Hume and True Skepticism: How Do We Know?

“Custom is the great guide of human life,” according to David Hume.1 The reasoning of rational thinkers and philosophers at work is a completely different way of thinking than the everyday common sense by which most people live their lives. If we as critical thinkers were to apply our skepticism to our everyday lives, they simply wouldn’t be able to function. We would be constantly walking directly into walls because we can’t be certain that the wall is in fact in front of them and that it constitutes a barrier to their progress and that running into a wall is likely to cause them to break our nose. We would be like Pyrrho, the ancient philosopher who was allegedly so skeptical that had to be stopped by his friends from walking out into the middle of traffic because he had no rational basis for proving it was dangerous to do so. Must the skeptical reason of the rational thinker and the customary common sense of the everyman be so completely divorced from each other?

Hume’s famous example of the inability of reason to justify the principles by which we customarily live our lives is in the principle of cause and effect, demonstrated with the relationship of billiard balls on a table. When we think we are observing one ball hitting another, thereby causing the second to move, we are in fact only observing the movement of one billiard ball followed by the movement of another billiard ball. The “law of cause-and-effect” is not something we see, we only see one thing happening, and then another thing happening immediately after. The cause-and-effect relationship is added by mind via the process of induction, but it is not contained in the observation of the billiard balls.

If the process of induction is not based on reason, where does it fall? According to Hume, “All reasonings may be divided into two kinds, namely, demonstrative reasoning, or that concerning relations of ideas, and moral reasoning, or that concerning matter of fact and existence.”2 Induction cannot be demonstrative reasoning, because it deals not just with relations of ideas, but with matters of fact. It cannot be moral reasoning, because it attempts to apply a universal idea to matters of fact.

This skepticism even applies to what Hume calls the “Uniformity Principle.” The Uniformity Principle is the common-sense idea that the world is uniform and subject to simple predictions. The example he gives is that we assume that eating bread will be nourishing (it provides calories) because it was nourishing every other time we ate it. We assume that the sun will come up tomorrow because it has come up every day for the duration of recorded history. This, Hume says, cannot be proven, and in fact is based on circular logic.

Painting of Hume by Allan Ramsay. Despite my extensive research, I have yet to find out why he’s wearing a shower cap.

Induction is the logical process by which we assume things like because the sun rose every day of our life, it will rise tomorrow. It is a probabilistic argument, and a practical one. But it’s also a circular argument, according to Hume. How do we know that induction is correct? Because it works. And how do we know it works? We know it inductively, because it worked every other time we used it. This is circular logic.

The principle of cause and effect, according to Hume, is a connection “that accompanies the imagination’s habitual move from observing one event to expecting another of the kind that usually follows it. That’s all there is to it.  Study the topic from all angles; you will never find any other origin for that idea.”3(emphasis added)

But why should we think of ideas in terms of their origins? If we cannot prove the existence of the cause-and-effect relationship in the material realm, why would we evaluate ideas on the strength of their causes? Hume’s basic epistemological framework evaluates things on the basis of their origins. Is this really how people think? More importantly, is this really how people should think? Is this the way of thinking most likely to identify truth or pragmatic value?

Any argument against an epistemological framework – a way of thinking about how we know what we know – must take the form of a new epistemological framework, capable of evaluating claims, evidence, postulates, etc. with more knowledge-finding ability and less risk of contradiction than the previous system. Kant, for example, created a system in which judgments of cause and effect or the Uniformity Principle are possible as “synthetic a priori” judgements. Hegel created a system in which cause-and-effect are part of a back-and-forth dialectic.

As an epistemological starting point, lets take a principle of the scientific method: it is not possible to prove a theory, only to try and fail to reject it. Science is a dialectical process, in which a hypothesis constitutes the thesis and the tests or experiments or challenges we put it through are the antithesis. In this scientific framework, the antithesis constitutes an attempt to negate the thesis.

The dialectical method associated with G.W.F. Hegel has a reputation as one of the most difficult concepts in philosophy, owing to his infamously jargon-laden texts. But simply put, the dialectic is the set of relationships and changes in relationships that constitute any kind of progress. The simplest application of the dialectic is in the realm of intellectual progress: first the intellectual community has an idea, which in Hegelian thought is called the “thesis.” Then others in the intellectual community, seeing the flaws with the mainstream thesis, come up with their own idea, called the “antithesis.” The thesis and antithesis remain in conflict until they can be “subsumed” into a system that incorporates both of them, which is called the “synthesis.” The synthesis becomes a new thesis and the process repeats again, continuously improving the state of knowledge.

The scientific method is not precisely the same as Hegel’s formulation of the dialectic, in which the thesis and antithesis become part of the synthesis. The scientific method is a dialectic with a binary fork; that is, every antithesis either causes the rejection of the thesis or fails to cause the rejection of the thesis. When a test or challenge fails to cause the thesis to be rejected, that test serves to strengthen, not to negate or transform, the thesis, because the thesis has shown that it can survive the test. If our test or challenge shows that the thesis causes contradiction, then we must reject it. We do not simply subsume the rejected hypothesis into the synthesis, we formulate a new hypothesis to replace it. Of course, it is possible that the new hypothesis is a similar but modified version to the hypothesis, but it sometimes is the case that the new hypothesis is radically different, or else evaluates things in terms of an entirely different framework.

The fundamental process behind the scientific method is dialectical testing for contradiction.

But the scientific method is constrained and focused by means of experimentation and controlled observation, and therefore applies only to problems with hypothetical answers that can be tested via experimentation. The general process of dialectical testing for contradiction, however, can be applied to all knowledge. Testing for contradictions is, in fact, the typical way by which we skeptically evaluate any proposition.

There is no proof in science, just testing for contradiction. In applying the principle of testing for contradiction to the rest of the knowledge, we must consider a radical proposition:

Proof does not exist as a means of attaining knowledge.      

We naturally have to clarify this claim. Proof can exist in the colloquial sense; in which whatever idea we try but fail to reject is thereby “proven.” Furthermore, a “deductive proof” might exist as process of analyzing statements already proven, but since a deductive proof’s premises are always unproven, a deductive proof is simply a mental exercise.

As an example of this, take the most common and basic example of a deductive proof: Socrates is a man, all men are mortal, therefore Socrates is mortal. Deductive logic may say that if Socrates is a man, and if all men are mortal, then Socrates is mortal. We have a “valid” proof, but we do not have proof. How do we prove that all men are mortal? We might say that every man we can find evidence of is mortal, and thereby induce that all men are mortal, but as Hume taught us, we cannot have inductive proof. If we cannot have inductive proof, then we cannot have a complete deductive proof.

But we wouldn’t say, then, that deduction is impossible, only that a positive proof via deduction is impossible. By the same token, we should say the same thing about induction.

What do deduction and induction give us then? Only a suggestion. When we see the conjunction between the movements of the billiard balls, induction gives us the suggestion of a cause-and-effect relationship. We then evaluate that suggestion with the dialectical testing for contradiction, both by experimenting with tossing billiard balls around the table ourselves, by analogizing it to other cause-and-effect relationships, and by formulating a theory of cause-and-effect that we can evaluate mentally for possible contradiction with other theories. Induction and deduction also serve to reduce suggestions into their constituent parts to make them easier to test.

So what are we doing when we evaluate claims in our minds? Let’s look at what has been claimed as the most basic and undoubtable proof of all: Descartes’s “I think, therefore, I am.” Just as a person cannot prove that they are not in the Matrix or the subject of their own personal “Truman Show” or a brain in a vat receiving all experiences through an electrical wire, Descartes took doubt to its logical conclusion. Descartes, probably as an intellectual exercise, doubted his own existence for a moment, but found a way out with the argument “I think, therefore, I am.”

Rene Descartes, detail of portrait by Frans Hals

Descartes justifies this claim not by defining “think,” “therefore,” and “I am,” which should be all that it takes to evaluate this claim if it were possible to truly prove things via definition. Instead, he attempts to disprove the claim. He challenges the claim by evaluating the possibility that perhaps an “supremely powerful and cunning deceiver” is fooling him into believing that he exists. He then challenges his own challenge, rejecting it by saying that “if he is deceiving me I undoubtedly exist: let him deceive me all he can, he will never bring it about that I am nothing while I think I am something.”4 Descartes didn’t “prove” he exists, he asserted it and tested it for contradiction. If a proof was actually proven, there would be no point in testing possible objections to it.

All “proof” is the survival of a suggestion when tested for contradiction.

Let’s take another extremely basic proof, A=A. How do we prove that A=A? We can’t, we can only disprove its opposite. We can say that if A sometimes does not equal A, then we wouldn’t be able to even talk about it, because without a law of identity we wouldn’t even be able to think or compare ideas. A≠A is so contradictory we can’t even conceptualize it. But we still can’t formulate positive proof of its opposite.

This is how Moroni tells us we should test ideas and principles for truth.

“4 And when ye shall receive these things, I would exhort you that ye would ask God, the Eternal Father, in the name of Christ, if these things are not true; and if ye shall ask with a sincere heart, with real intent, having faith in Christ, he will manifest the truth of it unto you, by the power of the Holy Ghost.”

–Moroni 10:4 (emphasis added)

Notice the use of the phrase “not true.” He is asking us to test if a proposition can be falsified, and it’s by not negating whatever proposition we bring to the Lord that he manifests the truth of it unto us.

This idea is embedded in how we think about evidence and proof, even though we don’t always directly acknowledge it. When we say things like “prove beyond a reasonable doubt” or “prove beyond all doubt” we are defining the proof by the strength of its possible negation. If positive proof were a reality, then the phrase “prove beyond all doubt” would just be “prove.” If custom is the guide of thought, philosophy should not be fundamentally different, the difference should only be that philosophers are willing to be more skeptical. To be skeptical is to search for the underlying principles or constituent parts of an argument and then to evaluate them by this negative dialectic.

Moroni in his natural habitat

Knowledge is always uncertain, meaning that it is subject to perpetual evaluation by the dialectical testing for contradiction. It’s a dialectic between everyday common sense, which takes a useful suggestion as truth, and philosophical skepticism, which asks “if your suggestion were truth, wouldn’t it cause these contradictions? How does your suggestion stand up to these possible tests”? Practicality takes any reasonably well-tested suggestion as truth until further notice.

Hume recognizes that when reason evaluates relations of ideas and matters of fact that it is attempting to find a contradiction or an applied contradiction in the ideas, terms, or evidence. Why then, should we ever deal in justification and proofs, inductive or deductive? We can only search for contradiction. He says “[the proposition] that the sun will not rise tomorrow is just as intelligible as—and no more contradictory than—the proposition that the sun will rise tomorrow” because it can be conceived by the mind easily and clearly.”5

But is evaluating whether a proposition “can be conceived… easily and clearly” really what human reason does, or ought to do, when it searches for contradictions? Descartes used the same concept, which he called a “clear and distinct notion,” in his proofs for the existence of God.6 But what is a “clear and distinct notion”? The notion of clear and distinct notions is a rather unclear and indistinct notion to my mind. Some people may disagree with me on this point, others may agree, but if the notion of clearness and distinctness was clear and distinct, then there shouldn’t be any disagreements.

What, then, are we actually doing when we search for contradictions? We are evaluating the implications of an argument to see if they cause contradictions. We are asking “if this were true, what would be the results?” The if-then evaluation I just made is just this type of argument. We cannot positively say that something does not cause contradictions, just as we cannot say that something is definitely “clear and distinct,” but we can point out contradictions, just as we can point out a lack of clarity and distinctness.

A possible contradiction is a reason to doubt a hypothesis. A doubt need not be proven, it only needs to be suggested, and like any suggestion, the doubt too is subject to the process of dialectical testing for contradiction. President Uchtdorf’s famous advice to “Doubt your Doubts” is a fundamental principle here. Of course, a doubt of a doubt can be doubted, which means that every theory we hold is attached to a “tree of doubts” (or a “tree of evidence,” viewed more optimistically) that we must evaluate along as part of the hypothesis.

The process of dialectical testing for contradiction can be applied to all types of judgements, not just experimental science. Even contemporary ethical reasoning uses this method regularly in the form of thought experiments. The trolley problem, for example, tests the ethical theory of utilitarianism by asking what someone would do when given the option of killing one person to save two. If utilitarianism were really as universal, obvious, common sense, etc., as its proponents believe, then we ought to be willing to push a bystander onto the tracks to prevent the trolley from hitting two others. We ought to be willing to kill one person as an unwilling organ donor for two others if utilitarianism were the obvious mode of ethical thinking.

This epistemology of skeptical doubting and testing for contradiction can be applied broadly across all fields of knowledge. It is, in fact, our natural way of arguing, though we humans have a bad habit of only applying it to that with which we disagree. Metaphysicians used the dialectic of testing for contradiction when they evaluated of necessary truths by the implications of their existence and of their non-existence. Mathematical “proofs” break a down a statement into its constituent parts, each of which are as subject to doubt as the law of identity as shown earlier.

This is why we should “become a seeker,” as Steven C. Harper entreated at his recent BYU devotional. He tells us that at the far side of the complexity of evaluating the thousands of doubts and caveats and pieces of evidence that question a claim is a “simplicity on the other side of complexity.”7 Every tree of doubts has a simple root truth, a simple principle that can guide everyday life without the need to consult every branch of the tree to evaluate whether or not we are actually in the Matrix before deciding whether to cross the street.

At root of this whole project is the problem of evaluating this epistemological system. How do we evaluate epistemological systems? We clearly can’t prove them mathematically, so the natural, obvious, pragmatic, and most logically sound way is to expose epistemological systems to a negative dialectic.

In giving examples of this epistemology in action throughout my argument, I naturally used this principle of testing for negation, mostly unconsciously. When I give examples of how this framework applies in a certain context, I am, in fact, testing this suggested framework to show that it can be applied without contradiction (in addition, of course, I am using examples to try to help the reader understand my thesis).

Hume was right when he said that “this idea of a necessary connection among events arises from a number of similar instances which occur of the constant conjunction of these events.” So what? Tracking ideas to their origin to find their proof is a futile approach to epistemology. It’s an example of the “genetic fallacy,” and if we were to evaluate all ideas by their origin, we would immediately find that they are all based in our imperfect minds. The human mind doesn’t have the power of “proof,” though it has the power to create unproven ideas, or “suggestions,” which range from useful to beautiful to intriguing to absurd to imperfect to evil. It’s up to us, individually and collectively, with the help of our senses, our minds, and revelation, to learn about and skeptically test those ideas.

Note: This essay incorporates some material written for BYU’s History of Philosophy course (PHIL 202) in March of 2021. I’m picking on Hume here because he’s probably the most intellectually influential skeptic who has ever lived, and it’s the brilliant ridiculousness of his famous billiard ball example that sparked the idea that led to my approach to epistemology.


1. David Hume, Enquiry Concerning Human Understanding (Early Modern Texts, 2017) 21,

2. Ibid., 16.

3. Ibid., 37.

4. Rene Descartes, Meditations on First Philosophy (Early Modern Texts, 2017), 4,

5. Enquiry, 11.

6. Meditations, 13-16.

7. Steven C. Harper, “How I Became A Seeker” (BYU Speeches, 8 June 2021)

Top Image: Detail of The Billiard Room by Nicolas Antoine Taunay, circa after 1810