Smith discusses various meanings of “belief” and “doubt.”

George H. Smith was formerly Senior Research Fellow for the Institute for Humane Studies, a lecturer on American History for Cato Summer Seminars, and Executive Editor of Knowledge Products. Smith’s fourth and most recent book, The System of Liberty, was published by Cambridge University Press in 2013.

In the previous two essays I discussed some aspects of “The Ethics of Belief,” an essay (1877) by the mathematician and freethinker W.K. Clifford. Before examining some of Clifford’s particular points in greater detail, it would be helpful to clarify what we mean by the word “belief.”


There are two major forms in which we express a belief, viz., “belief that” and “belief in.”

The second kind of belief—belief in rather than belief that—often conveys one’s devotion to someone or something of significant value. We see this kind of value‐​charged conviction in expressions like “I believe in democracy” or “I believe in freedom.” This kind of “belief in” frequently implies a corresponding “belief that” which underlies the reason for one’s commitment. The libertarian believes in freedom, because he believes that freedom is essential to individual happiness and a prosperous society. In such cases, to “believe in” can express something more than one’s assent to the truth of a proposition; it can also convey one’s personal commitment to something of significant value.

The value‐​laden nature of personal beliefs helps to explain why we tend to be more jealous of our personal beliefs than of our abstract knowledge claims, often defending the former with more vigor and passion. When someone criticizes my significant personal beliefs (i.e., my “beliefs in”), she is doing far more than challenging my abstract claim to know, for this knowledge claim constitutes the foundation of my most important value commitments. And because my sense of “who I am” is inextricably linked to my fundamental values, I will defend the knowledge on which these values depend with great passion, as if I were fighting for my very existence—as indeed, in a psychological sense, I am.

According to some philosophers, the fundamental meaning of “belief” is confidence in the testimony of other people. Thomas Hobbes, for example, maintained that belief “beginneth at some saying of another, of whose ability to know the truth, and of whose honesty in not deceiving, he doubteth not….” Belief is based on our trust in the credibility and veracity of another person. I believe p not because I can demonstrate its truth but because an authority has assured me that is true.

It can be disconcerting to reflect on the how many of our beliefs are based on the testimony of others, such as scientists, historians, and other “authorities.” The Thomistic philosopher Peter Coffey (Epistemology, or the Theory of Knowledge, 1917) was surely close to the mark when he wrote:

People do not generally realize—because it needs a little reflection to realize—what a vast proportion of our knowledge comes to each of us individually through this source. Through it each of us has all his knowledge of the facts of history, most of his knowledge of the facts of geography, all his knowledge of what is actually going on the world outside the very limited corner that falls under his own personal notice. [N]ine-tenths of the human race accept “scientific knowledge” only on authority, and that of the other tenth, the scientists themselves, each accepts nine‐​tenths of his scientific knowledge on the authority of his fellow scientists and without exploring or verifying it for himself,—inasmuch ten lifetimes, not to speak of one lifetime, would be too short to carry out such verification.

Some philosophers have followed the lead of Aristotle and other Greek philosophers who distinguished between knowledge (episteme) and opinion (doxa), or belief. John Locke, for example, equated “knowledge” with certainty and “belief” with probability. To know that is true is to be certain of its truth, whereas to believe that is true is to accept it with some degree of probability. Quoting Locke’s An Essay Concerning Human Understanding:

Probability is likeliness to be true, the very notation of the Word signifying such a proposition, for which there be Arguments or Proofs, to make it pass or be received for true. The entertainment the Mind gives this sort of Propositions, is called Belief, Assent, or Opinion, which is the admitting or receiving any Proposition for true, upon Arguments or Proofs that are found to persuade us to receive it as true, without certain knowledge that it is so. And herein lies the difference between Probability and Certainty, Faith and Knowledge

Knowledge, according to Locke, is based on reason, whereas belief derives from judgment. We believe when, in the absence of a conclusive demonstration, we judge that the evidence in favor of is sufficient to establish its truth with some degree of probability.

Locke’s distinction between knowledge and belief (which, as I said, can be traced to the ancient Greeks) is sometimes reflected in our everyday use of these terms. For example, if I am asked about the date of a friend’s birthday, I might say “I believe it is February 10th”—meaning, this is probably the correct date, but I’m not certain. But if I say, “I know it is February 10th,” this suggests a high degree of certainty, as if I have no doubt whatever about the truth of my statement. Contrariwise, it might sound peculiar to say “I believe that two‐​plus‐​two equals four,” as if the truth of this proposition were a matter of opinion rather than knowledge. Less peculiar would be the statement “I know that two‐​plus‐​two equals four.”

According to a third and more general meaning, “belief” is mental assent (to whatever degree) to the truth of a proposition. In this sense, to say “I believe p” is to say that I affirm the truth of p. It is this generic meaning of “belief”—which refers to a psychological act of assent—that W.K. Clifford had in mind when he maintained, “It is wrong in all cases to believe on insufficient evidence.”


What does it mean to doubt something? Perhaps the best way to approach this question is to contrast “doubt” with other concepts. In other words, if we don’t have doubt, what do we have instead? Doubt as opposed to what?

It might seem that “doubt” should be contrasted with “certainty”; we do not doubt a proposition when we are certain of its truth. But even if we accept this dichotomy, it does not suffice to explain the meaning of “doubt,” because “certainty” is itself an ambiguous term that has two distinct meanings.

Like many similar terms “certainty” has both psychological and epistemological meanings. The psychological (or subjective) meaning is reflected in statements like “I am certain that is true.” The epistemological (or objective) meaning is present in statements like “The truth of is certain.”

The difference between these two senses of “certain”—the subjective and the objective—can be illustrated in the following way. Suppose Jack says, “I am certain that at least one human being has been abducted by aliens from another planet.” Jack is here expressing his strong belief in the existence of alien visitors, and the reality of this psychological state of mind does not depend on whether Jack’s belief can be justified as objectively true. Even if Jack is wrong, even if aliens have never visited earth, the fact remains that Jack believes otherwise; he is subjectively certain that his belief is true.

Objective certainty, in contrast, refers not to a belief per se but to the cognitive value of that belief. A belief is objectively certain if, and only if, it has been sufficiently justified by rational methods. Objective certainty is the result of an epistemological judgment. To say “The truth of is certain” is to say that has been tested for coherence, evidence, etc., and has passed with flying colors.

It seems that “doubt” is most often contrasted with the subjective meaning of “certainty.” To say “I doubt p” is to express a mental attitude towards p, feeling of hesitancy or reservation about the truth of p. And this suggests that doubt is active rather than passive: to doubt is actively to question or challenge the truth of a belief.

This is what Thomas Hobbes had in mind when he described doubt as a chain of alternate opinions about the truth of a proposition. I think may be true, then I think may be false, then I again think p may be true, and so forth. These alternating judgments, according to Hobbes, constitute the mental process known as “doubt.”

This view suggests that we should differentiate between doubt and uncertainty. To be uncertain (in this context) is to believe with some degree of probability, while refusing to give one’s full assent to the truth of a proposition. Such uncertainty may be purely negative in character. It may result from a lack of information about (or interest in) the subject at hand, in which case the uncertainty would not be accompanied by positive doubt. For example, I might be uncertain about the truth of Einstein’s theory of relativity, since I lack the necessary information and skill to evaluate it for myself; but I would not normally be said to doubt its truth, since this would imply that I have positive reasons to question it.

Uncertainty is the absence of certainty, so an uncertain belief may be nothing more than a probable belief (which also indicates a lack of certainty) But to say that I regard as probably true is not necessarily to say that I doubt the truth of p. Uncertainty is negative, whereas doubt is positive. Uncertainty is a mental state in which full assent is lacking, whereas doubt is a mental process in which the truth of a belief is actively called into question. Doubt therefore has an aggressive quality that pure uncertainty does not. To doubt is actively to question the truth of a proposition.

Doubt may be divided into two broad categories: spontaneous and methodic.

Doubt is spontaneous when it arises, unplanned and sometimes unwanted, in the normal course of our lives. For instance, Jill may doubt the fidelity of her husband, Jack, owing to the many telltale clues she has inadvertently run across. Jill did not plan this doubt; she did not undertake a systematic investigation of Jack’s activities in order to find evidence of his infidelity. On the contrary, prior to her accidental discoveries, she had never suspected him of anything. Hence Jill’s doubt is spontaneous; it did not arise from foresight or design.

This is not the kind of doubt that characterizes philosophy. Of course, the philosopher is a human being who, like everyone else, will sometimes experience spontaneous doubt during the course of his life—but the doubt of the philosopher qua philosopher is methodic rather than spontaneous. In calling this doubt methodic, I mean that it is part of a systematic method used by the philosopher in his investigations. And this method—which Kant dubbed “procedure according to principle”—is the product of foresight and planning. Methodic doubt, in other words, in purposeful rather than spontaneous, systematic rather than incidental.

Methodic doubt stands in a category of its own; it is not what most people think of when they employ the word “doubt.” Methodic doubt, for example, is fully compatible with subjective certainty. I may be fully convinced that there exists a world external to my consciousness, yet I may subject this belief to methodic doubt nonetheless. In my case, the purpose of this inquiry would not be to attain a higher degree of certainty than I already have; rather, I might question the existence of an external world in order to determine whether this is a reasonable question in the first place, and, if it is not, why it is not. Or, like Descartes, I might apply methodic doubt to my commonsensical beliefs in order to identify the ultimate foundation of certainty. In any case, the philosopher will often apply methodic doubt to beliefs that would rarely, if ever, generate spontaneous doubt. And this use of methodic doubt does not necessarily mean that the philosopher has any subjective doubts about belief that is being scrutinized. It may simply mean that the philosopher expects (or hopes) that something of value – a fresh insight, perhaps, or a new way of looking at an old problem—will emerge as a byproduct of his methodic doubt.