Summa Theologica, Part I (Prima Pars) From the Complete...

But since besides the intellectual light which is in us, intelligible species, which are derived from things, are required in order for us to have knowledge of material things; therefore this same knowledge is not due merely to a participation of the eternal types, as the Platonists held, maintaining that the mere participation of ideas sufficed for knowledge. Wherefore Augustine says (De Trin. iv, 16): "Although the philosophers prove by convincing arguments that all things occur in time according to the eternal types, were they able to see in the eternal types, or to find out from them how many kinds of animals there are and the origin of each? Did they not seek for this information from the story of times and places?"

But that Augustine did not understand all things to be known in their "eternal types" or in the "unchangeable truth," as though the eternal types themselves were seen, is clear from what he says (QQ. 83, qu. 46)—viz. that "not each and every rational soul can be said to be worthy of that vision," namely, of the eternal types, "but only those that are holy and pure," such as the souls of the blessed.

Objection 1: It would seem that intellectual knowledge is not derived from sensible things. For Augustine says (QQ. 83, qu. 9) that "we cannot expect to learn the fulness of truth from the senses of the body." This he proves in two ways. First, because "whatever the bodily senses reach, is continually being changed; and what is never the same cannot be perceived." Secondly, because, "whatever we perceive by the body, even when not present to the senses, may be present to the imagination, as when we are asleep or angry: yet we cannot discern by the senses, whether what we perceive be the sensible object or the deceptive image thereof. Now nothing can be perceived which cannot be distinguished from its counterfeit." And so he concludes that we cannot expect to learn the truth from the senses. But intellectual knowledge apprehends the truth. Therefore intellectual knowledge cannot be conveyed by the senses.

Obj. 2: Further, Augustine says (Gen. ad lit. xii, 16): "We must not think that the body can make any impression on the spirit, as though the spirit were to supply the place of matter in regard to the body's action; for that which acts is in every way more excellent than that which it acts on." Whence he concludes that "the body does not cause its image in the spirit, but the spirit causes it in itself." Therefore intellectual knowledge is not derived from sensible things.

Obj. 3: Further, an effect does not surpass the power of its cause. But intellectual knowledge extends beyond sensible things: for we understand some things which cannot be perceived by the senses. Therefore intellectual knowledge is not derived from sensible things.

I answer that, On this point the philosophers held three opinions. For Democritus held that "all knowledge is caused by images issuing from the bodies we think of and entering into our souls," as Augustine says in his letter to Dioscorus (cxviii, 4). And Aristotle says (De Somn. et Vigil.) that Democritus held that knowledge is caused by a "discharge of images." And the reason for this opinion was that both Democritus and the other early philosophers did not distinguish between intellect and sense, as Aristotle relates (De Anima iii, 3). Consequently, since the sense is affected by the sensible, they thought that all our knowledge is affected by this mere impression brought about by sensible things. Which impression Democritus held to be caused by a discharge of images.

Plato, on the other hand, held that the intellect is distinct from the senses: and that it is an immaterial power not making use of a corporeal organ for its action. And since the incorporeal cannot be affected by the corporeal, he held that intellectual knowledge is not brought about by sensible things affecting the intellect, but by separate intelligible forms being participated by the intellect, as we have said above (AA. 4 ,5). Moreover he held that sense is a power operating of itself. Consequently neither is sense, since it is a spiritual power, affected by the sensible: but the sensible organs are affected by the sensible, the result being that the soul is in a way roused to form within itself the species of the sensible. Augustine seems to touch on this opinion (Gen. ad lit. xii, 24) where he says that the "body feels not, but the soul through the body, which it makes use of as a kind of messenger, for reproducing within itself what is announced from without." Thus according to Plato, neither does intellectual knowledge proceed from sensible knowledge, nor sensible knowledge exclusively from sensible things; but these rouse the sensible soul to the sentient act, while the senses rouse the intellect to the act of understanding.

Aristotle chose a middle course. For with Plato he agreed that intellect and sense are different. But he held that the sense has not its proper operation without the cooperation of the body; so that to feel is not an act of the soul alone, but of the "composite." And he held the same in regard to all the operations of the sensitive part. Since, therefore, it is not unreasonable that the sensible objects which are outside the soul should produce some effect in the "composite," Aristotle agreed with Democritus in this, that the operations of the sensitive part are caused by the impression of the sensible on the sense: not by a discharge, as Democritus said, but by some kind of operation. For Democritus maintained that every operation is by way of a discharge of atoms, as we gather from De Gener. i, 8. But Aristotle held that the intellect has an operation which is independent of the body's cooperation. Now nothing corporeal can make an impression on the incorporeal. And therefore in order to cause the intellectual operation according to Aristotle, the impression caused by the sensible does not suffice, but something more noble is required, for "the agent is more noble than the patient," as he says (De Gener. i, 5).

Not, indeed, in the sense that the intellectual operation is effected in us by the mere impression of some superior beings, as Plato held; but that the higher and more noble agent which he calls the active intellect, of which we have spoken above (Q. 79, AA. 3, 4) causes the phantasms received from the senses to be actually intelligible, by a process of abstraction.

According to this opinion, then, on the part of the phantasms, intellectual knowledge is caused by the senses. But since the phantasms cannot of themselves affect the passive intellect, and require to be made actually intelligible by the active intellect, it cannot be said that sensible knowledge is the total and perfect cause of intellectual knowledge, but rather that it is in a way the material cause.

Reply Obj. 1: Those words of Augustine mean that we must not expect the entire truth from the senses. For the light of the active intellect is needed, through which we achieve the unchangeable truth of changeable things, and discern things themselves from their likeness.

Reply Obj. 2: In this passage Augustine speaks not of intellectual but of imaginary knowledge. And since, according to the opinion of Plato, the imagination has an operation which belongs to the soul only, Augustine, in order to show that corporeal images are impressed on the imagination, not by bodies but by the soul, uses the same argument as Aristotle does in proving that the active intellect must be separate, namely, because "the agent is more noble than the patient." And without doubt, according to the above opinion, in the imagination there must needs be not only a passive but also an active power. But if we hold, according to the opinion of Aristotle, that the action of the imagination is an action of the "composite," there is no difficulty; because the sensible body is more noble than the organ of the animal, in so far as it is compared to it as a being in act to a being in potentiality; even as the object actually colored is compared to the pupil which is potentially colored. It may, however, be said, although the first impression of the imagination is through the agency of the sensible, since "fancy is movement produced in accordance with sensation" (De Anima iii, 3), that nevertheless there is in man an operation which by synthesis and analysis forms images of various things, even of things not perceived by the senses. And Augustine's words may be taken in this sense.

Objection 1: It would seem that the intellect can actually understand through the intelligible species of which it is possessed, without turning to the phantasms. For the intellect is made actual by the intelligible species by which it is informed. But if the intellect is in act, it understands. Therefore the intelligible species suffices for the intellect to understand actually, without turning to the phantasms.

Obj. 2: Further, the imagination is more dependent on the senses than the intellect on the imagination. But the imagination can actually imagine in the absence of the sensible. Therefore much more can the intellect understand without turning to the phantasms.

Obj. 3: There are no phantasms of incorporeal things: for the imagination does not transcend time and space. If, therefore, our intellect cannot understand anything actually without turning to the phantasms, it follows that it cannot understand anything incorporeal. Which is clearly false: for we understand truth, and God, and the angels.

I answer that, In the present state of life in which the soul is united to a passible body, it is impossible for our intellect to understand anything actually, except by turning to the phantasms. First of all because the intellect, being a power that does not make use of a corporeal organ, would in no way be hindered in its act through the lesion of a corporeal organ, if for its act there were not required the act of some power that does make use of a corporeal organ. Now sense, imagination and the other powers belonging to the sensitive part, make use of a corporeal organ. Wherefore it is clear that for the intellect to understand actually, not only when it acquires fresh knowledge, but also when it applies knowledge already acquired, there is need for the act of the imagination and of the other powers. For when the act of the imagination is hindered by a lesion of the corporeal organ, for instance in a case of frenzy; or when the act of the memory is hindered, as in the case of lethargy, we see that a man is hindered from actually understanding things of which he had a previous knowledge. Secondly, anyone can experience this of himself, that when he tries to understand something, he forms certain phantasms to serve him by way of examples, in which as it were he examines what he is desirous of understanding. For this reason it is that when we wish to help someone to understand something, we lay examples before him, from which he forms phantasms for the purpose of understanding.

Now the reason of this is that the power of knowledge is proportioned to the thing known. Wherefore the proper object of the angelic intellect, which is entirely separate from a body, is an intelligible substance separate from a body. Whereas the proper object of the human intellect, which is united to a body, is a quiddity or nature existing in corporeal matter; and through such natures of visible things it rises to a certain knowledge of things invisible. Now it belongs to such a nature to exist in an individual, and this cannot be apart from corporeal matter: for instance, it belongs to the nature of a stone to be in an individual stone, and to the nature of a horse to be in an individual horse, and so forth. Wherefore the nature of a stone or any material thing cannot be known completely and truly, except in as much as it is known as existing in the individual. Now we apprehend the individual through the senses and the imagination. And, therefore, for the intellect to understand actually its proper object, it must of necessity turn to the phantasms in order to perceive the universal nature existing in the individual. But if the proper object of our intellect were a separate form; or if, as the Platonists say, the natures of sensible things subsisted apart from the individual; there would be no need for the intellect to turn to the phantasms whenever it understands.

Reply Obj. 1: The species preserved in the passive intellect exist there habitually when it does not understand them actually, as we have said above (Q. 79, A. 6). Wherefore for us to understand actually, the fact that the species are preserved does not suffice; we need further to make use of them in a manner befitting the things of which they are the species, which things are natures existing in individuals.

Reply Obj. 3: Incorporeal things, of which there are no phantasms, are known to us by comparison with sensible bodies of which there are phantasms. Thus we understand truth by considering a thing of which we possess the truth; and God, as Dionysius says (Div. Nom. i), we know as cause, by way of excess and by way of remotion. Other incorporeal substances we know, in the present state of life, only by way of remotion or by some comparison to corporeal things. And, therefore, when we understand something about these things, we need to turn to phantasms of bodies, although there are no phantasms of the things themselves. _______________________

Objection 1: It would seem that the judgment of the intellect is not hindered by suspension of the sensitive powers. For the superior does not depend on the inferior. But the judgment of the intellect is higher than the senses. Therefore the judgment of the intellect is not hindered through suspension of the senses.

Obj. 2: Further, to syllogize is an act of the intellect. But during sleep the senses are suspended, as is said in De Somn. et Vigil. i and yet it sometimes happens to us to syllogize while asleep. Therefore the judgment of the intellect is not hindered through suspension of the senses.

On the contrary, What a man does while asleep, against the moral law, is not imputed to him as a sin; as Augustine says (Gen. ad lit. xii, 15). But this would not be the case if man, while asleep, had free use of his reason and intellect. Therefore the judgment of the intellect is hindered by suspension of the senses.

I answer that, As we have said above (A. 7), our intellect's proper and proportionate object is the nature of a sensible thing. Now a perfect judgment concerning anything cannot be formed, unless all that pertains to that thing's nature be known; especially if that be ignored which is the term and end of judgment. Now the Philosopher says (De Coel. iii), that "as the end of a practical science is action, so the end of natural science is that which is perceived principally through the senses"; for the smith does not seek knowledge of a knife except for the purpose of action, in order that he may produce a certain individual knife; and in like manner the natural philosopher does not seek to know the nature of a stone and of a horse, save for the purpose of knowing the essential properties of those things which he perceives with his senses. Now it is clear that a smith cannot judge perfectly of a knife unless he knows the action of the knife: and in like manner the natural philosopher cannot judge perfectly of natural things, unless he knows sensible things. But in the present state of life whatever we understand, we know by comparison to natural sensible things. Consequently it is not possible for our intellect to form a perfect judgment, while the senses are suspended, through which sensible things are known to us.

Reply Obj. 1: Although the intellect is superior to the senses, nevertheless in a manner it receives from the senses, and its first and principal objects are founded in sensible things. And therefore suspension of the senses necessarily involves a hindrance to the judgment of the intellect.

Reply Obj. 2: The senses are suspended in the sleeper through certain evaporations and the escape of certain exhalations, as we read in De Somn. et Vigil. iii. And, therefore, according to the amount of such evaporation, the senses are more or less suspended. For when the amount is considerable, not only are the senses suspended, but also the imagination, so that there are no phantasms; thus does it happen, especially when a man falls asleep after eating and drinking copiously. If, however, the evaporation be somewhat less, phantasms appear, but distorted and without sequence; thus it happens in a case of fever. And if the evaporation be still more attenuated, the phantasms will have a certain sequence: thus especially does it happen towards the end of sleep in sober men and those who are gifted with a strong imagination. If the evaporation be very slight, not only does the imagination retain its freedom, but also the common sense is partly freed; so that sometimes while asleep a man may judge that what he sees is a dream, discerning, as it were, between things, and their images. Nevertheless, the common sense remains partly suspended; and therefore, although it discriminates some images from the reality, yet is it always deceived in some particular. Therefore, while man is asleep, according as sense and imagination are free, so is the judgment of his intellect unfettered, though not entirely. Consequently, if a man syllogizes while asleep, when he wakes up he invariably recognizes a flaw in some respect. _______________________