St. Thomas Aquinas
Summa Contra Gentiles, Book I, chapters 1-9

Chapter 1:  The function of the wise man

My mouth shall discuss truth, and my lips shall detest the ungodly (Prov. vii,7). 

ACCORDING to established popular usage, which the Philosopher considers should be our guide in the naming of things, they are called 'wise' who put things in their right* order and control them well. Now, in all things that are to be controlled and put in order to an end, the measure of control and order must be taken from the end in view; and the proper end of everything is something good. Hence we see in the arts that art A governs and, as it were, lords it over art B, when the proper end of art B belongs to A.* Thus the art of medicine lords it over the art of the apothecary, because health, the object of medicine, is the end of all drugs that the apothecary's art compounds. These arts that lord it over others are called master-building,' or 'masterful arts'; and the 'master-builders' who practise them arrogate to themselves the name of 'wise men.' But because these persons deal with the ends in view of certain particular things, without attaining to the general end of all things, they are called 'wise in this or that particular thing,' as it is said, 'As a wise architect I have laid the foundation' (I Cor. iii, 10); while the name of 'wise' without qualification is reserved for him alone who deals with the last end of the universe, which is also the first beginning of the order of the universe. Hence, according to the Philosopher, it is proper to the wise man to consider the highest causes.

Now the last end of everything is that which is intended by the prime author or mover thereof. The prime author and mover of the universe is intelligence, as will be shown later (B. II, Chap. XXIII, XXIV).  Therefore the last end of the universe must be the good of the intelligence, and that is truth. Truth then must be the final end of the whole universe; and about the consideration of that end* wisdom must primarily be concerned. And therefore the Divine Wisdom, clothed in flesh, testifies that He came into the world for the manifestation of truth: For this was I born, and unto this I came into the World, to give testimony to the truth (John xvii, 37). The Philosopher also rules that the first philosophy is the science of truth, not of any and every truth, but of that truth which is the origin of all truth, and appertains to the first principle of the being of all things; hence its truth is the principle of all truth, for things are in truth as they are in being.

It is one and the same function to embrace either of two contraries and to repel the other. Hence, as it is the function of the wise man to discuss truth, particularly of the first beginning, so it is his also to impugn the contrary error. Suitably therefore is the double function of the wise man displayed in the words above quoted from the Sapiential Book, namely, to study, and upon study to speak out the truth of God, which of all other is most properly called truth, and this is referred to in the words, My mouth shall discuss truth, and to impugn error contrary to truth, as referred to in the words, And my lips shall detest the ungodly. 

Chapter 2:  The author's purpose

OF all human pursuits, the pursuit of wisdom is the more perfect, the more sublime, the more useful, and the more agreeable. The more perfect, because in so far as a man gives himself up to the pursuit of wisdom, to that extent he enjoys already some portion of true happiness. Blessed is the man that shall dwell in wisdom (Ecclus xiv, 22). The more sublime, because thereby man comes closest to the likeness of God, who hath made all things in wisdom (Ps. ciii, 24). The more useful, because by this same wisdom we arrive at the realm of immortality. The desire of wisdom shall lead to an everlasting kingdom (Wisd. vi, 21). The more agreeable, because her conversation hath no bitterness, nor her company any weariness, but gladness and joy (Wisd. viii, 16).

But on two accounts it is difficult to proceed against each particular error: first, because the sacrilegious utterances of our various erring opponents are not so well known to us as to enable us to find reasons, drawn from their own words, for the confutation of their errors: for such was the method of the ancient doctors in confuting the errors of the Gentiles, whose tenets they were readily able to know, having either been Gentiles themselves, or at least having lived among Gentiles and been instructed in their doctrines. Secondly, because some of them, as Mohammedans and Pagans, do not agree with us in recognising the authority of any scripture, available for their conviction, as we can argue against the Jews from the Old Testament, and against heretics from the New. But these receive neither: hence it is necessary to have recourse to natural reason, which all are obliged to assent to. But in the things of God natural reason is often at a loss. 

Chapter 3:  The truths we confess concerning God fall into two categories

BECAUSE not every truth admits of the same mode of manifestation, and "a well-educated man will expect exactness in every class of subject, according as the nature of the thing admits," as is very well remarked by the Philosopher (Eth. Nicom. I, 1094b), we must first show what mode of proof is possible for the truth that we have now before us. The truths that we confess concerning God fall under two modes. Some things true of God are beyond all the competence of human reason, as that God is Three and One. Other things there are to which even human reason can attain, as the existence and unity of God, which philosophers have proved to a demonstration under the guidance of the light of natural reason. That there are points of absolute intelligibility in God altogether beyond the compass of human reason, most manifestly appears. For since the leading principle of all knowledge of any given subject-matter is an understanding of the thing's innermost being, or substance -- according to the doctrine of the Philosopher, that the essence is the principle of demonstration -- it follows that the mode of our knowledge of the substance must be the mode of knowledge of whatever we know about the substance. Hence if the human understanding comprehends the substance of anything, as of a stone or triangle, none of the points of intelligibility about that thing will exceed the capacity of human reason.* But this is not our case with regard to God. The human understanding cannot go so far of its natural power as to grasp His substance, since under the conditions of the present life the knowledge of our understanding commences with sense; and therefore objects beyond sense cannot be grasped by human understanding except so far as knowledge is gathered of them through the senses. But things of sense cannot lead our understanding to read in them the essence of the Divine Substance, inasmuch as they are effects inadequate to the power that caused them. Nevertheless our understanding is thereby led to some knowledge of God, namely, of His existence and of other attributes that must necessarily be attributed to the First Cause. There are, therefore, some points of intelligibility in God, accessible to human reason, and other points that altogether transcend the power of human reason.* 

The same thing may be understood from consideration of degrees of intelligibility.* Of two minds, one of which has a keener insight into truth than the other, the higher mind understands much that the other cannot grasp at all, as is clear in the 'plain man' (in rustico), who can in no way grasp the subtle theories of philosophy. Now the intellect of an angel excels that of a man more than the intellect of the ablest philosopher excels that of the plainest of plain men (rudissimi idiotae). The angel has a higher standpoint in creation than man as a basis of his knowledge of God, inasmuch as the substance of the angel, whereby he is led to know God by a process of natural knowledge, is nobler and more excellent than the things of sense, and even than the soul itself, whereby the human mind rises to the knowledge of God. But the Divine Mind exceeds the angelic much more than the angelic the human. For the Divine Mind of its own comprehensiveness covers the whole extent of its substance, and therefore perfectly understands its own essence, and knows all that is knowable about itself; but an angel of his natural knowledge does not know the essence of God, because the angel's own substance, whereby it is led to a knowledge of God, is an effect inadequate to the power of the cause that created it. Hence not all things that God understands in Himself can be grasped by the natural knowledge of an angel; nor is human reason competent to take in all that an angel understands of his own natural ability. As therefore it would be the height of madness in a 'plain man' to declare a philosopher's propositions false, because he could not understand them, so and much more would a man show exceeding folly if he suspected of falsehood a divine revelation given by the ministry of angels, on the mere ground that it was beyond the investigation of reason.* 

The same thing manifestly appears from the incapacity which we daily experience in the observation of nature. We are ignorant of very many properties of the things of sense; and of the properties that our senses do apprehend, in most cases we cannot perfectly discover the reason. Much more is it beyond the competence of human reason to investigate all the points of intelligibility in that supreme excellent and transcendent substance of God. Consonant with this is the saying of the Philosopher, that "as the eyes of bats are to the light of the sun, so is the intelligence of our soul to the things most manifest by nature" (Aristotle, Metaphysics I, min. i). To this truth Holy Scripture also bears testimony. For it is said: Perchance thou wilt seize upon the traces of God, and fully discover the Almighty (Job xi, 7). And, Lo, God is great, and surpassing our knowledge (Job xxxvi, 26). And, We know in part (I Cor. xiii, 9). Not everything, therefore, that is said of God, even though it be beyond the power of reason to investigate, is at once to be rejected as false. 

Chapter 4: It is an advantage for the truths about God that are 
knowable by natural reason to be proposed to men 
to be believed on faith

IF a truth of this nature were left to the sole enquiry of reason, three disadvantages would follow. One is that the knowledge of God would be confined to few. The discovery of truth is the fruit of studious enquiry. From this very many are hindered. Some are hindered by a constitutional unfitness, their natures being ill-disposed to the acquisition of knowledge. They could never arrive by study to the highest grade of human knowledge, which consists in the knowledge of God. Others are hindered by the needs of business and the ties of the management of property. There must be in human society some men devoted to temporal affairs. These could not possibly spend time enough in the learned lessons of speculative enquiry to arrive at the highest point of human enquiry, the knowledge of God. Some again are hindered by sloth. The knowledge of the truths that reason can investigate concerning God presupposes much previous knowledge. Indeed almost the entire study of philosophy is directed to the knowledge of God. Hence, of all parts of philosophy, that part stands over to be learnt last, which consists of metaphysics dealing with points of Divinity.* Thus, only with great labour of study is it possible to arrive at the searching out of the aforesaid truth; and this labour few are willing to undergo for sheer love of knowledge. Another disadvantage is that such as did arrive at the knowledge or discovery of the aforesaid truth would take a long time over it, on account of the profundity of such truth, and the many prerequisites to the study, and also because in youth and early manhood, the soul, tossed to and fro on the waves of passion, is not fit for the study of such high truth: only in settled age does the soul become prudent and scientific, as the Philosopher says. Thus, if the only way open to the knowledge of God were the way of reason, the human race would dwell long in thick darkness of ignorance: as the knowledge of God, the best instrument for making men perfect and good, would accrue only to a few, and to those few after a considerable lapse of time.

A third disadvantage is that, owing to the infirmity of our judgement and the perturbing force of  imagination, there is some admixture of error in most of the investigations of human reason. This would be a reason to many for continuing to doubt even of the most accurate demonstrations, not perceiving the force of the demonstration, and seeing the divers judgements of divers persons who have the name of being wise men. Besides, in the midst of much demonstrated truth there is sometimes an element of error, not demonstrated but asserted on the strength of some plausible and sophistic reasoning that is taken for a demonstration. And therefore it was necessary for the real truth concerning divine things to be presented to men with fixed certainty by way of faith. Wholesome therefore is the arrangement of divine clemency, whereby things even that reason can investigate are commanded to be held on faith, so that all might easily be partakers of the knowledge of God, and that without doubt and error.* Hence it is said: Now ye walk not as the Gentiles walk in the vanity of their own notions, having the understanding darkened (Eph. iv, 17, 18); and, I will make all thy sons taught of the Lord (Isa. liv, 1, 5). 

Chapter 5:  It is an advantage for things that cannot be 
discovered by reason to be proposed as tenets of the faith

SOME may possibly think that points which reason is unable to investigate ought not to be proposed to man to believe, since Divine Wisdom provides for every being according to the measure of its nature; and therefore we must show the necessity of things even that transcend reason being proposed by God to man for his belief.

1. One proof is this. No one strives with any earnestness of desire after anything, unless it be known to him beforehand. Since, then, as will be traced out in the following pages (B.III, Chap.CXLVIII), Divine Providence directs men to a higher good than human frailty can experience in the present life, the mental faculties ought to be evoked and led onward to something higher than our reason can attain at present, learning thereby to desire something and earnestly to tend to something that transcends the entire state of the present life. And such is the special function of the Christian religion, which stands alone in its promise of spiritual and eternal goods, whereas the Old Law, carrying temporal promises, proposed few tenets that transcended the enquiry of human reason.*

2. Also another advantage is thence derived, to wit, the repression of presumption, which is the mother of error. For there are some so presumptuous of their own genius as to think that they can measure with their understanding the whole nature of the Godhead, thinking all that to be true which seems true to them, and that to be false which does not seem true to them. In order then that the human mind might be  delivered from this presumption, and attain to a modest style of enquiry after truth, it was necessary for certain things to be proposed to man from God that altogether exceeded his understanding.

3. There is also another evident advantage in this, that any knowledge, however imperfect, of the noblest objects confers a very high perfection on the soul. And therefore, though human reason cannot fully grasp truths above reason, nevertheless it is much perfected by holding such truths after some fashion at least by faith. And therefore it is said: Many things beyond the understanding of man are shown to thee (Ecclus iii, 23). And, The things that are of God, none knoweth but the Spirit of God: but to us God hath revealed them through his Spirit (I Cor. ii, 10, 11). 

Chapter 6:  It is not foolish to assent to the truths of the Faith,
even though they are beyond reason

THE Divine Wisdom, that knows all things most fully, has deigned to reveal these her secrets to men, and in proof of them has displayed works beyond the competence of all natural powers, in the wonderful cure of diseases, in the raising of the dead, and what is more wonderful still, in such inspiration of human minds as that simple and ignorant persons, filled with the gift of the Holy Ghost, have gained in an instant the height of wisdom and eloquence.* By force of the aforesaid proof, without violence of arms, without promise of pleasures, and, most wonderful thing of all, in the midst of the violence of persecutors, a countless multitude, not only of the uneducated but of the wisest men, flocked to the Christian faith, wherein doctrines are preached that transcend all human understanding, pleasures of sense are restrained, and a contempt is taught of all worldly possessions. That mortal minds should assent to such teaching is the greatest of miracles, and a manifest work of divine inspiration leading men to despise the visible and desire only invisible goods. Nor did this happen suddenly nor by chance, but by a divine disposition, as is manifest from the fact that God foretold by many oracles of His prophets that He intended to do this. The books of those prophets are still venerated amongst us, as bearing testimony to our faith. This argument is touched upon in the text: Which (salvation) having begun to be uttered by the Lord, was confirmed by them that heard him even unto us, God joining in the testimony by signs and portents and various distributions of the Holy Spirit (Heb. ii, 3, 4). This so wonderful conversion of the world to the Christian faith is so certain a sign of past miracles, that they need no further reiteration, since they appear evidently in their effects. It would be more wonderful than all other miracles, if without miraculous signs the world had been induced by simple and low-born men to believe truths so arduous, to do works so difficult, to hope for reward so high. And yet even in our times God ceases not through His saints to work miracles for the confirmation of the faith.* 

Chapter 7:  The truth of reason is not contrary to 
the truth of Christian Faith

THE natural dictates of reason must certainly be quite true: it is impossible to think of their being otherwise. Nor a gain is it permissible to believe that the tenets of faith are false, being so evidently confirmed by God.* Since therefore falsehood alone is contrary to truth, it is impossible for the truth of faith to be contrary to principles known by natural reason.

2. Whatever is put into the disciple's mind by the teacher is contained in the knowledge of the teacher, unless the teacher is teaching dishonestly, which would be a wicked thing to say of God. But the knowledge of principles naturally known is put into us by God, seeing that God Himself is the author of our nature. Therefore these principles also are contained in the Divine Wisdom. Whatever therefore is contrary to these principles is contrary to Divine Wisdom, and cannot be of God.

3. Contrary reasons fetter our intellect fast, so that it cannot proceed to the knowledge of the truth. If therefore contrary informations were sent us by God, our intellect would be thereby hindered from knowledge of the truth: but such hindrance cannot be of God.

4. What is natural cannot be changed while nature remains.* But contrary opinions cannot be in the same mind at the same time: therefore no opinion or belief is sent to man from God contrary to natural knowledge. And therefore the Apostle says: The word is near in thy heart and in thy mouth, that is, the word of faith which we preach (Rom. x, 8). But because it surpasses reason it is counted by some as contrary to reason, which cannot be. To the same effect is the authority of Augustine (Gen. ad litt. ii, 18) : " What truth reveals can nowise be contrary to the holy books either of the Old or of the New Testament." Hence the conclusion is evident, that any arguments alleged against the teachings of faith do not proceed logically from first principles of nature, principles of themselves known, and so do not amount to a demonstration; but are either probable reasons or sophistical; hence room is left for refuting them.* 

Chapter 8:  The relation of human reason to the truth of Faith

THE things of sense, from whence human reason takes its beginning of knowledge, retain in themselves some trace of imitation of God, inasmuch as they are, and are good; yet so imperfect is this trace that it proves wholly insufficient to declare the substance of God Himself. Since every agent acts to the producing of its own likeness, effects in their several ways bear some likeness to their causes: nevertheless the effect does not always attain to the perfect likeness of the agent that produces it. In regard then to knowledge of the truth of faith, which can only be thoroughly known to those who behold the substance of God, human reason stands so conditioned as to be able to argue some true likenesses to it: which likenesses however are not sufficient for any sort of demonstrative or intuitive comprehension of the aforesaid truth. Still it is useful for the human mind to exercise itself in such reasonings, however feeble, provided there be no presumptuous hope of perfect comprehension or demonstration. With this view the authority of Hilary agrees, who says (De Trinitate, ii, 10), speaking of such truth : "In this belief start, run, persist; and though I know that you will not reach the goal, still I shall congratulate you as I see you making progress. But intrude not into that sanctuary, and plunge not into the mystery of infinite truth; entertain no presumptuous hope of comprehending the height of intelligence, but understand that it is incomprehensible."

Chapter 9:  The order and mode of proceding in this work

THERE is then a twofold sort of truth in things divine for the wise man to study: one that can be attained by rational enquiry, another that transcends all the industry of reason. This truth of things divine I do not call twofold on the part of God, who is one simple Truth, but on the part of our knowledge, as our cognitive faculty has different aptitudes for the knowledge of divine things. To the declaration therefore of the first sort of truth we must proceed by demonstrative reasons that may serve to convince the adversary. But because such reasons are not forthcoming for truth of the second sort, our aim ought not to be to convince the adversary by reasons, but to refute his reasonings against the truth, which we may hope to do, since natural reason cannot be contrary to the truth of faith. The special mode of refutation to be employed against an opponent of this second sort of truth is by alleging the authority of Scripture confirmed from heaven by miracles. There are however some probable reasons available for the declaration of this truth, to the exercise and consolation of the faithful, but not to the convincing of opponents, because the mere insufficiency of such reasoning would rather confirm them in their error, they thinking that we assented to the truth of faith for reasons so weak.*

According then to the manner indicated we will bend our endeavour, first, to the manifestation of that truth which faith professes and reason searches out, alleging reasons demonstrative and probable, some of which we have gathered from the books of philosophers and saints, for the establishment of the truth and the confutation of the opponent. Then, to proceed from what is more to what is less manifest in our regard, we will pass to the manifestation of that truth which transcends reason, solving the arguments of opponents, and by probable reasons and authorities, so far as God shall enable us, declaring the truth of faith. Taking therefore the way of reason to the pursuit of truths that human reason can search out regarding God, the first consideration that meets us is of the attributes of God in Himself; secondly of the coming forth of creatures from God; thirdly of the order of creatures to God as to their last end.*