Chapter Six of Norm Geisler’s book Thomas Aquinas: An Evangelical Appraisal (Bastion Books, 2013, 2022)

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Behind the Christian belief that God has revealed himself to us in Scrip­ture (John 10:34-35) and nature (Rom. 1:19-20) is the conviction that knowl­edge about God is possible. With this the Christian as a believer is content. The Christian, however, also operates as a thinker, and so must ask the question: How do we know? Aquinas’s answer is that we know by means of basic principles of knowledge. There are certain fundamental axioms of thought by which thought is possible. For we cannot build a house of knowledge unless there is a foundation on which to erect it. Aquinas calls these foundational prin­ciples of knowing first principles.

The Importance of the First Principles

Aquinas believes that all knowledge is based on basic undeniable principles that provide the foundation for sure knowledge. Without these first principles there can be no true knowledge. As he puts it, “perfect knowledge requires cer­titude, and this is why we cannot be said to know unless we know what cannot be otherwise.”¹ That is, if there is to be certainty, then knowledge must be based ultimately on some principles about which there can be no question. This means that there can be no infinite regress in our knowledge, for “if there were an infi­nite regress in demonstrations, demonstration would be impossible, because the conclusion of any demonstration is made certain by reducing it to the first prin­ciple of demonstration.”² All knowledge, then, rests on certain undeniable first principles that we must study if we are to avoid ultimate skepticism.

The Meaning of the First Principles

In order to understand what the first principles of knowledge are, it is nec­essary to understand first what is meant by a principle. Then, the nature of first principles as “first” will be more readily understood.

The Definition of a Principle

According to Aquinas, “anything whence something proceeds in any way we call a principle.”³ That is, a principle is that from which something follows. A principle is to be distinguished from a cause, which is that from which some­thing else follows in dependence. A cause, in distinction from a principle, has diversity of substance and dependence of one on another, which is not implied in the principle. For in all kinds of causes there is always to be found between the cause and the effect a distance of perfection or of power: whereas we use the term principle even in things which have no such difference, but have only a certain order to each other; as when we say that a point is the principle of a line.⁴

So, then, there are two basic differences between a cause and a principle. First, a cause is not part of the effect, whereas a principle can be part of that which proceeds from it. Second, a principle is merely that from which some­thing follows; a cause is that from which something follows in dependence.

The Definition of a First Principle

A principle, by its very nature, is the first in its order, since all else within that order follows from it. “A first principle is, therefore, a first among firsts.”⁵ It may be first in the order of knowing, being, or becoming. That is, each of the various orders of knowledge or reality have their points of beginning; these are known as first principles if they have that irreducible premise upon which all else depends in that order. There may be other principles under this first prin­ciple, but the first principle is that from which conclusions may be drawn.⁶ Of course, a first principle “does not signify priority [in time], but origin.”⁷ It is logically (but not necessarily chronologically) prior to its sequent. It is the ulti­mate starting point from which all conclusions may be drawn in a given area of knowledge or reality. First principles are necessary constituents of all knowl­edge, but they do not supply any content of knowledge.

Kinds of First Principles

There are as many first principles as there are orders of knowledge and real­ity. Aquinas does not provide a complete list of first principles in any one place, but rather refers to the different principles by way of example.⁸ Since Aquinas is a realist, the realm of knowing is the realm of being. There is no disjunction between the rational and the real. Indeed, one cannot deny he knows reality without implying that he does. So then first principles will have both an epistemological and ontological dimension. Since a first prin­ciple is that from which everything else in its order follows, first principles of knowledge are those basic premises from which all else follows in the realm of knowing.

The most important first principles of knowledge are as follows.

The Principle of Identity. In the order of being (ontology) Aquinas states this principle in several ways: “being is being”; “every being is necessarily what it is”; “everything is identical with itself”; and “being and one are con­vertible.”⁹ Fundamentally, the principle of identity signifies the unity of things.

When this unity is applied in the order of knowing (epistemology), it takes on the form “being is intelligible.” For if it were not so, then “the human intellect is consigned to total absurdity, to the absolute inability to conceive anything whatever: every thought is unthinkable.”¹⁰

The Principle of Non-contradiction. The ontological aspect of this principle may be stated in several ways: “being is not nonbeing”; “it is necessary that being not be nonbeing”; and “it is impossible that being be nonbeing.” Epistemologically, there are at least two ways to express this principle: (1) it is impossible that contradictory statements be simultaneously true; (2) if one contradiction is true, the other is necessarily false.¹¹ Aquinas justifies this principle by pointing out that being is intelligible; nonbeing is unintelligible and whoever denies this uses it to make an intelligible statement.¹²

The Principle of Excluded Middle. This principle is the principle of either/or. Ontologically, something must either be or not be. It cannot both be and not be at the same time and in the same sense.

Epistemologically, a proposition must be either true or false. It cannot be both true and false simultaneously in the same sense. “Whatever the subject, either affirmation or negation is true,” although we do not always know which is the case. In this sense “it is merely the logical consequence of the absurdity implied in a simultaneous yes and no about the same thing con­sidered from the same aspect.”¹³

The Principle of Causality. Ontologically, the principle of causality is limited in its application to the realm of finite, contingent beings. In this regard, “everything which is capable of existing or not existing has some cause; because considered in itself it is indifferent to either alternative, and thus there has to be something else which determines it to be.” Therefore, “since there can be no process into infinity there has to be something necessary, which is the cause of all things capable of existing or not existing.”¹⁴ In general, then, causality says “everything contingent is caused.” Or “every efficient action, which is a passage from potency to act, is caused.” Since it is contingent, it is possible for it to not be, and therefore it is caused or dependent on another.¹⁵

Epistemologically, every proposition that is not self-evident depends for its truth on the truth of another. Whatever is not necessarily true is depen­dent on some other truth. Of course, not every proposition can depend for its truth on the truth of another. Hence, there must be some first, self-evi­dent principles that are simply true in themselves.

The Principle of Finality. “Every agent acts for an end.” This principle of finality (or teleology) is the one Aquinas uses to develop many of his great metaphysical and epistemological theses. In its ontological form, finality states that “being as agent is finalized.” He wrote, “I answer that it is in the nature of every act to communicate itself as far as possible.”¹⁶ To act for an end is to communicate oneself.

Epistemologically, the principle of finality takes the following forms: every proposition has an end in view; it is necessary that every proposition com­municate some meaning; and mind communicates what is intelligible.¹⁷

Priority among First Principles

All of the foregoing principles may be said to be “first” principles. The question is which principle is first among the first. As Aquinas says, “Not every principle is a first principle …. Nevertheless, it must be observed that a principle of movement may happen to be first in a genus, but not first absolutely.”¹⁸ That is, while all of the first principles are first relatively, only one can be first absolutely. Before a determination is made as to which principle is first among firsts, the qualifications for this high honor must be examined.

As Aristotle defines it, “the most certain principle of all is that about which it is impossible to be mistaken. For such a one must be the best known … and non-hypothetical.”¹⁹ He first sets down three conditions for the strongest principle. First, no one can lie or err in respect to it. Second, it is unconditional. Third, it is not acquired by demonstration or in another like manner, but it comes naturally.

It is important to note that Aquinas and Aristotle are speaking of the absolutely first principle of knowing, not of being. In this epistemological context there are two prime candidates for absolutely first principle: the prin­ciple of identity and the principle of non-contradiction.

Some contend that identity is first. Jacques Maritain insists that the prin­ciple of identity is first and that the principle of non-contradiction is only a negative form of it.²⁰ The reason usually offered in defense of the primacy of identity is that the positive must precede the negative. It is argued that first the mind has a concept of being. The first and most basic judgment, then, is that “being is being,” which is an assertion of identity.

Others object, contending that such a position confuses the order of judgments and concepts. The mind has a concept of being first; the next concept must be that of nonbeing, since that is all that is left. The first judg­ment, however, must be of the relation between being and nonbeing, which is one of contradiction and not of identity. “In short, the positive is prior to the negative only in the sense that a concept of the negative presupposes a concept of the positive; not in the sense that a positive judgment must pre­cede a negative one.”²¹

Aquinas holds that non-contradiction is first among first principles. Whatever may be said of other philosophers, scholastic or non-scholastic, it seems clear that for Aquinas (and for Aristotle before him) the principle of non-contradiction is absolutely first in the order of knowing. Aristotle writes: “It is clear, then, that such is the most certain principle of all; and what this is, we state as follows. For it is impossible that the same [attribute] should at the same time belong and not belong to the same thing, and under the same respect …. This indeed is the most certain of all the principles. Wherefore all who demonstrate argue back to this ulti­mate proposition; for by nature this is the principle of all the other axioms.”²² Aquinas agrees with this assertion: “And hence the first indemonstrable principle is, that it is not [possible] to affirm and deny at the same time; which is based on the nature of being and non-being; and on this principle all others are based.”²³

Summing up, the primacy of the principle of non-contradiction is manifest since the principles of identity and excluded middle are dependent aspects of it. For if contradictions were possible, then a thing would not have to be identical with itself (identity) nor would opposites have to be different from each other (excluded middle). The principle of causality is also reducible to the principle of non-contradiction, for on inspection of the terms it would be a contradiction to affirm that a contingent (dependent) being is uncaused (independent). Likewise, the principle of finality rests upon the principle of non-contradiction, since otherwise being could communicate something other than being; intelligence would communicate something other than the intelligible.²⁴

Finally, then, if non-contradiction is the only principle that is strictly first, why are these other principles necessary at all? Sullivan explains that “it is because they also may be called first principles, not indeed in the sense that no principle is prior to them, but in the sense that each is first as the source of that particular branch of human knowledge at the head of which it stands.”²⁵

The Necessity of First Principles: Certainty and Certitude

There could be no certainty in knowledge if there were no certain principles of knowledge on which to build. But there are two sides to this issue: the objective certainty of the principles and the subjective certitude of the knower who knows the principles.

Objective Certainty

First principles are considered certain and infallible by virtue of their very nature. In this regard the adjectives used by Aquinas to describe them are illuminating. He calls them “necessary,” “indemonstrable,” and “non-discur­sive.”

First principles are necessary and indemonstrable because an infinite regress of knowledge is impossible. “For it is impossible that there be entirely demonstration of all things; for it would proceed into infinity, so that not even in this way would there be demonstration.”²⁶ “For not every­thing can be demonstrated.” For “if everything were demonstrated, since the same thing is not demonstrated by itself but by another, there would have to be a circle in demonstrations. But this cannot be … or it would be necessary to proceed into infinity.” But, “if there were progress into infinity, there would be no demonstration; because every conclusion of demonstra­tion is rendered certain by its reduction to the first principle of demonstra­tion: which would not be if demonstration proceeded upward to infinity.” Therefore, it is evident “that not all things are demonstrable.”²⁷

Aquinas sheds further light on why an infinite regress in knowledge is impossible. “As in the intellect, when reasoning, the conclusion is compared with the principle, so in the intellect composing and dividing, the predicate is compared with the subject.” So “if our intellect were to see at once the truth of the conclusion in the principle, it would never understand by dis­cursion and reasoning.”²⁸

Since we in our present state of knowledge must think discursively, it would be impossible for us to know anything if there were an infinite regress of terms in our syllogisms. “The identity of the first object with that represented by the middle term cannot depend on an infinite series of middle terms; for no matter how far back one might go, there would still be a premise left to be proved.” Hence, “one must come at last to a premise which is self-evident: i.e., in which the identity or diversity of the two objective concepts is immediately clear to the intellect from a simple inspection of the concepts.”²⁹ “C. S. Lewis saw this clearly when he wrote: “But you cannot go on ‘explaining away’ forever: you will find that you have explained explanation itself away. You cannot go on ‘seeing through’ things forever …. It is no use trying to see through first principles. If you see through everything, then everything is transparent.  But a wholly transparent world is an invisible world. To ‘see through’ all things is the same as not to see” (The Abolition of Man, 91).

Certainty in knowledge, then, depends ultimately on propositions about whose terms there can be no question; their truth is immediately known by inspection. These are often called “analytic“ principles. Aquinas defines first principles as those “whose predicates are of the nature of the subjects prime propositions of themselves; in order that they may be known of themselves to all, the subjects and predicates must be known to all.”³⁰ And again, “the intellect is always right as regards first principles, since it is not deceived about them for the same reason that it is not deceived about what a thing is. For self-known principles are such as are known as soon as the terms are understood, from the fact that the predicate is contained in the definition of the subject.”³¹

It should be noted that although the predicate is contained in the notion of the subject, this does not mean that this is immediately obvious. It may become obvious by inspection of the terms of the premise or by a reduction of the predicate to the subject. For example, it is not necessarily obvious that every contingent being needs a cause until, upon closer inspection, it is seen that contingent means dependent and dependent beings must be dependent on something else (i.e., a cause).

In summary, the mind is always right about first principles, for they are known to be true as soon as the terms are understood, from the fact that the predicate is contained in the definition of the subject.³²

Since first principles are self-evident, there is a sense in which it is absurd to attempt a direct proof or demonstration of them. Since some people deny their validity, however, there is an indirect sense in which some attempt to prove them. This is done by showing that first principles cannot actually be denied without absurdity. Aristotle lists several arguments of this kind in defense of the first principle of non-contradiction:

1. To deny it would deprive words of their fixed meaning and render speech useless.
2. Reality of essences must be abandoned. There would be becoming without anything that becomes, flying without a bird, accidents with­out substance.
3. There would be no distinction between things. All would be one.
4. It would mean the destruction of truth, for truth and falsity would be the same.
5. It would destroy all thought, even opinion, for its affirmation would be its negation.
6. Desire and preference would be useless, for there would be no differ­ence between good and evil.
7. Everything would be equally true and false at the same time. No opin­ion would be more wrong than any other, even in degree.
8. It would make impossible all becoming, change, or motion, for all this implies a transition from one state to another, but all states would be the same, if contradiction is not true.³³

Despite the weighty defense given to this important principle, there are those who persist in denying its rational justifiability. Some insist that to defend the principle of non-contradiction by using the principle of non-con­tradiction is to argue in a circle. It is to argue against a position that denies contradiction by saying it is contradictory. But this is really to say nothing. It is simply saying that a denial of the principle of non-contradiction is con­tradictory.

But this objection confuses the issue. For the law of non-contradiction is not used as the basis of the indirect proof of its validity; it is simply used in the process of defending its validity. Take, for example, the statement “I can­not speak a word in English.” This statement is self-destructive, since it does what it says it cannot do. It uses English to deny that it can use English. So it disproves itself. The indirect proof for the law of non-contradiction is simi­lar. We cannot deny the law of non-contradiction without using it in the very sentence that denies it. For the sentence that denies non-contradiction is offered as a non-contradictory sentence. If it is not, then it makes no sense.

In like manner, if I say “I can utter a word in English,” it is obvious that I uttered a word in English in the process of doing so. But there is nothing self-defeating about using English to say I can use English. There is only something self-defeating about using English to deny I can use English. Likewise, there is nothing wrong with using the principle of non-contradic­tion to defend the principle of non-contradiction. There is only something wrong about using the principle of non-contradiction to deny that principle.

The basic laws of thought are self-evidently true. They are known to be such by inspecting their terms to see if the predicate is reducible to the sub­ject. The only direct “proof” of them is to state clearly their meaning, so that their self-evident nature becomes intuitively or immediately obvious.

First principles, then, are not only indemonstrable but are actually unde­niable. That is, they are objectively certain, regardless of the subjective cer­tainty we may have (or may not have) about them.

Subjective Certitude

Certainty and certitude are different. Certainty is objective while certitude is subjective. A principle is certain if it is self-evident, whether a person is sure about it or not. Certitude, on the other hand, involves a knower’s assent to that which is certain; it is a subjective response to what is objec­tively so.

According to Aquinas, once the terms of first principles are known the mind must assent. Assent to first principles, however, is not “free.” “Now the assent of science is not subject to free choice, because the knower is obliged to assent by the force of the demonstration.”³⁴ Assent is “natural“: “If, therefore, that which the reason apprehends is such that it naturally assents thereto, e.g., first principles, it is not in our power to assent to it or to dissent. For in such cases, assent follows naturally, and consequently, properly speaking is not subject to our command.”³⁵ In fact, Aquinas even speaks of a natural, unconscious inclination of the intellect to truth, a kind of “instinct for truth.” For “truth is the intellect’s good and the term of its natural ordination; and just as things without knowledge are moved toward their end without knowing it, so sometimes does the human intellect by a natural inclination tend toward truth although it does not perceive its nature.”³⁶

The reason for this natural predisposition to truth is as follows: “Every­thing that comes from God receives from Him a certain nature by which it is related to its final end.” But “it is also true that every power has a natural appetite … with respect to its proper good … and with the exception of the will all the powers of the soul are necessitated by the objects.”³⁷

Certitude about first principles, then, comes from the fact that the intel­lect is naturally determined to truth and we are not free to dissent. The rea­son the mind must assent to first principles is that they are reducible to the principle of non-contradiction to which the mind cannot dissent. “The first of all assents is stated as a modal proposition; on the other hand, all other immediate assents imply this first assent, and mediate assents can exist only insofar as they can be reduced to the principle of noncontradiction.”³⁸

Certainty is always accompanied by assent. That is, the mind always assents to propositions that are certain, provided that it understands them. Not all assent, however, is accompanied by certitude. We may assent to something as being only probable and not necessary, as is the case in our everyday life. “In the business affairs of men, there is no such thing as demonstrative and infallible proof, and we must be content with a certain conjectural probability, such as that which an orator employs to persuade.”³⁹ We may also assent to the conclusion of a probable induction but not with certitude. “For he who proceeds inductively through singulars to a universal, does not demonstrate, nor does he construct a necessarily conclusive syllogism.”⁴⁰

There are different kinds of certainty, which can be summarized as fol­lows: “Assent with intellectual certitude is threefold: (a) metaphysical, wherein there is absolutely no possibility for the truth of the opposite; (b) physical; and (c) moral, wherein there is a remote possibility for the truth of the contrary, but we have no sufficient reason to think this possibility will be fulfilled in the situation at hand.”⁴¹ Further, “a man can possess intellectual certitude about a proposition and still fail to possess subjective or emotional certitude. He can emotionally fear the opposite, even though he cannot think the opposite to be a possibility.” For example, “a man can be abso­lutely certain that God exists and still feel His absence. Subjective certitude often works in the opposite direction as well. A feeling of conviction can so invade the rational powers that the will moves the intellect to assent where there is no sufficient evidence or where there is no real evidence at all.”⁴²

Aquinas even acknowledges that a person may have certitude that some­thing is true when in fact it is not. “Now certitude of adherence does not belong properly to the act of faith; first because it also belongs to the intel­lectual virtues of science, wisdom and understanding; then because it is common to both true and false faith … for men do not adhere any less firmly to truth than to falsity.”⁴³ It should be carefully noted, however, that Aquinas is not speaking here of intellectual certitude about first principles, about which there can be no error, but about matters of faith that are not rationally demonstrable. He is speaking of “certitude of adherence” or the tenacity of belief, not the veracity of it.

Nevertheless, Aquinas does admit that “error seems to be even more nat­ural to men as they actually are than knowledge because men are easily deceived and because the soul is longer in error than in truth during its life.”⁴⁴ Frederick Wilhelmsen lists the causes of error as follows: “Diseased sensation, incomplete consciousness resulting from a lack of union between intellect and sensation, the drive of the will toward the good, the necessity to act without compelling evidence in the practical order—these are the causes of error.”⁴⁵ But whatever the causes of error, it is not a question of error with regard to first principles.

Although Aquinas speaks of an unconscious “natural inclination” to the truth, properly speaking the assent to certitude is a conscious activity. We can be certain if we know that the truth is a first principle or reducible to it, which necessitates a rational awareness. Accordingly, Aquinas makes it a pre­requisite of assent that the relationship between subject and predicate be perceived as necessary:

Sometimes, again, the possible intellect is so determined that it adheres to one member without reservation. This happens sometimes because of the intelli­gible object and sometimes because of the will. Furthermore, the intelligible object sometimes acts immediately, sometimes mediately. It acts immediately when the truth of the proposition is unmistakably clear to the intellect from the intelligible objects themselves. This is the state of one who understands principles, which are known as soon as the terms are known …. In this case the very nature of the terms immediately determines the intellect to proposi­tions of this sort.⁴⁶

So if we “understand” the principle and when the truth is “unmistakably clear” to the mind assent is necessitated and certitude is guaranteed. “Assent is a conscious discernment and commitment to the truth … assent is the mind’s ratification of the proposition formed.”⁴⁷

But if assent is always conscious, why does Aquinas refer to an “uncon­scious” inclination of a subject or agent to its divinely appointed object or end?⁴⁸ This point will be discussed more fully later, but here it might be observed that there is a difference between a natural “inclination” to truth, which may at times be unconscious, and a conscious “assent” to truth. It is when we consciously reflect on this natural inclination and the necessary nature of the proposition that assent comes “naturally,” albeit consciously.

Since certitude involves a conscious assent to the certainty of the truth for which we have an unconscious appetite, then the possession of this truth by the intellect is the reward of certitude. In short, “to reflection, certitude appears as the repose in the possession of this good, a sort of partial beati­tude of the intellect caused by the presence of truths, of which nothing in the world could ever deprive it.”⁴⁹ The reward for the hunger for truth is the meat of certitude that we consciously enjoy when we perceive the cer­tainty and necessity of the truth possessed.

The Ontological Basis of First Principles

Itis because first principles are self-evident and analytic that they are unde­niable. For Aquinas, however, analytic does not necessarily mean a priori or independent of experience. First principles are known because the mind knows reality. In fact, these epistemological principles have an ontological basis in reality. For “a system of valid philosophy cannot be devel­oped from a priori principles alone by pure deduction, as Spinoza tried to do. Trust must be attained by a combination of principles, sense judgments, and the conclusions derived by reasoning from either or both.”⁵⁰

For Aquinas it is sufficient that we know being (or that we know that we know being) and that in reality our knowledge of first principles is based in our most fundamental knowledge of being. He sees no need to justify this knowledge any more than we could directly demonstrate a first principle.

For thought is based in thing. That is, knowing is based in being. This does not mean, however, that there is a direct one-to-one cor­respondence between the thing-in-itself and the thing-as-known. In two clear passages, Aquinas spells out his view on this subject. “Whence, to the composition and division of the intellect there corresponds something on the part of reality; nevertheless, it is constituted in reality in a manner dif­ferent from that in which it is in the intellect. For the proper object of the human intellect is the whatness (quiddity) of the material thing which falls under [the perception of] sense and imagination …. Nevertheless, the com­position by the intellect differs from the composition in reality. For the things which are combined in reality are diverse; whereas the composition of the intellect is a sign of the identity of the things which are combined.”⁵¹ “So, since the true is in the intellect in so far as the intellect is conformed to the thing understood, the aspect of the true must needs pass from the intel­lect to the thing understood, so that also the thing understood is said to be true in so far as it has some relation to the intellect.”⁵²

According to Aquinas, being is the beginning of knowing. It is a given, undemonstrable fact that ultimately thought is based in thing and that what the rational knows is the real. This is why Aquinas sees no need to elaborate his epistemology before his natural theology. He begins with a knowledge of finite being and on that builds his knowledge of Infinite Being. Further­more, as Eric Mascall pointedly observes, it is as unnecessary to expound one’s epistemology before beginning to talk about God as it is to under­stand human physiology before beginning to walk.⁵³

Aquinas does not ask how we know that we know reality. It is obvious that we do. That is a question philosophers have asked since the time of Kant. Aquinas’s followers respond in two basic ways to this question. There is the school of Neo-Thomists, like Cardinal Mercier, Leon Noel, G. Picard, and others at the University of Louvain, known as critical realists. They claim that although we are certain that there is a world of things beyond our thought, nevertheless we must subject this certainty to criticism and establish the truth that we really know being. They contend that even though the mind begins with things, in philosophy we must begin with the mind’s knowledge of things and not the things themselves. Epistemology is a critique and a critique is logically prior to a metaphysics.⁵⁴

Other Thomists feel that post-Kantian philosophy has made a gratuitous assumption in denying what is most obvious to the realist, namely, that it is something that we know and not nothing. Wilhelmsen, for example, says: “A Thomistic realist has no need to follow the fortunes of critical realism as it attempts to ‘bridge the gap’ between the mind and things. The Thomist refuses to admit any gap between mind and things. Therefore, he refuses to build a bridge where there is no need for one. He refuses to separate sense knowledge from intellectual knowledge because he finds them together, not separate.”⁵⁵

It is argued that to justify our knowledge of reality by anything other than reality itself is to base knowledge on non-reality. “But if the truth about being is not strong enough to act as the foundation of philosophy, if it is not evident, if it needs a prior truth to guarantee it, then the absolutely first principle of philosophy will not be the truth about being; it will be some other truth.”⁵⁶ Wilhelmsen continues: “It follows, therefore, that a ‘critical realism‘ is a contradiction in terms. Either the truth that ‘being exists’ is first among all evident truths, or it is not. If it is not, then realism is not realism …. The ‘critical’ swallows up the ‘realist’ in a philosophical comedy in which metaphysicians attempt to justify that what they say lies beyond all need of justification.”⁵⁷

As a realist, Aquinas himself would respond to the question of how we know first principles are based in reality by noting that it is undeniable. For one cannot know about reality that he cannot know anything about reality, unless he does know something about reality. For Aquinas first principles of all kinds, whether logical or ontological, are not directly demonstrable, but they are indirectly undeniable. If they were capable of being proved, they would not be first principles. The only possible way to defend them is indirectly, by showing that it is self-defeating to deny them, or better, by indicating that every denial that we can know reality itself presupposes and affirms a knowl­edge of reality. “Just as the first principles are indemonstrable insofar as they are first, so also any direct demonstration of the ontological validity of first principles is impossible.”⁵⁸

Garrigou-Lagrange argues that “this ontological validity cannot be demonstrated by a direct method, for, like the necessity of first principles, it is an immediately evident truth. The immediately connected subject and predicate do not admit of a demonstrative middle term.” Hence, “all that we can do is to explain the meaning of the subject and the predicate … for an attempt at demonstration would result merely in a vicious circle, since one would have to assume as true what remains to be proved, to wit, the ontological validity of first principles.”⁵⁹ First principles may not be directly demonstrable, but they are actually undeniable. And it is a first principle that being is that which is, and that which is can be known.

The Epistemological Origin of First Principles

In some ways this discussion is logically prior to the preceding, since we can­not discuss what we do not in some way possess. It is also true to the order of Aquinas, however, to discuss what we know before we know exactly how we know it. In a very clear and complete passage, Aquinas explains the whole psychological process of the origin of our knowledge of first princi­ples:

So inquiry in all the speculative sciences works back to something first given, which one does not have to learn or discover (otherwise we would have to go on to infinity), but which he knows naturally. Such are the indemonstrable principles of demonstration (for example, every whole is greater than its part, and the like), to which all demonstrations in the sciences are reducible. Such too are the first conceptions of the intellect (for example, being, one, and the like), to which all definitions in the sciences must be reduced. From this it is clear that the only things we can know in the speculative sciences, either through demonstration or definition, are those that lie within the range of these naturally known principles. Now these principles are revealed to man by the light of the agent intellect, which is something natural to him …. So our knowledge of the above-mentioned principles begins in the senses and mem­ory, as is evident from the philosopher [Aristotle, Posterior analytics, II, 19, 100a 3-9]. Consequently, these principles do not carry us beyond that which we can know from the objects grasped by the senses.⁶⁰

All Knowledge Begins in Sensation

All knowledge begins in sensation, but the mind’s ability to know is logically prior to sensation. That is, everything that is in the mind was first in the senses, except the mind itself. “But the human intellect … is in potentiality with regard to things intelligible, and is at first like a clean tablet on which nothing is written, as the philosopher⁶¹ says.”⁶² Again, “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.”⁶³ Nevertheless, the mind has an innate, a priori capacity to know, otherwise it would be impossible for it to know even first principles. This ability rests in what Aquinas calls agent (active) intellect.

The Need for the Agent Intellect

In order to arise from sensation, which provides as such only a knowledge of singulars, to a knowledge of universal principles, it is necessary to have the action of something that is superior to sensation as the universal is superior to the individual. Such is the agent intellect. “Truth is not entirely from the senses. For the light of the agent intellect is needed, through which we know the truth of changeable things unchangeably, and discern things themselves from their likeness.”⁶⁴ Aquinas further describes the role of the agent intellect as follows: “The possible intellect cannot have actual knowl­edge of principles except through the active intellect. For the knowledge of principles is received from sensible things …. But intelligibles cannot be received from sensible things except through the abstraction of the active intellect.”⁶⁵

In summary, then, the whole process of knowing first principles is this: “For the first principles become known by the natural light of the active intellect itself; they are not acquired through reasoning, but only through this … that their terms become known. Which comes about by this [pro­cess]: memory is taken from sensible things, and experience is taken from memory, and knowledge of those terms is derived from experience; and when these [terms] are known, common propositions of this kind become known which are the principles of the arts and sciences.”⁶⁶

First Principles Arise from Judgments, Not Apprehensions

It should not be thought that the mind apprehends (first act of the intellect) first principles by abstracting their nature from sensible things. Properly speaking, first principles, as well as other knowledge, are to be found in judg­ments (second act of the intellect). “The human intellect must of necessity understand by composition and division [judgment].” It “does not acquire perfect knowledge of a thing by the first apprehension; but it first apprehends something of the thing, such as the quiddity, which is the first and proper object of the intellect; and then it understands the properties, accidents, and various dispositions affecting the essence. Thus it necessarily relates one thing with another by composition or division; and from one composition and divi­sion it necessarily proceeds to another, and this is reasoning.”⁶⁷

“For the perfection of the intellect is truth as known. Therefore, properly speaking, truth resides in the intellect composing and dividing; and not in the sense, nor in the intellect knowing what a thing is. But the intellect can know its own conformity with the intelligible thing; yet it does not apprehend it by knowing of a thing what a thing is. When, however, it judges that a thing corresponds to the form which it apprehends about that thing, then it first knows and expresses truth. This it does by composing and dividing.”⁶⁸

The intellectual knowledge of first principles rests in judgment made about the objects of sensible knowledge. First, by way of sensation and abstraction the intellect apprehends that things are and something of what they are. Then, by way of judgment the mind knows that being is and non-being is not. From these judgments arises the first principle of knowledge that “being is not nonbeing,” which is the principle of non-contradiction.

An Epistemological Problem

Out of a study of first principles, their certainty, and our certitude of them, there are posed several problems for a Thomistic theory of knowledge. Three of them will be briefly considered here.

How Can First Principles Be Known from Sensation?

“It may be objected that according to Aristotle [Posterior analytics, I, c. 13], the universal judgment from which all deductions proceed can be known only by an induction. And even St. Thomas might be thought to support this view [Posterior analytics, I, lect. 29].”⁶⁹ This is the problem that was later accentuated by Hume and the empiricists: if all knowledge begins in sensation, if the mind is a tabula rasa at birth, if we have no innate ideas, then how can our knowledge ever rise above the flux of empirical experi­ence? Only flux can come from flux!

The answer, of course, is the agent intellect, which enables us to get more out of the data of sense than sense data. “In every man there is a certain principle of knowledge, namely the light of the agent intellect, through which certain universal principles of all the sciences are naturally understood as soon as proposed to the intellect.”⁷⁰ Although all knowledge begins in sensation, it is not limited to sensible knowledge.⁷¹ What the mind knows by sensation is not merely its sensations but the essence of things in and through the sensations by the activity of the agent intellect. In fact, “there is nothing in the intellect that has not first been in sense, except intellect itself.”⁷²

In What Sense are First Principles “Natural” Knowledge?

Aquinas refers to the knowledge of first principles as “natural“ and “not by acquisition.”⁷³ Yet if these general principles must be known first in order to know anything at all, and if ultimately they find their origin, like all knowl­edge, in the senses, are we not arguing in a vicious circle? Are we not driven to accept either the Platonic theory of innate knowledge or Locke’s argu­ment that the principles are not known first at all and do not serve as the foundation of any other knowledge? As a matter of fact, Aquinas even uses the word “innate” of first principles. “The first principles of which we have innate cognition are certain likenesses of uncreated truth. When we judge about other things through these likenesses, we are said to judge about things through unchangeable principles or through uncreated truth.”⁷⁴

Elsewhere, first principles are said to be acquired by a “natural habit,”⁷⁵ a “natural inclination,”⁷⁶ as “divinely instilled in us by God.”⁷⁷ We are said to have natural knowledge of first principles⁷⁸ or an understanding of first principles that “follows human nature itself “⁷⁹ Aquinas speaks of the mind being “naturally endowed” with principles “not known by investigation” but which are “bestowed on us by nature.”⁸⁰

All of this stress on “natural“ and “innate” would seem to be quite con­trary to the clear assertion that all knowledge begins in the senses. How can it be both a posteriori and yet innate? Gilson answers this problem by noting that “these pre-formed germs of which we have natural knowledge are the first principles …. To say that they preexist, does not mean that the intellect possesses them actually, independently of the action which bodies exercise on our soul; it simply means that they are the first intelligibles which our intellect can reach in starting from sensible experience. The intellection of these principles is no more innate than the conclusions of deductive argu­ments, but whereas we discover the former naturally, we have to reach the latter by an effort of search.”⁸¹

Other commentators are not so sure that this is all that Aquinas means by these statements on our natural inclination to truth. Sullivan suggests that “innate” and “natural” knowledge may also mean that first principles are regu­lative principles in all intellectual processes, and that they are in the mind when it begins to act.⁸² They do not precede consciousness but they are there in our nature when we begin to act and have God as their Exemplar Cause.

What Is the Precondition for Knowing First Principles?

The answer to the first two epistemological problems has given rise to a third, namely, if it is necessary to posit the agent intellect and a natural inclination to truth to account for the certitude about first principles, then how can we account for the agent intellect’s ability to recognize first principles or for our natural appetite for truth? Is this saying that the intellect unconsciously uses first principles (from the moment it begins to act) to come to a conscious knowledge of first principles? That is, are first principles the very categories of thought (to borrow a Kantian term) that are impressed upon our nature by God and with which we are able to think about things? There is a sense in which Aquinas gives an affirmative answer to this question. For first principles are the very structure of the rational by which the real is known and hence the intellect by virtue of its very nature is predisposed to truth.

Sullivan seems to make this point: “The person who makes the sense judgment is not explicitly aware of these principles at the time, but they may be easily and instantly elicited from him by questioning. Hence he may be said to possess principles virtually or habitually, from the beginning of his cognitive life. This is what is meant by saying that first principles ‘come by nature, and are known naturally.”⁸³

If we take seriously what Aquinas says about “natural inclination” he would seem to be holding a form of realism, somewhere between an intu­itionism and a pure empiricism. He says, for example, “to say that a natural inclination is not well regulated, is to derogate from the author of nature …. In the same way, the truth of natural knowledge is of one kind, and the truth of infused or acquired knowledge is of another.”⁸⁴

Natural knowledge is neither “infused” a priori nor “acquired” a poste­riori. It is known naturally because we have the natural capacity or “form” for it. Aquinas defines natural inclination this way: “Each power of the soul is a form of nature, and has a natural inclination to something. Hence, each power desires, by natural appetite, that object which is suitable to itself.”⁸⁵

Furthermore, this inclination is not voluntary but is impressed upon its very nature by God. “Every inclination of anything, whether natural or vol­untary, is nothing but an impulse received from the archer. Hence, every agent, whether natural or voluntary, attains to its divinely appointed end, as though of its own accord.”⁸⁶ It is described as a “natural appetite,” which again shows that it is part of our very nature to tend to the truth of first principles. “Natural appetite is that inclination which each thing has, of its own nature, for something; wherefore by its natural appetite each power desires what is suitable to itself.”⁸⁷

Aquinas makes a significant contribution to epistemology. By a unique syn­thesis, he unites both the a priori and a posteriori elements of knowledge. Humans have an innate, natural capacity or form for the truth of first prin­ciples that is ingrained into their very nature by God. They have first prin­ciples in a kind of virtual and natural way as a precondition of all cognitive activity. And when this innate capacity is filled with the content of sense experience, we are able by conscious reflection to come to a knowledge of the very first principles, which as a fundamental part of our nature, enable us to have a consciousness of them.⁸⁸ That is to say, we can only know first principles if we are exercising first principles to know them, otherwise, we would have no means by which they could be known. We have them by way of operation before we know them by way of consciousness.

Notes

1. Thomas Aquinas, Peri Hermeneias, I, lect. 8.
2. Thomas Aquinas, Commentary on the Metaphysics of Aristotle, trans. John P. Rowan (Chicago: Henry Regnery, 1961), 244.
3. Thomas Aquinas, Summa Theologiae, la. 33, 1.
4. Ibid.
5. L. M. Regis, Epistemology (New York: Macmillan, 1946), 378.
6. T. Aquinas, Summa Theologiae, la2ae. 66, 5, ad 4.
7. Ibid., la. 33, ad 3.
8. James Bacon Sullivan, An Examination of First Principles in Thought and Being in the Light of Aristotle and Aquinas (Washington, D.C.: Catholic University of America Press, 1939), p. 235.
9. Thomas Aquinas, Metaphysics, V, lect. 11, n. 912; WI, leer. 17, n. 1652; Sent., I, 19, 1, 1, ad 2.
10. Regis, Epistemology, 395.
11. Ibid., 388-89.
12. T. Aquinas, Metaphysics, IV, lects. 6-17; XI, 5-7.
13. T. Aquinas, Peri Hermeneias, I, lect. 11, n. 5.
14. Thomas Aquinas, Summa contra Gentiles, II, 15.
15. Aquinas never refers to the so-called principle of sufficient reason. According to some scholastic philosophers, however, he uses it in his “Third Way” when he argues from contingency to necessity (Sullivan, Examination of First Principles, p. 80). Others say that it is an epistemological aspect of the principle of contradiction. Regis, for example, says: “But the principle of raison d’etre, or sufficient reason, is nothing but the thing’s capacity to account for itself to human reason . . to be intelligible and understood …. However, the unintelligible has no raison d’etre, guarantees nothing, therefore is the foundation for no truth, however small” (Epistemology, p. 390). Leibnitz originated the phrase “sufficient reason” in his Theodicae (I, n. 44) and defended it, when challenged, in his Fifth Letter to Mr. Samuel Clarke (n. 125). Sullivan considers it to be broader than the principle of con­tradiction since the latter applies only to finite contingent being whereas “sufficient reason” applies to God. It says only that “everything must have a sufficient reason” (whether in itself or another). Causality, on the other hand, demands that every contingent being does not have a sufficient reason for its existence in itself but must find its cause in another. Sufficient reason, says Sullivan, tells us that there is a reason; causality shows us where it is (Examination of First Principles, pp. 80-83).
16. Thomas Aquinas, De potentia, II, I. c; Sent., II, 34, 1, 3.
17. Regis, Epistemology, p. 402.
18. T. Aquinas, Summa Theologiae, la2ae. 6, 1, ad 1.
19. T. Aquinas, Metaphysics, IV, c. 3, 10056.
20. See his Elements of Philosophy, vol 2.
21. Sullivan, Examination of First Principles, 58.
22. T. Aquinas, Metaphysics, IV, c. (1005b).
23. T. Aquinas, Summa Theologiae, I a2ae. 94, 2.
24. According to Sullivan, those scholastic philosophers who contend for the primacy of identity over non-contradiction “use quotations from St. Thomas favoring the primacy of the principle of Contradiction to confirm their contentions, and maintain that it is the same thing as the principle of Identity” (Examination of First Principles, p. 56). Sullivan traces the influence of the primacy of identity among philosophers to Sir William Hamilton (Lectures on Logic [1886]).

Regis observes that since Kant and the rule of the mathematic method, identity has had primacy. This is an exaltation rooted in a philosophical idealism that denies the concrete and makes existential being the starting point for philosophical reflection (Epistemology, p. 391). Along this same line Sullivan notes that even Descartes‘ cogito ergo sum depends on noncontradiction for its certainty because “consciousness can be doubted as well as any other source of knowledge. Moreover, even the judgment ‘I think’ presupposes the principle of Contradiction, for in making it one presupposes that one cannot at that time be not thinking; if one could, this judgment would be useless as a principle of further knowledge” (Examination of First Principles, p. 40).

• Ibid., 96.
• T. Aquinas, Metaphysics, IV,c.4 (1006a).
• Ibid., IV, lect. 2.
• T. Aquinas, Summa Theologiae, 1a58, 4.
• Sullivan, Examination of First Principles, 26.
• T. Aquinas, Metaphysics, XI, lect. 4.
• T. Aquinas, Summa Theologiae, la. 17, 3, ad 2.
• See Regis, Epistemology, 374.
• See Sullivan, Examination of First Principles, 121-22.
• T. Aquinas, Summa theolagiae, la2ae. 2, 10, ad 2.
• Ibid., la2ae. 17, 7.
• T. Aquinas, Physics, lect. 10, n. 5.
• T. Aquinas, Sent., III, 27, 1, 2, c.
• T. Aquinas, Peri Hermeneias, I, lect. 8, nn. 11, 15.
• T. Aquinas, Summa Theologiae, la2ae. 105, 2, ad 8.
• Cardinal John Newman speaks of a series of converging probabilities that can yield certainty and demand assent (Grammar of Assent [London: Longmans, Green, 1987], 72), butAquinas does not speak of rational certitude in this sense.
• Frederick D. Wilhelmsen, Man’s Knowledge of Reality: An Introduction to Thomistic Epistemology (Englewood Cliffs, N.J.: Prentice-Hall, 1956), 171.
• Ibid., pp. 172-73.
• T. Aquinas, Peri Hermeneias, I, lect. 8, nn. 8,19-21.
• T. Aquinas, De anima, III, lect. 4, n. 624; cf. Summa contra Gentiles, I, 4.
• Wilhelmsen, Man’s Knowledge of Reality, 179.
• T. Aquinas, De veritate, XIV, 1, c.
• Wilhelmsen, Man’s Knowledge of Reality, 157.
• T. Aquinas, Physics, lect. 10, n. 5.
• Wilhelmsen, Man’s Knowledge of Reality, 419.
• Sullivan, Examination of First Principles,30.
• T. Aquinas, Summa Theologiae, la. 85, 5, ad 3.
• Ibid., la. 16, 1.
• Thomas Aquinas, Existence and Analogy (New York: Longmans, 1945), 45.
• See Fernand Van Steenberghen, Epistemology (New York: Wagner, 1949), for further elaboration of this view.
• Wilhelmsen, Man’s Knowledge of Reality, 40.
• Ibid.
• Ibid., 41.
• Sullivan, Examination of First Principles, 98.
• R. Garrigou-Lagrange, God: His Existence and His Nature (St. Louis, Mo.: Herder, 1945), vol. 1, 117.
• T. Aquinas, De Trinitate, V, 4, c.
• Aristotle, De anima, III, 4 (430a 1).
• T. Aquinas, Summa theologise, la. 79,2.
• Ibid., la. 84, 8, ad 1.
• Ibid., la. 84, 6, ad 1.
• Aristotle, De anima, III, 4, ad 6.
• T. Aquinas, Metaphysics, IV, lect. 6, c 599.
• T. Aquinas, Summa Theologiae, la. 85, 5.
• Ibid., la. 16, 2.
• Sullivan, Examination of First Principles, p. 21.
• Regis, Epistemology, 376.
• T. Aquinas, Summa Theologiae, la. 84, 6, ad 1.
• Etienne Gilson, The Unity of Philosophical Experience (New York: Charles Scribner’s Sons, 1965), 308.
• T. Aquinas, Metaphysics, IV, lect. 6, 2 (476B).
• T. Aquinas, De veritate, 10, 6, ad 6.
• T. Aquinas, Summa Theologiae, la2ae. 51, 1.
• Ibid., la. 103, 8; 80, 1, ad 3; 77, 3; 78, 1, ad 3.
• T. Aquinas, Summa contra Gentiles, I, 7; III, 47; Summa Theologiae, I, 105, 3.
• T. Aquinas, Commentary on the Book of Job, lect. 3.
• T. Aquinas, Summa Theologiae, la2ae. 5, 4, ad 3.
• Ibid., la. 79, 12.
• Etienne Gilson, The Philosophy of St. Thomas Aquinas (St. Louis, Mo.: Herder, 1937), 246.
• Sullivan, Examination of First Principles, 136.
• Ibid., 33.
• T. Aquinas, Summa Theologiae, la. 60, 1, ad 3.
• Ibid., la. 80, 1, ad 3; cf. 77, 3; 78, 1, ad 3.
• Ibid., la. 103, 8.
• Ibid., la. 78, 1, ad 3.