point is that if sight can go wrong even about its proper object, then there is no hope for sensory certainty elsewhere. See the note to objection 4 for the notion of a species. 4
Next, [this same conclusion] is argued for with respect to the intellect. 12. Our intellect is dependent on the senses for its understanding. Therefore, if we do not have certainty by means of the senses, as was said, it follows that neither do we have it through the intellect. 13. The intellect has to be moved by an external thing or things if it is to understand them, but this is impossible, because the mover has to be nobler than the thing moved, as Aristotle says in De Anima III. 14. Further, the species of sensible things, before they can reach the intellect, have to pass through many intermediaries, such as the internal senses, common sense and imagination. And it is possible that in these intermediaries they become quite transmuted and so cannot represent things to the intellect with certainty. 15. It is argued with regard to [first] principles that these principles are made known through experience, and these experiences are deceptive, as is clear [first] from Hippocrates. Second, it is proved that they are deceptive, for experiences do not have the force to reach a universal principle as their conclusion, unless by means of induction over many [singular cases]. But a universal proposition never follows by induction unless the induction covers all singular cases of that universal, which is impossible. So, suppose that whenever you have touched fire you have always felt it to be hot; therefore, by experience you judge that a fire you have never touched is hot, and so on, and thus finally you judge that every fire is hot. Suppose, then, that by the will of God whenever you have touched a piece of iron you have felt it to be hot. It is clear by parity of reasoning that when you see a piece of iron that is cold you will judge it to be hot, and that ultimately you will judge every piece of iron to be hot. And these would be false judgments, although at that point you would have just as much This reference to Hippocrates, the famous physician, is quite obscure, and is perhaps a mistake in the text. 5
experience about iron as you now in fact have about fire. 16. By Plato s reasoning, if the intellect never knew a first principle, and it happened on it, it would no more assent to it than to its opposite, in accordance with his example about the fugitive slave and his master [who would not be able to recognize his slave if he had never seen him]; therefore, we will not be certain about the first principles. 17. Next, the same is argued about conclusions, because if we were to have certainty about conclusions this would be either through the teaching of another or solely through our own discovery. If through discovery then this would be quite meager, as Aristotle says. If through the teaching of another then there would already have to be things previously known to us, through what was signified of these things. Therefore, [these further things] are known to us either through discovery or through teaching. Not through discovery, as is obvious, because we did not introduce their meaning. If through teaching then this again was through other words already known to us. We would ask about these as before, and would proceed to infinity in teachings, which is absurd. 18. Neither a conclusion nor an effect can be known through its cause, nor can a cause be known through its effect, because the cause is not contained essentially or virtually in its effect. And an effect cannot be known through its cause because causes are less known to us. And if you say that they are better known by nature, that is irrelevant to the question, because we are inquiring about our learning and not about nature s. It seems, indeed, that we can never achieve evidentness about one thing through another, because the only evidentness is through reduction to the first principle, which is grounded in contradiction. But we can never reach a contradiction concerning two distinct things. For suppose that they are A and B. It is not a contradiction that A exists and B does not exist, or that A is white and B is not white. So there will never be an evident This is perhaps a reference to Meno 71b, which raises ssues in this vicinity. But there s no mention there of a fugitive slave. Buridan s knowledge of Plato is largely second hand, since Plato s works were mostly not available in the Middle Ages. This is the most obscure argument in this whole series, and the Latin is even more obscure than the translation reflects. The idea seems to be that we cannot learn from teaching, because that requires prior knowledge to understand the words and concepts that the teacher uses, and then we can ask where that knowledge comes from, and thus we are led into an infinite regress. There s a similar sort of argument in Augustine s On the Teacher (De magistro). Much more about evidentness below. 6
Speaking of they here suggests that these objections are not simply Buridan s inventions, but that there are specific opponents Buridan has in mind. And, indeed, there were a lot of people in the early fourteenth century who found skepticism to be at least a tempting doctrine. For some information about this, see Henrik Lagerlund (ed.), Rethinking the History of Skepticism: The Missing Medieval Background (Brill, 2010). Metaphysics II.1, 993a30. inference concluding that B exists from the fact that A exists, and so on for other cases. 19. It can be argued that the intellect does not have certain knowledge (certam notitiam) that is, scientia about external things, since these are singular and changeable and there is no scientia about these. And if we do not have certain scientia about these then even less will we have it about things that are in the soul, since these are less known. These are the arguments through which they have denied that a certain comprehension of the truth about things is possible for us. [On the Contrary] The opposite is argued from Aristotle, who says that [the certain comprehension of truth] is in one way easy and in another way difficult. Thus he regards it as possible. The Commentator too argues as follows: that for which we have a natural desire is possible, since nothing with a foundation in nature is in vain. But we do have a natural desire for knowledge, and consequently for the certain comprehension of truth, as Aristotle said in the preface of the Metaphysics. Therefore etc. There s no good way to capture in English the various Latin terms in the vicinity of knowledge. Here Buridan makes it clear that he understands scientia to be equivalent to a notitia that is certain. In what follows, however, very little rests on the epistemic language we use. The translation indicates whenever it seems important to know what the underlying Latin is. [Main Reply] [The Possibility of Comprehending the Truth] To clarify the question, we have to explain, to some extent, its terms. I will discuss incomplex truth in detail in other books. But for now I am concerned only with complex truth, on account of which a proposition is said to be true. And we can disregard spoken and written propositions, for these are said 7
to be true or false only because of the true or false mental propositions they represent, just as a urine sample is said to be healthy or sick because it signifies the health or sickness of an animal. Furthermore, I assume for the time being that the truth of a mental proposition is nothing other than the true mental proposition itself, although the names true and truth connote that a mental proposition of this sort conforms to the things it signifies in the way that will be explained elsewhere. Next we should look at how we are to understand comprehension of truth. It should by now be clear from the aforesaid that the comprehension of truth is nothing other than the comprehension of a true proposition. But comprehension of truth can be taken in three ways. In one way, the comprehension of truth is nothing but the formation or existence of a proposition within the soul, and then again the comprehension of a true proposition is nothing other than the true proposition itself, and it is obvious that this is possible. So understood, then, we have to conclude that the comprehension of truth is possible within us. In a second way, the comprehension of truth is the same as the understanding of a true proposition in the manner of an object, so that we understand a proposition in just the way that we comprehend or understand a stone. And it is clear that this too is possible for us, since we understand both terms and propositions, and thus we know how to say many things about them. Thus we should conclude that in this way, too, the comprehension of truth is possible for us. In a third way, comprehension of truth is taken for the adherence or assent by which we assent or adhere to a true proposition, and once again it is clear that this is possible for us. Indeed, not only can we assent to true propositions, but we also often assent to false ones, namely, when we stubbornly persist in false opinions. Therefore, we should conclude that in this way too the comprehension of truth is possible for us. This and the following paragraph nail down some preliminary technicalities regarding what truth is. In short, to comprehend the truth, as that will be understood here, is to comprehend a true mental proposition. This is possible because all it requires is the existence of a true thought (= mental proposition) within the soul. And we are all bound to have one of those, sooner or later, if only as a matter of luck. The next two ways of comprehending truth aren t much more demanding. 8
The Latin word is catholica, but in the medieval context it refers to something like the orthodox faith, in contrast to the pseudo-christianity of the heretic. [The Possibility of Comprehending the Truth with Certainty] The objections made above, however, raise doubts over whether such an assent to the truth is possible for us with certainty. So we should note that assent to the truth with certainty requires [A] firmness of truth and [B] firmness of assent. Now [A1] firmness of truth is possible in one way absolutely (simpliciter), as in the proposition God exists, which can in no case be falsified. But [A2] there is also firmness of truth on the assumption of the common course of nature, and in this way it is a firm truth that the heavens are in motion, that fire is hot, and so on for other propositions and conclusions of natural science, notwithstanding the fact that God could make fire cold, which would falsify the proposition that every fire is hot. So it is clear, then, that firmness of truth is possible. But [B] firmness of assent is that whereby we adhere and assent to a proposition without fear of the opposite, and this can occur in three ways. [B1] In one way, it arises from the will without any natural appearance, and in this way Christians assent and adhere firmly to the articles of the catholic faith, and also some heretics adhere to their false opinions, so much so that they would rather die than deny them, and such is the experience of the saints who were willing to die for the faith of Christ. And so it is obvious that firmness of assent is possible for us. [B2] In a second way, firmness arises in us from natural appearances through reasons of some sort, and in this way too it is possible that we can assent firmly not only to truth but also to falsity. For many people believing and holding false opinions take themselves to have firm knowledge (firmam scientiam), just as Aristotle says in Ethics VII that many people adhere to what they opine no less firmly than to what they know. [B3] In a third way, firmness of assent arises in us from evidentness. [B3a] This is said to be the evidentness of a proposition absolutely (simpliciter) when, because of the nature of sense or intellect, a person is compelled or necessitated to assent to a proposition in such a way that he cannot dissent from 9 Here we get into the case that really matters: can we have certainty? Firmness of truth is, in effect, the necessity of the proposition. Fear of the opposite is a standard way to talk about the case where one does not feel able to rule out alternative possibilities. Firmness of assent, then, occurs when we do feel able to rule out all the alternatives. The test is, notice, wholly subjective: this is Buridan s way of invoking subjective certainty. Evidentness (evidentia) is the key concept of this whole question, and indeed of medieval epistemology in general. What it means, to a first approximation, is what Buridan goes on to say: a proposition is evident to me if I cannot dissent from it. But what s striking and original in the discussion that follows is that Buridan distinguishes three levels of evidentness.
995a15-16 it. Evidentness of this sort belongs to the first complex principle [the principle of non-contradiction], according to Aristotle in Metaphysics IV. [B3b] In another way, evidentness is taken qualifiedly (secundum quid), on the assumption stated earlier [at A2] that things obey the common course of nature. In this way it is evident to us that every fire is hot and that the heavens are in motion, even though the contrary is possible by God s power. And this sort of evidentness suffices for the principles and conclusions of natural science. [B3c] There is still another, weaker evidentness, which suffices for acting well morally. Namely, if someone, having seen and investigated all the attendant circumstances that one can investigate with diligence, judges in accord with the demands of such circumstances, then that judgment will be evident with an evidentness sufficient for acting well morally even if that judgment were false on account of invincible ignorance concerning some circumstance. For instance, it would be possible for a judge to act well and meritoriously in hanging a holy man because through testimony and other documents it sufficiently appeared to him in accord with his duty that that good man was an evil murderer. Therefore, we conclude as a corollary that some people speak very badly, wanting to destroy natural and moral knowledge (scientias) on the grounds that there is no absolute evidentness in many of their principles and conclusions, given that they can be falsified in supernaturally possible cases. This is wrong, because such knowledge does not require absolute evidentness; instead, the aforementioned evidentness suffices, which is qualified or based on a supposition. So Aristotle rightly says in Metaphysics II that mathematical precision is not to be sought in all sciences. And since it has now become apparent that firmness of truth and firmness of assent are possible for us in all the aforesaid ways, we should thus conclude with what was sought: namely, that the comprehension of truth with certainty is possible for us. This seems to be the origin of the famous notion of moral certainty, which plays a very large role in early modern philosophy. 10
And then we should respond to the [initial] arguments [Reply to 1, 2, 4.] To the arguments made about the donkey, dogs, the young and old, and men and women that some judge [certain things] to taste good while others judge [those things] not to taste good, and so on in other cases I say in brief that they all judge well about flavors, since they all say there is sweetness where there is sweetness, and bitterness where there is bitterness. But suited and unsuited, or good and bad, are said relatively. For this is good and suited to a human being which is unsuited to a donkey, and vice versa, on account of their distinct constitutions. Thus a donkey judges well in judging that thistles have a good flavor, because they have a flavor that is good and suited to a donkey. A human being also judges well in judging that that they have a bad flavor because they have a flavor that is unsuited to a human. But if a human being were to judge that thistles have a flavor that is bad and unsuited to a donkey, then he would judge badly. So it is too for the other aforesaid cases, on account of the distinct constitutions of those who are judging. [Reply to 3-4.] To the argument from sickness, I say that the sick person judges well in judging bitterness or even in judging that the thing he tastes is bitter. But if someone were to give him sweet honey he would be deceived in judging that this honey is bitter. Instead, the bitter thing that he senses is the putrid humor attached to his tongue, namely the bile produced by the chewing and grinding of food. And so it is happily granted that sweetness never gives rise on the tongue to the species of bitterness. It should also be said that in order to judge well there is no need for an organ or medium that is perfectly well disposed. For we do not judge colors, sizes, and flavors according to the precise (punctualem) degree of their length or intensity. Instead, often, it is enough for us to judge that this is white and that this is more than two feet long. For these judgments are absolutely Buridan does not reply as one might expect, by saying that the bitterness and sweetness are in the perceiver. That s the response associated with the ancient atomists, which would later become standard from Descartes forward. Buridan, in contrast, is committed to the claim that flavors, odors, sounds, heat, and colors (all the proper objects of sense) are objectively in the world. Hence the thistles really are bitter. It s just that the donkey likes that taste! Here s something else that Buridan wants to insist on: that if the quality out in the world is sweetness, then the species of sweetness is what will arrive on the tongue. But this isn t what the sick person is tasting. 11
(simpliciter) true, even if we do not know the precise extent of the degree. [Reply to 5.] To the argument from the motion of trees it is said that we should believe the one who stands next to the tree and not the one who is in the boat, because he is deceived. But when it is said that he sees motion since he judges that motion, I say that he does not see motion. Instead, the internal estimative power judges that the tree is moving because it does not perceive the eye s motion. Thus it should be known that if the eye is at rest and the object is in motion then the species from the object are in the eye according to distinct positions. Because of this, the estimative power judges that the object is in motion. If, however, the object is at rest and the eye is in motion then the species may just as well be in the eye according to distinct positions. In that case the estimative power, if it judges that the eye is not in motion, judges that the distinctness in positions arises from the object s motion, and it is deceived. This is a reference to the theory of the internal senses that goes back to Avicenna. [Reply to 6.] To the next it is granted that a difference in mediums changes the appearances, because sometimes what is seen is not just the object, when it is so remote, but also the colors that are mixed into the medium. Now, a light that is seen with black appears red, inasmuch as red is caused by the mixture of light and black. Thus coals that are black when not on fire appear red when they are on fire. So too when above the earth there are smoky exhalations raised up from the earth that are themselves black, and those alone are seen with the sun s ray passing through them, then the sun appears red. But the intellect, seeing the cause of such an appearance, corrects the sense s error. [Reply to 7.] Also, when the close and distant were spoken of, I say that things that serve well to terminate sight, like wood and stones, are judged better up close. Other things, however, are too rarified and do not serve perfectly as a terminus unless through a great and far-off aggregation. Such things, then, are 12
These remarks show that Buridan has the correct scientific account of rainbows. The theory had been worked out just a few decades earlier, by Theodoric of Freiberg. not judged by sight to be near or at the distance where sight terminates. For example, even if we were within a cloud, it would appear to us only as a kind of snow. But due to the cloud s greatness, sight would eventually terminate at a far distance, and black or red or clouds would rightly appear there. So it is too for the iridescent raindrops that cause a rainbow. Those that occur close to us make a kind of reflection of light back toward us. Then there are others that are some distance farther away. Sight has those as its terminus, and we judge that there is a rainbow there, rather than up close. For the raindrops that are close do not suffice on their own to terminate sight, even though they act to reflect light just as well as do those that are distant. [Reply to 8.] To the next I say that we need not judge that this human being or this air is totally the same as before. Instead, it suffices that it is partially the same. Also, it suffices that this is a human being. For these [things] are unities absolutely speaking (simpliciter), notwithstanding the fact that the thing is continuously generated and corrupted partially. As these brief remarks suggest, Buridan has surprising views about the identity over time of material substances. It is hard to judge, based on this passage alone, what his view amounts to, but he discusses it in far more detail in Generation I.13 and Physics I.10. [Reply to 9.] To the next, concerning greenness, I say that the color green is composed from yellow and blue or azure. So if species from those glasses are multiplied confusedly toward the eye, it will appear as if the whole is green. But the intellect seeing each one separately can correct this judgment. [Reply to 10.] To the next I grant that we do not judge with certainty the precise degree (punctualem gradum) of whiteness or blackness, but yet with the highest certainty we judge that it is white. [Reply to 11.] To the next I say that if the senses are naturally deceived, then the intellect has the task of inquiring into whether or not a person is in front of you, and correcting illusory judgments. If it is supposed that God acted by some absolute miracle, then we should conclude that he can do so. 13
The printed edition of the Latin used here does not contain Buridan s response to objections 16-17. Accordingly, the only evidentness is conditional [B3b], as was said, and this suffices for natural science. [Reply to 12.] To the next I grant that the intellect depends on the senses for its first, simple apprehension. But later on the intellect can compose and divide [these apprehensions] and can discern beyond the senses. [Reply to 13.] When it is further said that the thing cannot move the intellect because the mover has to be nobler than the thing acted on, I say that this has to be understood as concerning the principal mover, which is what the agent intellect is. This claim is not true for all movers. [Reply to 14.] When it is further said that a species passing through several intermediaries can be transmuted, I say that in a well-disposed intermediary they are not transmuted for the worse; rather, they are refined in the inner senses, in order to represent their objects better or more clearly. [Reply to 15.] To the next objection, which says that experiences are insufficient to reach a conclusion about a universal principle, I say that [such induction] is not a formally valid inference. Yet the intellect, on account of its natural inclination to the truth, predisposed by experiences, assents to the universal principle. It can be granted that experiences of this sort are not sufficient for absolute evidentness, but they are sufficient for the evidentness that suffices for natural science [B3b]. Along with this there are also other principles that, based on the inclusion or repugnance of their terms or propositions, do not need experience [for their confirmation]. This is so for the first principle. Indeed, it is evidently true that a chimera exists or does not exist, that a goatstag exists or does not exist, and that a human being is an animal, provided one knows the signification of the terms. [Reply to 18.] To the next I say that, through their causes, ef- This is the familiar idea that the terms composing a proposition (or the propositions composing an argument) can be seen, on their face, either to include each other in such a way that the proposition must be true (or the argument must be valid) or be repugnant in such a way that the proposition must be false (or the premises are contradictory). 14
fects are known so as to know the reason why (propter quid), because the cause is better known, even to us, than is the reason why the effect is. Likewise, through its effect, a cause is known so as to know that it is (quia), because the effect bears some likeness to its cause, and so can represent its cause along with the intellect s natural inclination to the truth. And when it is further said that one thing cannot conclusively be known from another, I deny this, and I say that there is a virtual (quasi) infinity of principles known per se, either through the senses or through experience or through the inclusion of terms without having to be demonstrated through the first principle. Indeed, in the Posterior Analytics Aristotle demonstrates that there are nearly as many indemonstrable principles as there are demonstrable conclusions. This invokes the distinction Aristotle draws in the Posterior Analytics between arguments that run from cause to effect (propter quid) and arguments that run from effects to cause (quia). [Reply to 19.] To the last objection I say that we do indeed have knowledge (scientia) even about things existing singularly that are changeable. But this is so inasmuch as they are signified by common terms forming unchangeable propositions. Hence they are not falsifiable nor do they lack necessity. 15