Summa Th. I EN Qu.115 a.4

Article: 4 Whether the heavenly bodies are the cause of human actions?

Objection: 1. It would seem that the heavenly bodies are the cause of human actions. For since the heavenly bodies are moved by spiritual substances, as stated above (Question [110], Article [3]), they act by virtue thereof as their instruments. But those spiritual substances are superior to our souls. Therefore it seems that they can cause impressions on our souls, and thereby cause human actions.

2. Further, every multiform is reducible to a uniform principle. But human actions are various and multiform. Therefore it seems that they are reducible to the uniform movements of heavenly bodies, as to their principles.

3. Further, astrologers often foretell the truth concerning the outcome of wars, and other human actions, of which the intellect and will are the principles. But they could not do this by means of the heavenly bodies, unless these were the cause of human actions. Therefore the heavenly bodies are the cause of human actions.

On the contrary Damascene says (De Fide Orth. ii, 7) that "the heavenly bodies are by no means the cause of human actions."

I answer that The heavenly bodies can directly and of themselves act on bodies, as stated above (Article [3]). They can act directly indeed on those powers of the soul which are the acts of corporeal organs, but accidentally: because the acts of such powers must needs be hindered by obstacles in the organs; thus an eye when disturbed cannot see well. Wherefore if the intellect and will were powers affixed to corporeal organs, as some maintained, holding that intellect does not differ from sense; it would follow of necessity that the heavenly bodies are the cause of human choice and action. It would also follow that man is led by natural instinct to his actions, just as other animals, in which there are powers other than those which are affixed to corporeal organs: for whatever is done here below in virtue of the action of heavenly bodies, is done naturally. It would therefore follow that man has no free-will, and that he would have determinate actions, like other natural things. All of which is manifestly false, and contrary to human habit. It must be observed, however, that indirectly and accidentally, the impressions of heavenly bodies can reach the intellect and will, forasmuch, namely, as both intellect and will receive something from the inferior powers which are affixed to corporeal organs. But in this the intellect and will are differently situated. For the intellect, of necessity, receives from the inferior apprehensive powers: wherefore if the imaginative, cogitative, or memorative powers be disturbed, the action of the intellect is, of necessity, disturbed also. The will, on the contrary, does not, of necessity, follow the inclination of the inferior appetite; for although the passions in the irascible and concupiscible have a certain force in inclining the will; nevertheless the will retains the power of following the passions or repressing them. Therefore the impressions of the heavenly bodies, by virtue of which the inferior powers can be changed, has less influence on the will, which is the proximate cause of human actions, than on the intellect.

To maintain therefore that heavenly bodies are the cause of human actions is proper to those who hold that intellect does not differ from sense. Wherefore some of these said that "such is the will of men, as is the day which the father of men and of gods brings on" (Odyssey xviii 135). Since, therefore, it is manifest that intellect and will are not acts of corporeal organs, it is impossible that heavenly bodies be the cause of human actions.

Reply to Objection: 1. The spiritual substances, that move the heavenly bodies, do indeed act on corporeal things by means of the heavenly bodies; but they act immediately on the human intellect by enlightening it. On the other hand, they cannot compel the will, as stated above (Question [111], Article [2]).

2. Just as the multiformity of corporeal movements is reducible to the uniformity of the heavenly movement as to its cause: so the multiformity of actions proceeding from the intellect and the will is reduced to a uniform principle which is the Divine intellect and will.

3. The majority of men follow their passions, which are movements of the sensitive appetite, in which movements of the heavenly bodies can cooperate: but few are wise enough to resist these passions. Consequently astrologers are able to foretell the truth in the majority of cases, especially in a general way. But not in particular cases; for nothing prevents man resisting his passions by his free-will. Wherefore the astrologers themselves are wont to say that "the wise man is stronger than the stars" [*Ptolemy, Centiloquium, prop. 5], forasmuch as, to wit, he conquers his passions.

Article: 5 Whether heavenly bodies can act on the demons?

Objection: 1. It would seem that heavenly bodies can act on the demons. For the demons, according to certain phases of the moon, can harass men, who on that account are called lunatics, as appears from Mt 4,24 Mt 17,14. But this would not be if they were not subject to the heavenly bodies. Therefore the demons are subject to them.

2. Further, necromancers observe certain constellations in order to invoke the demons. But these would not be invoked through the heavenly bodies unless they were subject to them. Therefore they are subject to them.

3. Further, heavenly bodies are more powerful than inferior bodies. But the demons are confined to certain inferior bodies, namely, "herbs, stones, animals, and to certain sounds and words, forms and figures," as Porphyry says, quoted by Augustine (De Civ. Dei x, 11). Much more therefore are the demons subject to the action of heavenly bodies.

On the contrary The demons are superior in the order of nature, to the heavenly bodies. But the "agent is superior to the patient," as Augustine says (Gn ad lit. xii, 16). Therefore the demons are not subject to the action of heavenly bodies.

I answer that, There have been three opinions about the demons. In the first place the Peripatetics denied the existence of demons; and held that what is ascribed to the demons, according to the necromantic art, is effected by the power of the heavenly bodies. This is what Augustine (De Civ. Dei x, 11) relates as having been held by Porphyry, namely, that "on earth men fabricate certain powers useful in producing certain effects of the stars." But this opinion is manifestly false. For we know by experience that many things are done by demons, for which the power of heavenly bodies would in no way suffice: for instance, that a man in a state of delirium should speak an unknown tongue, recite poetry and authors of whom he has no previous knowledge; that necromancers make statues to speak and move, and other like things.

For this reason the Platonists were led to hold that demons are "animals with an aerial body and a passive soul," as Apuleius says, quoted by Augustine (De Civ. Dei viii, 16). And this is the second of the opinions mentioned above: according to which it could be said that demons are subject to heavenly bodies in the same way as we have said man is subject thereto (Article [4]). But this opinion is proved to be false from what we have said above (Question [51], Article [1]): for we hold that demons are spiritual substances not united to bodies. Hence it is clear that they are subject to the action of heavenly bodies neither essentially nor accidentally, neither directly nor indirectly.

Reply to Objection: 1. That demons harass men, according to certain phases of the moon, happens in two ways. Firstly, they do so in order to "defame God's creature," namely, the moon; as Jerome (In Matt. iv, 24) and Chrysostom (Hom. lvii in ) say. Secondly, because as they are unable to effect anything save by means of the natural forces, as stated above (Question [114], Article [4], ad 2) they take into account the aptitude of bodies for the intended result. Now it is manifest that "the brain is the most moist of all the parts of the body," as Aristotle says [*De Part. Animal. ii, 7: De Sens. et Sensato ii: De Somn. et Vigil. iii]: wherefore it is the most subject to the action of the moon, the property of which is to move what is moist. And it is precisely in the brain that animal forces culminate: wherefore the demons, according to certain phases of the moon, disturb man's imagination, when they observe that the brain is thereto disposed.

2. Demons when summoned through certain constellations, come for two reasons. Firstly, in order to lead man into the error of believing that there is some Divine power in the stars. Secondly, because they consider that under certain constellations corporeal matter is better disposed for the result for which they are summoned.

3. As Augustine says (De Civ. Dei xxi, 6), the "demons are enticed through various kinds of stones, herbs, trees, animals, songs, rites, not as an animal is enticed by food, but as a spirit by signs"; that is to say, forasmuch as these things are offered to them in token of the honor due to God, of which they are covetous.

Article: 6 Whether heavenly bodies impose necessity on things subject to their action?

Objection: 1. It would seem that heavenly bodies impose necessity on things subject to their action. For given a sufficient cause, the effect follows of necessity. But heavenly bodies are a sufficient cause of their effects. Since, therefore, heavenly bodies, with their movements and dispositions, are necessary beings; it seems that their effects follow of necessity.

2. Further, an agent's effect results of necessity in matter, when the power of the agent is such that it can subject the matter to itself entirely. But the entire matter of inferior bodies is subject to the power of heavenly bodies, since this is a higher power than theirs. Therefore the effect of the heavenly bodies is of necessity received in corporeal matter.

3. Further, if the effect of the heavenly body does not follow of necessity, this is due to some hindering cause. But any corporeal cause, that might possibly hinder the effect of a heavenly body, must of necessity be reducible to some heavenly principle: since the heavenly bodies are the causes of all that takes place here below. Therefore, since also that heavenly principle is necessary, it follows that the effect of the heavenly body is necessarily hindered. Consequently it would follow that all that takes place here below happens of necessity.

On the contrary The Philosopher says (De Somn. et Vigil. [*De Divin. per Somn. ii]): "It is not incongruous that many of the signs observed in bodies, of occurrences in the heavens, such as rain and wind, should not be fulfilled." Therefore not all the effects of heavenly bodies take place of necessity.

I answer that This question is partly solved by what was said above (Article [4]); and in part presents some difficulty. For it was shown that although the action of heavenly bodies produces certain inclinations in corporeal nature, the will nevertheless does not of necessity follow these inclinations. Therefore there is nothing to prevent the effect of heavenly bodies being hindered by the action of the will, not only in man himself, but also in other things to which human action extends.

But in natural things there is no such principle, endowed with freedom to follow or not to follow the impressions produced by heavenly agents. Wherefore it seems that in such things at least, everything happens of necessity; according to the reasoning of some of the ancients who supposing that everything that is, has a cause; and that, given the cause, the effect follows of necessity; concluded that all things happen of necessity. This opinion is refuted by Aristotle (Metaph. vi, Did. v, 3) as to this double supposition.

For in the first place it is not true that, given any cause whatever, the effect must follow of necessity. For some causes are so ordered to their effects, as to produce them, not of necessity, but in the majority of cases, and in the minority to fail in producing them. But that such cases do fail in the minority of cases is due to some hindering cause; consequently the above-mentioned difficulty seems not to be avoided, since the cause in question is hindered of necessity.

Therefore we must say, in the second place, that everything that is a being "per se," has a cause; but what is accidentally, has not a cause, because it is not truly a being, since it is not truly one. For (that a thing is) "white" has a cause, likewise (that a man is) "musical" has not a cause, but (that a being is) "white-musical" has not a cause, because it is not truly a being, nor truly one. Now it is manifest that a cause which hinders the action of a cause so ordered to its effect as to produce it in the majority of cases, clashes sometimes with this cause by accident: and the clashing of these two causes, inasmuch as it is accidental, has no cause. Consequently what results from this clashing of causes is not to be reduced to a further pre-existing cause, from which it follows of necessity. For instance, that some terrestrial body take fire in the higher regions of the air and fall to the earth, is caused by some heavenly power: again, that there be on the surface of the earth some combustible matter, is reducible to some heavenly principle. But that the burning body should alight on this matter and set fire to it, is not caused by a heavenly body, but is accidental. Consequently not all the effects of heavenly bodies result of necessity.

Reply to Objection: 1. The heavenly bodies are causes of effects that take place here below, through the means of particular inferior causes, which can fail in their effects in the minority of cases.

2. The power of a heavenly body is not infinite. Wherefore it requires a determinate disposition in matter, both as to local distance and as to other conditions, in order to produce its effect. Therefore as local distance hinders the effect of a heavenly body (for the sun has not the same effect in heat in Dacia as in Ethiopia); so the grossness of matter, its low or high temperature or other such disposition, can hinder the effect of a heavenly body.

3. Although the cause that hinders the effect of another cause can be reduced to a heavenly body as its cause; nevertheless the clashing of two causes, being accidental, is not reduced to the causality of a heavenly body, as stated above.


We come now to the consideration of fate. Under this head there are four points of inquiry:

(1) Is there such a thing as fate?

(2) Where is it?

(3) Is it unchangeable?

(4) Are all things subject to fate?

Article: 1 Whether there be such a thing as fate?

Objection: 1. It would seem that fate is nothing. For Gregory says in a homily for the Epiphany (Hom. x in Evang.): "Far be it from the hearts of the faithful to think that fate is anything real."

2. Further, what happens by fate is not unforeseen, for as Augustine says (De Civ. Dei v, 4), "fate is understood to be derived from the verb 'fari' which means to speak"; as though things were said to happen by fate, which are "fore-spoken" by one who decrees them to happen. Now what is foreseen is neither lucky nor chance-like. If therefore things happen by fate, there will be neither luck nor chance in the world.

On the contrary What does not exist cannot be defined. But Boethius (De Consol. iv) defines fate thus: "Fate is a disposition inherent to changeable things, by which Providence connects each one with its proper order."

I answer that In this world some things seem to happen by luck or chance. Now it happens sometimes that something is lucky or chance-like as compared to inferior causes, which, if compared to some higher cause, is directly intended. For instance, if two servants are sent by their master to the same place; the meeting of the two servants in regard to themselves is by chance; but as compared to the master, who had ordered it, it is directly intended.

So there were some who refused to refer to a higher cause such events which by luck or chance take place here below. These denied the existence of fate and Providence, as Augustine relates of Tully (De Civ. Dei v, 9). And this is contrary to what we have said above about Providence (Question [22], Article [2]).

On the other hand, some have considered that everything that takes place here below by luck or by chance, whether in natural things or in human affairs, is to be reduced to a superior cause, namely, the heavenly bodies. According to these fate is nothing else than "a disposition of the stars under which each one is begotten or born" [*Cf. St. Augustine De Civ. Dei v, 1,8,9]. But this will not hold. First, as to human affairs: because we have proved above (Question [115], Article [4]) that human actions are not subject to the action of heavenly bodies, save accidentally and indirectly. Now the cause of fate, since it has the ordering of things that happen by fate, must of necessity be directly and of itself the cause of what takes place. Secondly, as to all things that happen accidentally: for it has been said (Question [115], Article [6]) that what is accidental, is properly speaking neither a being, nor a unity. But every action of nature terminates in some one thing. Wherefore it is impossible for that which is accidental to be the proper effect of an active natural principle. No natural cause can therefore have for its proper effect that a man intending to dig a grace finds a treasure. Now it is manifest that a heavenly body acts after the manner of a natural principle: wherefore its effects in this world are natural. It is therefore impossible that any active power of a heavenly body be the cause of what happens by accident here below, whether by luck or by chance.

We must therefore say that what happens here by accident, both in natural things and in human affairs, is reduced to a preordaining cause, which is Divine Providence. For nothing hinders that which happens by accident being considered as one by an intellect: otherwise the intellect could not form this proposition: "The digger of a grave found a treasure." And just as an intellect can apprehend this so can it effect it; for instance, someone who knows a place where a treasure is hidden, might instigate a rustic, ignorant of this, to dig a grave there. Consequently, nothing hinders what happens here by accident, by luck or by chance, being reduced to some ordering cause which acts by the intellect, especially the Divine intellect. For God alone can change the will, as shown above (Question [105], Article [4]). Consequently the ordering of human actions, the principle of which is the will, must be ascribed to God alone.

So therefore inasmuch as all that happens here below is subject to Divine Providence, as being pre-ordained, and as it were "fore-spoken," we can admit the existence of fate: although the holy doctors avoided the use of this word, on account of those who twisted its application to a certain force in the position of the stars. Hence Augustine says (De Civ. Dei v, 1): "If anyone ascribes human affairs to fate, meaning thereby the will or power of God, let him keep to his opinion, but hold his tongue." For this reason Gregory denies the existence of fate: wherefore the first objection's solution is manifest.

2. Nothing hinders certain things happening by luck or by chance, if compared to their proximate causes: but not if compared to Divine Providence, whereby "nothing happens at random in the world," as Augustine says (Questions. 83, qu. 24).

Article: 2 Whether fate is in created things?

Objection: 1. It would seem that fate is not in created things. For Augustine says (De Civ. Dei v, 1) that the "Divine will or power is called fate." But the Divine will or power is not in creatures, but in God. Therefore fate is not in creatures but in God.

2. Further, fate is compared to things that happen by fate, as their cause; as the very use of the word proves. But the universal cause that of itself effects what takes place by accident here below, is God alone, as stated above (Article [1]). Therefore fate is in God, and not in creatures.

3. Further, if fate is in creatures, it is either a substance or an accident: and whichever it is it must be multiplied according to the number of creatures. Since, therefore, fate seems to be one thing only, it seems that fate is not in creatures, but in God.

On the contrary Boethius says (De Consol. iv): "Fate is a disposition inherent to changeable things."

I answer that As is clear from what has been stated above (Question [22], Article [3]; Question [103], Article [6]), Divine Providence produces effects through mediate causes. We can therefore consider the ordering of the effects in two ways. Firstly, as being in God Himself: and thus the ordering of the effects is called Providence. But if we consider this ordering as being in the mediate causes ordered by God to the production of certain effects, thus it has the nature of fate. This is what Boethius says (De Consol. iv): "Fate is worked out when Divine Providence is served by certain spirits; whether by the soul, or by all nature itself which obeys Him, whether by the heavenly movements of the stars, whether by the angelic power, or by the ingenuity of the demons, whether by some of these, or by all, the chain of fate is forged." Of each of these things we have spoken above (Article [1]; Question [104], Article [2]; Question [110], Article [1]; Question [113]; Question [114]). It is therefore manifest that fate is in the created causes themselves, as ordered by God to the production of their effects.

Reply to Objection: 1. The ordering itself of second causes, which Augustine (De Civ. Dei v, 8) calls the "series of causes," has not the nature of fate, except as dependent on God. Wherefore the Divine power or will can be called fate, as being the cause of fate. But essentially fate is the very disposition or "series," i.e. order, of second causes.

2. Fate has the nature of a cause, just as much as the second causes themselves, the ordering of which is called fate.

3. Fate is called a disposition, not that disposition which is a species of quality, but in the sense in which it signifies order, which is not a substance, but a relation. And if this order be considered in relation to its principle, it is one; and thus fate is one. But if it be considered in relation to its effects, or to the mediate causes, this fate is multiple. In this sense the poet wrote: "Thy fate draws thee."

Article: 3 Whether fate is unchangeable?

Objection: 1. It seems that fate is not unchangeable. For Boethius says (De Consol. iv): "As reasoning is to the intellect, as the begotten is to that which is, as time to eternity, as the circle to its centre; so is the fickle chain of fate to the unwavering simplicity of Providence."

2. Further, the Philosopher says (Topic. ii, 7): "If we be moved, what is in us is moved." But fate is a "disposition inherent to changeable things," as Boethius says (De Consol. iv). Therefore fate is changeable.

3. Further, if fate is unchangeable, what is subject to fate happens unchangeably and of necessity. But things ascribed to fate seem principally to be contingencies. Therefore there would be no contingencies in the world, but all things would happen of necessity.

On the contrary Boethius says (De Consol. iv) that fate is an unchangeable disposition.

I answer that The disposition of second causes which we call fate, can be considered in two ways: firstly, in regard to the second causes, which are thus disposed or ordered; secondly, in regard to the first principle, namely, God, by Whom they are ordered. Some, therefore, have held that the series itself or dispositions of causes is in itself necessary, so that all things would happen of necessity; for this reason that each effect has a cause, and given a cause the effect must follow of necessity. But this is false, as proved above (Question [115], Article [6]).

Others, on the other hand, held that fate is changeable, even as dependent on Divine Providence. Wherefore the Egyptians said that fate could be changed by certain sacrifices, as Gregory of Nyssa says (Nemesius, De Homine). This too has been disproved above for the reason that it is repugnant to Divine Providence.

We must therefore say that fate, considered in regard to second causes, is changeable; but as subject to Divine Providence, it derives a certain unchangeableness, not of absolute but of conditional necessity. In this sense we say that this conditional is true and necessary: "If God foreknew that this would happen, it will happen." Wherefore Boethius, having said that the chain of fate is fickle, shortly afterwards adds---"which, since it is derived from an unchangeable Providence must also itself be unchangeable."

From this the answers to the objections are clear.

Article: 4 Whether all things are subject to fate?

Objection: 1. It seems that all things are subject to fate. For Boethius says (De Consol. iv): "The chain of fate moves the heaven and the stars, tempers the elements to one another, and models them by a reciprocal transformation. By fate all things that are born into the world and perish are renewed in a uniform progression of offspring and seed." Nothing therefore seems to be excluded from the domain of fate.

2. Further, Augustine says (De Civ. Dei v, 1) that fate is something real, as referred to the Divine will and power. But the Divine will is cause of all things that happen, as Augustine says (De Trin. iii, 1 seqq.). Therefore all things are subject to fate.

3. Further, Boethius says (De Consol. iv) that fate "is a disposition inherent to changeable things." But all creatures are changeable, and God alone is truly unchangeable, as stated above (Question [9], Article [2]). Therefore fate is in all things.

On the contrary Boethius says (De Consol. iv) that "some things subject to Providence are above the ordering of fate."

I answer that As stated above (Article [2]), fate is the ordering of second causes to effects foreseen by God. Whatever, therefore, is subject to second causes, is subject also to fate. But whatever is done immediately by God, since it is not subject to second causes, neither is it subject to fate; such are creation, the glorification of spiritual substances, and the like. And this is what Boethius says (De Consol. iv): viz. that "those things which are nigh to God have a state of immobility, and exceed the changeable order of fate." Hence it is clear that "the further a thing is from the First Mind, the more it is involved in the chain of fate"; since so much the more it is bound up with second causes.

Reply to Objection: 1. All the things mentioned in this passage are done by God by means of second causes; for this reason they are contained in the order of fate. But it is not the same with everything else, as stated above.

2. Fate is to be referred to the Divine will and power, as to its first principle. Consequently it does not follow that whatever is subject to the Divine will or power, is subject also to fate, as already stated.

3. Although all creatures are in some way changeable, yet some of them do not proceed from changeable created causes. And these, therefore, are not subject to fate, as stated above.


We have next to consider those things which pertain to the action of man, who is composed of a created corporeal and spiritual nature. In the first place we shall consider that action (in general) and secondly in regard to the propagation of man from man. As to the first, there are four points of inquiry:

(1) Whether one man can teach another, as being the cause of his knowledge?

(2) Whether man can teach an angel?

(3) Whether by the power of his soul man can change corporeal matter?

(4) Whether the separate soul of man can move bodies by local movement?

Article: 1 Whether one man can teach another?

Objection: 1. It would seem that one man cannot teach another. For the Lord says (Mt 23,8): "Be not you called Rabbi": on which the gloss of Jerome says, "Lest you give to men the honor due to God." Therefore to be a master is properly an honor due to God. But it belongs to a master to teach. Therefore man cannot teach, and this is proper to God.

2. Further, if one man teaches another this is only inasmuch as he acts through his own knowledge, so as to cause knowledge in the other. But a quality through which anyone acts so as to produce his like, is an active quality. Therefore it follows that knowledge is an active quality just as heat is.

3. Further, for knowledge we require intellectual light, and the species of the thing understood. But a man cannot cause either of these in another man. Therefore a man cannot by teaching cause knowledge in another man.

4. Further, the teacher does nothing in regard to a disciple save to propose to him certain signs, so as to signify something by words or gestures. But it is not possible to teach anyone so as to cause knowledge in him, by putting signs before him. For these are signs either of things that he knows, or of things he does not know. If of things that he knows, he to whom these signs are proposed is already in the possession of knowledge, and does not acquire it from the master. If they are signs of things that he does not know, he can learn nothing therefrom: for instance, if one were to speak Greek to a man who only knows Latin, he would learn nothing thereby. Therefore in no way can a man cause knowledge in another by teaching him.

On the contrary The Apostle says (1Tm 2,7): "Whereunto I am appointed a preacher and an apostle . . . a doctor of the Gentiles in faith and truth."

I answer that On this question there have been various opinions. For Averroes, commenting on De Anima iii, maintains that all men have one passive intellect in common, as stated above (Question [76], Article [2]). From this it follows that the same intelligible species belong to all men. Consequently he held that one man does not cause another to have a knowledge distinct from that which he has himself; but that he communicates the identical knowledge which he has himself, by moving him to order rightly the phantasms in his soul, so that they be rightly disposed for intelligible apprehension. This opinion is true so far as knowledge is the same in disciple and master, if we consider the identity of the thing known: for the same objective truth is known by both of them. But so far as he maintains that all men have but one passive intellect, and the same intelligible species, differing only as to various phantasms, his opinion is false, as stated above (Question [76], Article [2]).

Besides this, there is the opinion of the Platonists, who held that our souls are possessed of knowledge from the very beginning, through the participation of separate forms, as stated above (Question [84], Articles [3],4); but that the soul is hindered, through its union with the body, from the free consideration of those things which it knows. According to this, the disciple does not acquire fresh knowledge from his master, but is roused by him to consider what he knows; so that to learn would be nothing else than to remember. In the same way they held that natural agents only dispose (matter) to receive forms, which matter acquires by a participation of separate substances. But against this we have proved above (Question [79], Article [2]; Question [84], Article [3]) that the passive intellect of the human soul is in pure potentiality to intelligible (species), as Aristotle says (De Anima iii, 4).

We must therefore decide the question differently, by saying that the teacher causes knowledge in the learner, by reducing him from potentiality to act, as the Philosopher says (Phys. viii, 4). In order to make this clear, we must observe that of effects proceeding from an exterior principle, some proceed from the exterior principle alone; as the form of a house is caused to be in matter by art alone: whereas other effects proceed sometimes from an exterior principle, sometimes from an interior principle: thus health is caused in a sick man, sometimes by an exterior principle, namely by the medical art, sometimes by an interior principle as when a man is healed by the force of nature. In these latter effects two things must be noticed. First, that art in its work imitates nature for just as nature heals a man by alteration, digestion, rejection of the matter that caused the sickness, so does art. Secondly, we must remark that the exterior principle, art, acts, not as principal agent, but as helping the principal agent, but as helping the principal agent, which is the interior principle, by strengthening it, and by furnishing it with instruments and assistance, of which the interior principle makes use in producing the effect. Thus the physician strengthens nature, and employs food and medicine, of which nature makes use for the intended end.

Now knowledge is acquired in man, both from an interior principle, as is clear in one who procures knowledge by his own research; and from an exterior principle, as is clear in one who learns (by instruction). For in every man there is a certain principle of knowledge, namely the light of the active intellect, through which certain universal principles of all the sciences are naturally understood as soon as proposed to the intellect. Now when anyone applies these universal principles to certain particular things, the memory or experience of which he acquires through the senses; then by his own research advancing from the known to the unknown, he obtains knowledge of what he knew not before. Wherefore anyone who teaches, leads the disciple from things known by the latter, to the knowledge of things previously unknown to him; according to what the Philosopher says (Poster. i, 1): "All teaching and all learning proceed from previous knowledge."

Now the master leads the disciple from things known to knowledge of the unknown, in a twofold manner. Firstly, by proposing to him certain helps or means of instruction, which his intellect can use for the acquisition of science: for instance, he may put before him certain less universal propositions, of which nevertheless the disciple is able to judge from previous knowledge: or he may propose to him some sensible examples, either by way of likeness or of opposition, or something of the sort, from which the intellect of the learner is led to the knowledge of truth previously unknown. Secondly, by strengthening the intellect of the learner; not, indeed, by some active power as of a higher nature, as explained above (Question [106], Article [1]; Question [111], Article [1]) of the angelic enlightenment, because all human intellects are of one grade in the natural order; but inasmuch as he proposes to the disciple the order of principles to conclusions, by reason of his not having sufficient collating power to be able to draw the conclusions from the principles. Hence the Philosopher says (Poster. i, 2) that "a demonstration is a syllogism that causes knowledge." In this way a demonstrator causes his hearer to know.

Reply to Objection: 1. As stated above, the teacher only brings exterior help as the physician who heals: but just as the interior nature is the principal cause of the healing, so the interior light of the intellect is the principal cause of knowledge. But both of these are from God. Therefore as of God is it written: "Who healeth all thy diseases" (Ps 102,3); so of Him is it written: "He that teacheth man knowledge" (Ps 93,10), inasmuch as "the light of His countenance is signed upon us" (Ps 4,7), through which light all things are shown to us.

2. As Averroes argues, the teacher does not cause knowledge in the disciple after the manner of a natural active cause. Wherefore knowledge need not be an active quality: but is the principle by which one is directed in teaching, just as art is the principle by which one is directed in working.

3. The master does not cause the intellectual light in the disciple, nor does he cause the intelligible species directly: but he moves the disciple by teaching, so that the latter, by the power of his intellect, forms intelligible concepts, the signs of which are proposed to him from without.

4. The signs proposed by the master to the disciple are of things known in a general and confused manner; but not known in detail and distinctly. Therefore when anyone acquires knowledge by himself, he cannot be called self-taught, or be said to have his own master because perfect knowledge did not precede in him, such as is required in a master.

Summa Th. I EN Qu.115 a.4