Summa Theologica, by St. Thomas Aquinas, [1947], at sacred-texts.com
We now have to consider what our intellect knows in material things. Under this head there are four points of inquiry:
(1) Whether it knows singulars?
(2) Whether it knows the infinite?
(3) Whether it knows contingent things?
(4) Whether it knows future things?
Objection 1: It would seem that our intellect knows singulars. For whoever knows composition, knows the terms of composition. But our intellect knows this composition; "Socrates is a man": for it belongs to the intellect to form a proposition. Therefore our intellect knows this singular, Socrates.
Objection 2: Further, the practical intellect directs to action. But action has relation to singular things. Therefore the intellect knows the singular.
Objection 3: Further, our intellect understands itself. But in itself it is a singular, otherwise it would have no action of its own; for actions belong to singulars. Therefore our intellect knows singulars.
Objection 4: Further, a superior power can do whatever is done by an inferior power. But sense knows the singular. Much more, therefore, can the intellect know it.
On the contrary, The Philosopher says (Phys. i, 5), that "the universal is known by reason; and the singular is known by sense."
I answer that, Our intellect cannot know the singular in material things directly and primarily. The reason of this is that the principle of singularity in material things is individual matter, whereas our intellect, as have said above (Q[85], A[1]), understands by abstracting the intelligible species from such matter. Now what is abstracted from individual matter is the universal. Hence our intellect knows directly the universal only. But indirectly, and as it were by a kind of reflection, it can know the singular, because, as we have said above (Q[85], A[7]), even after abstracting the intelligible species, the intellect, in order to understand, needs to turn to the phantasms in which it understands the species, as is said De Anima iii, 7. Therefore it understands the universal directly through the intelligible species, and indirectly the singular represented by the phantasm. And thus it forms the proposition "Socrates is a man." Wherefore the reply to the first objection is clear.
Reply to Objection 2: The choice of a particular thing to be done is as the conclusion of a syllogism formed by the practical intellect, as is said Ethic. vii, 3. But a singular proposition cannot be directly concluded from a universal proposition, except through the medium of a singular proposition. Therefore the universal principle of the practical intellect does not move save through the medium of the particular apprehension of the sensitive part, as is said De Anima iii, 11.
Reply to Objection 3: Intelligibility is incompatible with the singular not as such, but as material, for nothing can be understood otherwise than immaterially. Therefore if there be an immaterial singular such as the intellect, there is no reason why it should not be intelligible.
Reply to Objection 4: The higher power can do what the lower power can, but in a more eminent way. Wherefore what the sense knows materially and concretely, which is to know the singular directly, the intellect knows immaterially and in the abstract, which is to know the universal.
Objection 1: It would seem that our intellect can know the infinite. For God excels all infinite things. But our intellect can know God, as we have said above (Q[12], A[1]). Much more, therefore, can our intellect know all other infinite things.
Objection 2: Further, our intellect can naturally know "genera" and "species." But there is an infinity of species in some genera, as in number, proportion, and figure. Therefore our intellect can know the infinite.
Objection 3: Further, if one body can coexist with another in the same place, there is nothing to prevent an infinite number of bodies being in one place. But one intelligible species can exist with another in the same intellect, for many things can be habitually known at the same time. Therefore our intellect can have an habitual knowledge of an infinite number of things.
Objection 4: Further, as the intellect is not a corporeal faculty, as we have said (Q[76], A[1]), it appears to be an infinite power. But an infinite power has a capacity for an infinite object. Therefore our intellect can know the infinite.
On the contrary, It is said (Phys. i, 4) that "the infinite, considered as such, is unknown."
I answer that, Since a faculty and its object are proportional to each other, the intellect must be related to the infinite, as is its object, which is the quiddity of a material thing. Now in material things the infinite does not exist actually, but only potentially, in the sense of one succeeding another, as is said Phys. iii, 6. Therefore infinity is potentially in our mind through its considering successively one thing after another: because never does our intellect understand so many things, that it cannot understand more.
On the other hand, our intellect cannot understand the infinite either actually or habitually. Not actually, for our intellect cannot know actually at the same time, except what it knows through one species. But the infinite is not represented by one species, for if it were it would be something whole and complete. Consequently it cannot be understood except by a successive consideration of one part after another, as is clear from its definition (Phys. iii, 6): for the infinite is that "from which, however much we may take, there always remains something to be taken." Thus the infinite could not be known actually, unless all its parts were counted: which is impossible.
For the same reason we cannot have habitual knowledge of the infinite: because in us habitual knowledge results from actual consideration: since by understanding we acquire knowledge, as is said Ethic. ii, 1. Wherefore it would not be possible for us to have a habit of an infinity of things distinctly known, unless we had already considered the entire infinity thereof, counting them according to the succession of our knowledge: which is impossible. And therefore neither actually nor habitually can our intellect know the infinite, but only potentially as explained above.
Reply to Objection 1: As we have said above (Q[7], A[1]), God is called infinite, because He is a form unlimited by matter; whereas in material things, the term 'infinite' is applied to that which is deprived of any formal term. And form being known in itself, whereas matter cannot be known without form, it follows that the material infinite is in itself unknowable. But the formal infinite, God, is of Himself known; but He is unknown to us by reason of our feeble intellect, which in its present state has a natural aptitude for material objects only. Therefore we cannot know God in our present life except through material effects. In the future life this defect of intellect will be removed by the state of glory, when we shall be able to see the Essence of God Himself, but without being able to comprehend Him.
Reply to Objection 2: The nature of our mind is to know species abstracted from phantasms; therefore it cannot know actually or habitually species of numbers or figures that are not in the imagination, except in a general way and in their universal principles; and this is to know them potentially and confusedly.
Reply to Objection 3: If two or more bodies were in the same place, there would be no need for them to occupy the place successively, in order for the things placed to be counted according to this succession of occupation. On the other hand, the intelligible species enter into our intellect successively; since many things cannot be actually understood at the same time: and therefore there must be a definite and not an infinite number of species in our intellect.
Reply to Objection 4: As our intellect is infinite in power, so does it know the infinite. For its power is indeed infinite inasmuch as it is not terminated by corporeal matter. Moreover it can know the universal, which is abstracted from individual matter, and which consequently is not limited to one individual, but, considered in itself, extends to an infinite number of individuals.
Objection 1: It would seem that the intellect cannot know contingent things: because, as the Philosopher says (Ethic. vi, 6), the objects of understanding, wisdom and knowledge are not contingent, but necessary things.
Objection 2: Further, as stated in Phys. iv, 12, "what sometimes is and sometimes is not, is measured by time." Now the intellect abstracts from time, and from other material conditions. Therefore, as it is proper to a contingent thing sometime to be and sometime not to be, it seems that contingent things are not known by the intellect.
On the contrary, All knowledge is in the intellect. But some sciences are of the contingent things, as the moral sciences, the objects of which are human actions subject to free-will; and again, the natural sciences in as far as they relate to things generated and corruptible. Therefore the intellect knows contingent things.
I answer that, Contingent things can be considered in two ways; either as contingent, or as containing some element of necessity, since every contingent thing has in it something necessary: for example, that Socrates runs, is in itself contingent; but the relation of running to motion is necessary, for it is necessary that Socrates move if he runs. Now contingency arises from matter, for contingency is a potentiality to be or not to be, and potentiality belongs to matter; whereas necessity results from form, because whatever is consequent on form is of necessity in the subject. But matter is the individualizing principle: whereas the universal comes from the abstraction of the form from the particular matter. Moreover it was laid down above (A[1]) that the intellect of itself and directly has the universal for its object; while the object of sense is the singular, which in a certain way is the indirect object of the intellect, as we have said above (A[1]). Therefore the contingent, considered as such, is known directly by sense and indirectly by the intellect; while the universal and necessary principles of contingent things are known only by the intellect. Hence if we consider the objects of science in their universal principles, then all science is of necessary things. But if we consider the things themselves, thus some sciences are of necessary things, some of contingent things.
From which the replies to the objections are clear.
Objection 1: It would seem that our intellect knows the future. For our intellect knows by means of intelligible species abstracted from the "here" and "now," and related indifferently to all time. But it can know the present. Therefore it can know the future.
Objection 2: Further, man, while his senses are in suspense, can know some future things, as in sleep, and in frenzy. But the intellect is freer and more vigorous when removed from sense. Therefore the intellect of its own nature can know the future.
Objection 3: The intellectual knowledge of man is superior to any knowledge of brutes. But some animals know the future; thus crows by their frequent cawing foretell rain. Therefore much more can the intellect know the future.
On the contrary, It is written (Eccl. 8:6, 7), "There is a great affliction for man, because he is ignorant of things past; and things to come he cannot know by any messenger."
I answer that, We must apply the same distinction to future things, as we applied above (A[3]) to contingent things. For future things considered as subject to time are singular, and the human intellect knows them by reflection only, as stated above (A[1]). But the principles of future things may be universal; and thus they may enter the domain of the intellect and become the objects of science.
Speaking, however, of the knowledge of the future in a general way, we must observe that the future may be known in two ways: either in itself, or in its cause. The future cannot be known in itself save by God alone; to Whom even that is present which in the course of events is future, forasmuch as from eternity His glance embraces the whole course of time, as we have said above when treating of God's knowledge (Q[14], A[13]). But forasmuch as it exists in its cause, the future can be known by us also. And if, indeed, the cause be such as to have a necessary connection with its future result, then the future is known with scientific certitude, just as the astronomer foresees the future eclipse. If, however, the cause be such as to produce a certain result more frequently than not, then can the future be known more or less conjecturally, according as its cause is more or less inclined to produce the effect.
Reply to Objection 1: This argument considers that knowledge which is drawn from universal causal principles; from these the future may be known, according to the order of the effects to the cause.
Reply to Objection 2: As Augustine says (Confess. xii [*Gen. ad lit. xii. 13]), the soul has a certain power of forecasting, so that by its very nature it can know the future; hence when withdrawn from corporeal sense, and, as it were, concentrated on itself, it shares in the knowledge of the future. Such an opinion would be reasonable if we were to admit that the soul receives knowledge by participating the ideas as the Platonists maintained, because in that case the soul by its nature would know the universal causes of all effects, and would only be impeded in its knowledge by the body, and hence when withdrawn from the corporeal senses it would know the future.
But since it is connatural to our intellect to know things, not thus, but by receiving its knowledge from the senses; it is not natural for the soul to know the future when withdrawn from the senses: rather does it know the future by the impression of superior spiritual and corporeal causes; of spiritual causes, when by Divine power the human intellect is enlightened through the ministry of angels, and the phantasms are directed to the knowledge of future events; or, by the influence of demons, when the imagination is moved regarding the future known to the demons, as explained above (Q[57], A[3]). The soul is naturally more inclined to receive these impressions of spiritual causes when it is withdrawn from the senses, as it is then nearer to the spiritual world, and freer from external distractions. The same may also come from superior corporeal causes. For it is clear that superior bodies influence inferior bodies. Hence, in consequence of the sensitive faculties being acts of corporeal organs, the influence of the heavenly bodies causes the imagination to be affected, and so, as the heavenly bodies cause many future events, the imagination receives certain images of some such events. These images are perceived more at night and while we sleep than in the daytime and while we are awake, because, as stated in De Somn. et Vigil. ii [*De Divinat. per somn. ii.], "impressions made by day are evanescent. The night air is calmer, when silence reigns, hence bodily impressions are made in sleep, when slight internal movements are felt more than in wakefulness, and such movements produce in the imagination images from which the future may be foreseen."
Reply to Objection 3: Brute animals have no power above the imagination wherewith to regulate it, as man has his reason, and therefore their imagination follows entirely the influence of the heavenly bodies. Thus from such animals' movements some future things, such as rain and the like, may be known rather from human movements directed by reason. Hence the Philosopher says (De Somn. et Vig.), that "some who are most imprudent are most far-seeing; for their intelligence is not burdened with cares, but is as it were barren and bare of all anxiety moving at the caprice of whatever is brought to bear on it."