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Why Reason Demands Absolute Certitudes

The concept of certitude itself is not very popular today. Most skeptics, agnostics, and atheists view natural science as providing the surest available rational knowledge, and yet, because of this very fact, view all knowledge, at best, to be a matter of very high degrees of probability – never of absolute certitude.

The inherent epistemological limitation of natural science is its inductive method, since observation of particular events can never produce universal certitudes – as famously argued by the Scottish skeptic, David Hume. Thus, those thinkers who claim to attain absolute certitude today tend to be viewed as being epistemologically naïve.

I aim here, not to demonstrate all possible absolute certitudes, nor even the most important conclusions of speculative philosophy, such as God’s existence or the human soul’s spirituality and immortality. Rather, I propose to show that (1) some cognitive starting points of philosophy entail certitudes, and (2) the proper use of reason necessarily implies some absolute certitudes about reality, which constitute universal metaphysical first principles.

What is certitude? Formal certitude is the firm assent of the mind to a proposition together with clear knowledge that the evidence for the assent excludes error and the possibility of error. Such knowledge will be made evident in the examples that follow.

Scio Aliquid Esse

The seventeenth century French philosopher, René Descartes, insisted that what we first know is expressed as Cogito, ergo sum; I think, therefore, I am.” In so doing, he recognized that, in the act of knowing, there is reflexive consciousness of the self as an existing knower. But what Descartes missed is that in every perceptive act of knowing – the kind first experienced in sensation – what is immediately known is given as an extramental object.1

The equally French contemporary Thomistic philosopher, Jacques Maritain, corrects Descartes’ omission by restating the initial proposition as “Scio aliquid esse; I know something to be.”2 In so saying, he affirms what is first and primarily known is something presented to the knower as an extramental sense object. It is solely in knowing such an object that I become conscious of my own act of knowing – and thereby, reflexively, of myself as the knower. In fact, direct experience tells us that both intramental and extramental objects are known clearly and distinctly, while they are also known as radically distinct from each other.3

Contemporary naturalists would argue that what is known is not directly something in the physical world, but rather some sort of representation inside the brain, presumably caused by an external object. And yet, to know that some internal image constitutes a representation of an external object would require somehow knowing both terms of that relationship. That logically requires directly knowing the external object at some point.

In fact, that is how we developed the scientific explanation of perception – by first knowing the extramental physical world, and from studying it, coming to the conclusion that sensation begins in an external stimulus caused by external objects and tracing the causal sequence from the external sense organs along nerve pathways to the inside of the brain, where, it is assumed, an image is somehow formed and known.

Of course, were it actually true that all the mind knows is the internal images of the brain, then the whole of science would be about images in the brain – in which case science would tell us nothing at all about an extramental physical world.

Still, prescinding from the question of whether what we directly know is some external object or merely an image inside the brain, it is immediately evident that what we know is not thought itself, but thought about something that is real in some way. Scio aliquid esse: I know something to be.4

This immediate experience is sufficient to give us absolute certitude that something is known to exist – regardless of its exact ontological status.

Even skeptics pay tribute to the realization that true knowledge consists in conforming the mind to reality. Error arises when what is known by the self does not conform to the really existing thing. Doubt arises when we fear that what we know may not conform to reality itself. But, in the act of perception, there can be no lack of union between the knower and the known, or else, no knowledge at all would occur. Knowledge actually occurs solely when there is union of knower and thing known.

This is why we cannot doubt the immediately known contents of perception.5 As children, we never doubted that the physical things around us were anything but real, since they are given to us as external in sensation – except for such internally-experienced entities as images or emotions. The external world around us is the primary given of sense experience, intellectually judged as such.

But Descartes rejected this whole world of things as his starting point. Rather, he took thought itself as his immediate knowledge – even forcing himself by convoluted reasoning to prove that extramental things exist, only after proving to himself God’s existence! Small wonder that Descartes’ inquiry, which starts with a subjective intramental starting point, namely, thought itself, inevitably led to subjective idealism – the denial that the mind can reach objective extramental reality.

Realist epistemology insists that Descartes made the fundamental blunder of thinking that the mind’s primary object is its own thought. Rather, thought is always of something other than the act of knowing itself. For this reason, we naturally distinguish between knowing a cow and knowing an image of a cow. The image is always secondary to direct experience of the extramental object.

Still, whether Descartes is right or wrong, what is absolutely certain is that in any act of knowing, what is immediately given is something existing in some way. That is sufficient for our purposes.

The Rules of Reason

No debate about reality’s nature can escape the use of reason. Defenders of naturalism and skeptics alike are bound by the rules of reason. One need not be a professional logician to know that (1) he cannot contradict himself and that (2) claims need to be supported in some fashion.

To say that contradictions are not permitted means that the same predicate cannot be both affirmed and denied of the same subject. I cannot simultaneously say that the moon is made of green cheese and that it is not, when speaking either of the whole moon or of its exact same part.

Even so-called “paraconsistent logics” are merely ones that try to maintain some limited coherence while ignoring or side-stepping such inconsistencies by denying the so-called “principle of explosion” that says that “if contradictories are true, then any statement is also true.” Yet, it is precisely because admission of any contradictories does lead to all statements being true that no exceptions to the principle of explosion can be admitted – ever. Or, as has been pointed out, in paraconsistent logic, “negation” is not really negation, but merely a subcontrary-forming operator.

Obviously, statements become unintelligible if they are ever permitted to mean the contradictory of what they say! Thus, the intelligibility of every possible proposition requires that its content be affirmed and not denied, thereby making the governing rule here absolutely universal.

Nor is it merely an axiom to forbid contradictions, since it is impossible even to posit an axiom without affirming what is proposed and denying its contradictory – and that includes positing the principle of non-contradiction itself. Thus, even to posit ¬(p ∧ ¬p) as an axiom is to presuppose that its contradictory is false, and hence, is to presuppose the very principle being posited.

To say that claims need support means that a statement or proposition must somehow be either (1) immediately evident or (2) that some extrinsic evidence proves it is true. A statement may be immediately evident either (1) because it is self-evident, as when I say every triangle has three angles, or else, (2) because it is immediately-known as when I say that I am presently experiencing some form of change or motion.

Failing to be immediately evident, any claim must have extrinsic evidence, or else, will rightly be dismissed as mere assertion with no reason to be believed at all. If you make a claim that is neither immediately evident nor supported by any extrinsic evidence, no one will listen to you. Nor should they.

These are the basic rules of reason that govern all rational arguments about anything. Everyone, including skeptics and agnostics, are bound by these rules – or else will have their statements derided as not worthy of any belief at all. These rules apply to all rational discourse between intelligent agents – governing all coherent communication between human beings, be they natural scientists, philosophers, theologians, or the proverbial man in the street.

The Rules of Reason are the Rules of Reality

But, do these rules of reason apply merely to logic and the mind alone? Why is reason the sole natural instrument we use to discover the true nature of reality? If these rules of reason do not apply to reality as well as to reasoning, then what is the usefulness of reason? If the rules of reason must apply infallibly within reason itself, must they not also apply to all extramental things as well?

If reason says that I cannot affirm and deny the same predicate of the same subject in making meaningful statements, then it must be that things, if they are real beings, cannot both be and not be in the same respect. Why is this so? Because, if affirming a predicate of a subject is to have any meaning in the real world, it must mean that some existential property is real in relation to whatever the subject of the proposition refers to in reality. And, if denying the predicate means anything in the real world it must mean that some existential property is not real in relation to that same subject. Hence, to say you cannot affirm and deny the same predicate of the same subject in a proposition must mean that, in the real world, you cannot have a property be both real and not real at the same time.

In a word, this necessarily implies the reality of the principle of non-contradiction. To deny what I have just said would amount to saying that the logical form of non-contradiction has no meaning in terms of reality – in which case the mental reasoning process would be absolutely irrelevant and useless in respect to reality, as I just stated above. Moreover, just as the rational principle must apply to every possible predicate – or else, every possible statement is true, so must its metaphysical corollary, the principle of non-contradiction, apply to every possible being. That is, just as its logical formulation must be universal, so too, must the ontological formulation of the principle of non-contradiction be universal. Otherwise, reason would not equivalently reflect reality – leaving rational understanding of the world unattainable.

The Principle of Sufficient Reason

As for the need for reasons in the domain of reason itself, nothing gets a “free pass” here. Reason itself demands a sufficient reason for every claim or assertion that goes beyond what is immediately evident in sense experience itself. Claims of “brute facts” are simply not accepted in the realm of reasoning. Every truth claim that goes beyond what is immediately evident in sense experience itself requires some rational justification.

Moreover, if such a need applies solely to reason itself, then reason becomes irrelevant to the search for ontological meaningfulness. If reason does not match reality, why deceive ourselves by using this fraudulent power at all?

As shown above, some statements are immediately evident, such as those that are self-evident or those immediately evident from experience. I gave an example above of a self-evident statement: “Every triangle has three angles.” This statement must be true since the definition of triangle entails having three angles.

So, too, if the nature of a real being requires a certain property, it is self-evident that it possesses that property. For example, if man’s nature is that of a rational animal, then rationality must be an existential property of man. Or, if a thing’s essential nature included its existence, such a being would necessarily exist. Essential definitions of real beings would be meaningless if they failed to include every necessarily entailed existential property of the thing defined.

It is also immediately evident that the reality of motion or change is “self-manifested” in the very act of being experienced.

That is to say, the fact of motion is immediately evident. But, what of statements about the facts of immediate experience which make claims beyond the facts themselves? For example, what if one says that motion is perfectly self-explanatory? Or, on the contrary, that motion requires a mover other than what itself is in motion?

There is a critical distinction here between the immediately evident fact that the motion itself is real and, on the other hand, any explanation as to why the motion is real. For it is not self-evident that self-motion is inherent in the essential nature of motion. That is, if it is claimed that motion’s self-movement is an essential property of motion, that claim itself must be rationally justified before it can be accepted as true.

Thus, even self-evident statements require an active defense of their self-evidence, or else they will not be acknowledged as self-evident. They cannot be merely asserted as a “brute fact.” Rather, the reason that they are their own evidence for being true must be defended by reason.

In the examples given, claims are being made beyond what is immediately given in experience. Whether motion is self-explanatory or needs a cause is not immediately evident merely from apprehending motion itself. As said earlier, such claims demand actual proof – or else, no sane person would simply accept one of these mutually-exclusive claims without adequate evidence of its truth. In a word, such claims demand a sufficient reason for belief beyond the mere statement itself.

Now, that sufficient reason could simply be that a given property belongs to the subject’s very nature, in which case, the claim would be proven to be self-evident. But if the property in question does not belong to the subject’s nature, then the claim is still in need of some explanation for the property being present, even though it does not belong to the essence or nature of the subject of the claim. In either case, some sufficient reason for the claim must be produced, which is the opposite of a “brute fact” – since “brute facts” require no explanation at all.

Therefore, claims about reality, which go beyond what is immediately given in sense experience itself, must either offer a reason why they are self-evident (1) because something about the very nature of the subject requires them to be true or (2) (hypothetically) some other intrinsic reason not based on the nature of the subject must explain why they are true. Or else, such claims are not self-evident, but rather (3) require extrinsic reasons explaining why they are true. In every possible case, some sufficient reason for the claim must be given – or else, there is no reason to take the claim seriously, since it is being made with no adequate reason or reasons for accepting it as true.

Since all such claims are claims about reality, the reasons needed for them to be true must be reasons pertaining to the real order of being, or else, such “reasons” are totally irrelevant as to why they would support the truth of the claim about reality being made, since it is a claim about reality, not merely about the way reason functions.

Moreover, this analysis is universally true and universally applicable, since it belongs to the very nature of how statements are understood and rationally defended.

If all this is true, then all beings must have a sufficient reason for their nature, properties, and existence either (1) within themselves (intrinsically) or (2) from something outside themselves (extrinsically, from a cause). This is, in fact, a statement of the metaphysical first principle of sufficient reason.

Moreover, if the above given inferences are not true, there is no reason to reason about anything, since the rational order would have absolutely no direct correlation to the real order of existing things.

Implications for the Science of Metaphysics

What all this means is that the universal fundamental rules of reason must also be the universal fundamental rules of reality or being. If statements cannot contradict each other and if statements must either be actively self-explanatory, or else, be explained by other statements, then this necessarily implies that beings cannot both be and not be and that beings must either be their own reason for being, or else, something else (a cause) must be their reason for being.

In a word, the principles of non-contradiction and sufficient reason must be universally affirmed by anyone who recognizes the rules of reason are correct and applicable to the real world. And if these most basic metaphysical principles are universally applicable to reality, as are the basic rules of reason, then they may be employed by metaphysicians as universal first principles necessarily applicable to all beings.

This means that the universal foundational principles of metaphysics are secure and certain. It means that the search for the ultimate existential basis for all finite beings is well-founded both in reason and in reality. It should then be no surprise that classical metaphysics employs these basic existential truths to lead the mind from the reality of finite beings, whose essences do not include their existence, back through a search of extrinsic reasons for their contingent existence, and ultimately to a First Cause for everything finite, the Infinite Being -- the God of classical theism.

But, it is All Backwards!

This essay has shown that the very intelligibility of rational thinking requires that the rules of reason are universally applicable to extramental reality: that logic has no meaning at all unless its rules that correspond to the metaphysical principles of “non-contradiction,” “sufficient reason,” and “causality” apply with equal universality and certitude to the real world as well as to the mental world.6

But, the Thomistic metaphysician – while accepting the truth of this necessary connection between the rules of reason and the rules of being – would immediately point out that this whole explanation is reversed from reality.

That is to say, it is not the rules of reason that dictate the rules of being, but it is the rules of being that dictate the rules of reason.

What the mind of man first knows is the concept of being – abstracted somewhat confusedly, but with certitude, from the first things it encounters in experience. This applies even to the subjective certitudes of experience described at the beginning of this essay. It is these initial certitudes (1) that being cannot be non-being and (2) that being must have reasons either in itself or from another, which force the mind to form the rules of reason that govern all our correct thinking and rational communication with others.7

Still, whether non-Thomists will accept this explanation or not, the very fact that all rational persons must accept the basic rules of reason, and that these rules of reason necessarily imply their universal application to extramental reality or being, is enough to establish with certitude the foundations for that very metaphysical science which is so anathema to contemporary naturalism. And it also shows us how certain absolute certitudes are possible.


  1. Benignus Gerrity, Nature, Knowledge, and God (Bruce Publishing Company, 1947), 308-313.
  2. Jacques Maritain, The Degrees of Knowledge (Charles Scribner’s Sons, 1959), 71-81.
  3. Benignus Gerrity, Nature, Knowledge, and God, 382-384.
  4. Jacques Maritain, The Degrees of Knowledge, 75-76.
  5. Benignus Gerrity, Nature, Knowledge, and God, 308-310.
  6. Benignus Gerrity, Nature, Knowledge, and God, 400-401.
  7. Jacques Maritain, A Preface to Metaphysics (Sheed & Ward, 1939), 90-105; Reginald Garrigou-Lagrange, God, His Existence and His Nature, Vol. I (B. Herder Book Co., 1934), 156-198.
Dr. Dennis Bonnette

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Dr. Dennis Bonnette retired as a Full Professor of Philosophy in 2003 from Niagara University in Lewiston, New York. He taught philosophy there for thirty-six years and served as Chairman of the Philosophy Department from 1992 to 2002. He lives in Youngstown, New York, with his wife, Lois. They have seven adult children and twenty-five grandchildren. He received his doctorate in philosophy from the University of Notre Dame in 1970. Dr. Bonnette taught philosophy at the college level for 40 years, and is now teaching free courses at the Aquinas School of Philosophy in Lewiston, New York. He is the author of two books, Aquinas' Proofs for God's Existence (The Hague: Martinus-Nijhoff, 1972) and Origin of the Human Species (Ave Maria, FL: Sapientia Press, third edition, 2014), and many scholarly articles.

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