Aquinas: We Can't Know Perfectly Even the Nature of a Single Fly (and Related Texts)

Aquinas: We Can't Know Perfectly Even the Nature of a Single Fly (and Related Texts)

St. Thomas famously says that we cannot naturally know what God is directly as He is in Himself, but only what He is not, and only through His effects, etc.  This is a commonplace in discussions among Thomists.

Interestingly, he also speaks of our significant limitations in our ability to know the essences of created things.  We don't really know what the essential principles, substantial forms, specific differences (differentiae), or even the true nature of the properties of ordinary sensible things are.  We remedy this ignorance by citing accidental properties.  That is why we speak of the difference between humans and other animals as being one between 'quadrupeds' and 'bipeds', which are quite accidental features of being a human or an animal.  St. Thomas goes so far as to say that we cannot perfectly know the essence of a single fly (!)   

For your convenience and future reference, here are the texts in question.  (The 'not-a-single-fly' text is at the end.)  If you know of a text that is missing from the list, please comment below.

This group of texts should represent a generously dosed "humility pill" for us philosophers.

From St. Thomas' De ente et essentia, Ch. 5:

Et quia in istis substantiis quiditas non est idem quod esse, ideo sunt ordinabiles in praedicamento, et propter hoc invenitur in eis genus et species et differentia, quamvis earum differentiae propriae nobis occultae sint. In rebus enim sensibilibus etiam ipsae differentiae essentiales ignotae sunt, unde significantur per differentias accidentales, quae ex essentialibus oriuntur, sicut causa significatur per suum effectum, sicut bipes ponitur differentia hominis. Accidentia autem propria substantiarum immaterialium nobis ignota sunt; unde differentiae earum nec per se nec per accidentales differentias a nobis significari possunt.

94. And because quiddity in these substances is not the same as existence, they are orderable within a predicament. And this is why they have a genus, a species, and a difference, although their proper differences are hidden from us. For even in the case of sensible things, the essential differences themselves are not known; whence they are signified through accidental differences which rise out of the essential ones, as a cause is signified through its effect; this is what is done when biped, for example, is given as the difference of man. But the proper accidents of immaterial substances are unknown to us; whence their differences cannot be signified by us either through themselves or through accidental differences.

From St. Thomas' Quaestiones disputatae de veritate, q. 4, a. 1, ad 8 ("Can there be a word, properly speaking, in God?"): 

Ad octavum dicendum, quod nomen dicitur ab aliquo imponi dupliciter: aut ex parte imponentis nomen, aut ex parte rei cui imponitur. Ex parte autem rei nomen dicitur ab illo imponi per quod completur ratio rei quam nomen significat; et hoc est differentia specifica illius rei. Et hoc est quod principaliter significatur per nomen. Sed quia differentiae essentiales sunt nobis ignotae, quandoque utimur accidentibus vel effectibus loco earum, ut VIII Metaph. dicitur; et secundum hoc nominamus rem; et sic illud quod loco differentiae essentialis sumitur, est a quo imponitur nomen ex parte imponentis, sicut lapis imponitur ab effectu, qui est laedere pedem. Et hoc non oportet esse principaliter significatum per nomen, sed illud loco cuius hoc ponitur. Similiter dico, quod nomen verbi imponitur a verberatione vel a boatu ex parte imponentis, non ex parte rei.

8. A name is derived from two sources: from the one who uses the word or from the thing to which it has been applied. A word is said to be derived from a thing in so far as it signifies that by which the notion of the thing is completed, that is, the thing’s specific difference; and this is what a word principally signifies. But, since we do not know essential differences, sometimes, as is said in the Metaphysics, we use accidents or effects in their place, and name a thing accordingly. Hence, in so far as something other than the essential difference of a thing is used as the source of a word, the word is said to be derived from the one who uses it. An example of this is the word lapis (stone) which is derived from its effect, laedere pedem (to bruise the foot). Now, this effect should not be taken as that which the word principally signifies, but merely as that which takes the place of what is signified. Similarly, verbum (word) is derived from verberatio (a disturbing) or from boatus (shout) because of those who use it—not because of the thing it signifies.

From St. Thomas' Quaestiones disputatae de veritate, q. 10, a. 1, ad 6 ("Is the mind... the essence of the soul or one of its powers?"): 

Ad sextum dicendum, quod secundum philosophum in VIII Metaph., quia substantiales rerum differentiae sunt nobis ignotae, loco earum interdum definientes accidentalibus utuntur, secundum quod ipsa accidentia designant vel notificant essentiam, ut proprii effectus notificant causam: unde sensibile, secundum quod est differentia constitutiva animalis, non sumitur a sensu prout nominat potentiam, sed prout nominat ipsam animae essentiam, a qua talis potentia fluit. Et similiter est de rationali, vel de eo quod est habens mentem.

6. Since, according to the Philosopher, we do not know the substantial differences of things, those who make definitions sometimes use accidental differences because they indicate or afford knowledge of the essence as the proper effects afford knowledge of a cause. Therefore, when sensible is given as the constitutive difference of animal, it is not derived from the sense power, but the essence of the soul from which that power comes. The same is true of rational, or of that which has mind.

From St. Thomas' Commentary on Aristotle's De anima, Book 1, Lect. 1, no. 15:

Consequenter etiam cum dicit videtur autem ponit difficultates, quae emergunt quantum ad illa quae sunt in adiutorium definitionis animae. Quia in definitione oportet non solum cognoscere principia essentialia, sed etiam accidentalia. Si enim recte definirentur et possent cognosci principia essentialia, definitio non indigeret accidentibus. Sed quia principia essentialia rerum sunt nobis ignota, ideo oportet quod utamur differentiis accidentalibus in designatione essentialium: bipes enim non est essentiale, sed ponitur in designatione essentialis. Et per eas, scilicet per differentias accidentales, devenimus in cognitionem essentialium. Et ideo difficile est, quia oportet nos prius cognoscere quod quid est animae, ad cognoscendum facilius accidentia animae: sicut in mathematicis valde utile est praeaccipere quodquid erat esse recti et curvi et plani ad cognoscendum quod rectis trianguli anguli sint aequales. E converso etiam accidentia, si praeaccipiantur, multum conferunt ad cognoscendum quod quid erat esse, ut dictum est. Si quis ergo assignet definitionem, per quam non deveniatur in cognitionem accidentium rei definitae, illa definitio non est realis, sed remota et dialectica. Sed illa definitio per quam devenitur in cognitionem accidentium, est realis, et ex propriis, et essentialibus rei.

15. Next, at ‘Now it seems’, he states the difficulties that arise with regard to those accidental qualities which contribute to a definition of the soul. These are relevant here because a definition ought to reveal a thing’s accidental qualities, as well as its essential principles. If indeed the latter could be known and correctly defined there would be no need, to define the former; but since the essential principles of things are hidden from us we are compelled to make use of accidental differences as indications of what is essential. Thus to be two-footed is not of the essence of anything, yet it helps to indicate an essence. By such accidental differences we are led towards knowledge of the essential ones. It would indeed be easier to grasp even What is accidental to the soul if we could only first understand its essence, just, as in mathematics, it is a great help towards understanding that the angles of a triangle are equal to (two) right angles to know first what is meant by straight, curved and plane. Hence the difficulty of our present position. On the other hand a prior examination of the accidental factors is a considerable help towards knowing the essence, as has been said. if, therefore, one were to propose a definition from which no knowledge of the accidental attributes of the defined thing could be derived, such a definition would not be real, but abstract and hypothetical. But one from which a knowledge of the accidents flows is a real definition, based on what is proper and essential to the thing.

From St. Thomas' Commentary on Aristotle's Metaphysics, Book 7, Lect. 12, no. 1552:

Et ideo, quia habenti pedes accidit habere alas, non est dicendum, dividendo differentiam, quod habentis pedes aliud est alatum, aliud non alatum, si homo bene velit dicere divisionem differentiarum. Sed tamen quandoque aliquis dividens differentias facit hoc ut scilicet dividat per ea quae sunt secundum accidens, propter hoc quod non potest invenire proprias et per se differentias. Aliquando enim necessitas cogit, ut utamur, loco per se differentiarum, differentiis per accidens, inquantum sunt signa quaedam differentiarum essentialium nobis ignotarum.

1552. Therefore, since it is accidental to a thing having feet to have wings, it must not be said, in dividing the difference, that among those things which have feet, one kind is winged and another wingless, if a man wants to express correctly the division of the differences. Yet when someone in dividing differences “does this,” in such a way that he divides it by means of those attributes which are accidental, this is why he cannot find proper and essential differences. For sometimes necessity compels us to use accidental differences in place of essential differences inasmuch as accidental differences are the signs of certain essential differences unknown to us.

From St. Thomas' Quaestiones disputatae de spiritualibus creaturis, a. 11 ad 3 ("Are the powers of the soul the same as the essence of the soul"):

Ad tertium dicendum quod formae substantiales per seipsas sunt ignotae; sed innotescunt nobis per accidentia propria. Frequenter enim differentiae substantiales ab accidentibus sumuntur, loco formarum substantialium, quae per huiusmodi accidentia innotescunt; sicut bipes et gressibile et huiusmodi; et sic etiam sensibile et rationale ponuntur differentiae substantiales. Vel potest dici, quod sensibile et rationale, prout sunt differentiae, non sumuntur a ratione et a sensu secundum quod nominant potentias, sed ab anima rationali, et ab anima sensitiva.

As to the third, it must be said that because substantial forms in themselves are unknown but become known to us by their proper accidents, substantial differences are frequently taken from accidents instead of from the substantial forms which become known through such accidents; as, for example, "biped" and "able to walk" and the like; and so also "sensible" and "rational" are put down as substantial differences. Or it may be said that "sensible" and "rational", insofar as they are differences, are not derived from reason and sense according as these are names of powers, but from the rational soul and from the sentient soul.

From St. Thomas' Summa contra gentiles, Book 1, ch. 3 ("On the way in which divine truth is to be made known"):

Adhuc idem manifeste apparet ex defectu quem in rebus cognoscendis quotidie experimur. Rerum enim sensibilium plurimas proprietates ignoramus, earumque proprietatum quas sensu apprehendimus rationes perfecte in pluribus invenire non possumus. Multo igitur amplius illius excellentissimae substantiae omnia intelligibilia humana ratio investigare non sufficit.

[5] The same thing, moreover, appears quite clearly from the defect that we experience every day in our knowledge of things. We do not know a great many of the properties of sensible things, and in most cases we are not able to discover fully the natures of those properties that we apprehend by the sense. Much more is it the case, therefore, that the human reason is not equal to the task of investigating all the intelligible characteristics of that most excellent substance.

From St. Thomas' Exposition on the Apostle's Creed, prologue:

Sed dicit aliquis: stultum est credere quod non videtur, nec sunt credenda quae non videntur. Respondeo. Dicendum, quod hoc dubium primo tollit imperfectio intellectus nostri: nam si homo posset perfecte per se cognoscere omnia visibilia et invisibilia, stultum esset credere quae non videmus; sed cognitio nostra est adeo debilis quod nullus philosophus potuit unquam perfecte investigare naturam unius muscae: unde legitur, quod unus philosophus fuit triginta annis in solitudine, ut cognosceret naturam apis. Si ergo intellectus noster est ita debilis, nonne stultum est nolle credere de Deo, nisi illa tantum quae homo potest cognoscere per se? Et ideo contra hoc dicitur Iob XXXVI, 26: ecce Deus magnus, vincens scientiam nostram. Secundo potest responderi, quia dato quod aliquis magister aliquid diceret in sua scientia, et aliquis rusticus diceret non esse sicut magister doceret, eo quod ipse non intelligeret, multum reputaretur stultus ille rusticus. Constat autem quod intellectus Angeli excedit magis intellectum optimi philosophi, quam intellectus optimi philosophi intellectum rustici. Et ideo stultus est philosophus si nolit credere ea quae Angeli dicunt; et multo magis si nolit credere ea quae Deus dicit. Et contra hoc dicitur Eccli. III, 25: plurima supra sensum hominum ostensa sunt tibi.

“The Evidence of Things that Appear Not.”—But someone will say that it is foolish to believe what is not seen, and that one should not believe in things that he cannot see. I answer by saying that the imperfect nature of our intellect takes away the basis of this difficulty. For if man of himself could in a perfect manner know all things visible and invisible, it would indeed be foolish to believe what he does not see. But our manner of knowing is so weak that no philosopher could perfectly investigate the nature of even one little fly. We even read that a certain philosopher spent thirty years in solitude in order to know the nature of the bee. If, therefore, our intellect is so weak, it is foolish to be willing to believe concerning God only that which man can know by himself alone. And against this is the word of Job: “Behold, God is great, exceeding our knowledge” [Job 36:26]. One can also answer this question by supposing that a certain master had said something concerning his own special branch of knowledge, and some uneducated person would contradict him for no other reason than that he could not understand what the master said! Such a person would be considered very foolish. So, the intellect of the Angels as greatly exceeds the intellect of the greatest philosopher as much as that of the greatest philosopher exceeds the intellect of the uneducated man. Therefore, the philosopher is foolish if he refuses to believe what an Angel says, and far greater fool to refuse to believe what God says. Against such are these words: “For many things are shown to you above the understanding of men” [Sir 3:25].