There are approximately nine possible sources of belief: sensory perception, induction, deduction, extrasensory perception, intuition, common sense, introspection, direct experience, and imagination. Except for some propositions based on deduction, propositions based on these sources are always synthetic because their truth or untruth always depends on interactions with the world of experience. Now, these sources are not all equally reliable. As I will discuss, some of them are not valid sources of apparently justified apparently true belief.

SENSORY PERCEPTION

Sensory perception refers to the processing of sensations that originate in one of the sense organs (eyes, ears, tongue, nose, skin, or internal organs) by the brain. Sensory perception does not occur in a vacuum. All sensory perceptions depend on one’s prior interpretations of the world and can either reinforce or modify those interpretations. They are never purely empirical. Beliefs can be formed either as a direct consequence of sensory perception or from inferences drawn therefrom. An example of the former is the belief that fire is hot. An example of the latter, at least for those of us who live south of the Arctic Circle or north of the Antarctic Circle, is the belief that the sun will rise on all future mornings.

Sensory perception is one of the most obvious and direct sources of belief, and beliefs formed based on sensory perception are often apparently justified and apparently true. The belief that fire is hot and that the sun will rise on all future mornings on earth south of the Arctic Circle or north of the Antarctic Circle are examples of beliefs that are apparently justified and apparently true. The possibility that the foregoing beliefs are mistaken is so remote that it is hardly worthy of serious consideration. However, there are cases in which the possibility is not nearly as remote. For example, a person who sees a rope in a dark path may mistakenly believe it is a snake.

INDUCTION

Induction means reasoning from particular facts or observations to general principles. It is one of the main ways scientific explanations are developed. However, the reliability of induction is limited. One of the most illustrative examples of this is the case of the black swan. Prior to 1697, when European explorers first saw black swans in Australia, Europeans universally believed that all swans were white because all swans that had ever been seen by Europeans, as well as all other people except Aboriginal Australians, were white. Before 1697, Europeans had observed countless swans, and all had been white. From that, Europeans induced that all swans were white.

The “black swan” example illustrates that there is never any way to have certainty that the general principles one formulates based on numerous consistent experiences, sensory or otherwise, accurately reflect the way things are. However, there are numerous examples of inductions drawn from multiple consistent experiences that have not been falsified. Despite the uncertainty of whether beliefs based on induction are ultimately correct, they can be said to be apparently justified and apparently true on the grounds there are no compelling reasons to doubt their validity at present. However, those who value truth and the pursuit of truth should always keep in mind that such beliefs could turn out to be wrong.

DEDUCTION

Deduction refers to drawing logical inferences from valid premises. While deduced propositions can be purely analytical, and they often are in mathematics, most deduced propositions in philosophy can only be confirmed or disconfirmed based on interactions with the world outside their terms. For example, take the syllogism “All men are mortal. Socrates is a man. Therefore, Socrates is mortal.” It is not possible to understand what a man is, what it means to be mortal, or who Socrates was without interacting with the world of experience. To be a man is to be a member of the species homo sapiens sapiens, to be considered to have passed into manhood by the conventions of one’s society, and to identify as a male, all of which can only be understood based on actual interactions with men. To be mortal is to die. Mortality, like manhood, can only understood by reference to actual experiences of organisms dying. Finally, the person represented by the word Socrates can only be understood by reference to either historical texts or orally transmitted stories about him.

A deduced proposition can also consist of an empirical inference as to the nature of a heretofore unexperienced situation by comparison with a situation one has previously experienced. For example, where one sees smoke emanating from a forest but does not at the same time see fire, one can still deduce that there is a fire based on previous experiences in which one has observed fire producing smoke. To be reliable, empirical inferences must be based on previously established apparently justified apparently true beliefs. In the given example, for the inference that smoke in a forest means there is fire to be reliable, the twin beliefs that fire causes smoke and that smoke in a forest indicates the forest is on fire must be apparently justified and apparently true for the deduced inference to be valid. Here, the second belief is not apparently justified and apparently true in all situations, as it is possible for there to be smoke without fire. However, smoke usually indicates the presence of fire, particularly in a forest. Thus, the deduced inference is likely apparently justified and apparently true, though the only way to confirm this would be through directly sensory perception of an actual fire in the forest (seeing flames, hearing crackling, etc.).  

EXTRASENSORY PERCEPTION

Extrasensory perception refers to perception by means other than the physical senses. These means have never been proven to exist by any convincing evidence, but it is possible that they exist, and many people claim to have had extrasensory perceptive experiences. However, due to the immense difficulty of publicly testing such purported experiences and the utter lack of repeatable non-anecdotal evidence for even one such experience, it is impossible to say whether such experiences are products of the human imagination, experiences of aspects of reality of which humans aren’t ordinarily aware, or hallucinations.

Given this uncertainty, it is prudent for those who believe they have had extrasensory perceptual experiences to consider the possibility that they are mistaken. However, it is also prudent for those of us who haven’t had such experiences to acknowledge the possibility that there is more to existence than what can be experienced by the physical senses. Nonetheless, the possibility that there is more to existence than what is perceived by the physical senses does not mean that there is any justification for believing that the objects of purported extrasensory perception, such as ghosts, have any objective existence or that their existence can be intersubjectively confirmed. Two of the three factors that must be present for a belief to be apparently justified and apparently true—consistency with already held apparently justified apparently true beliefs and intersubjective confirmation—are lacking for experiences that purportedly involve extrasensory perception.

In addition, some people claim to have experienced paranormal phenomena with the physical senses rather than by means of extrasensory perception. But with these experiences too, there has never been a single case in which a purported experience of paranormal phenomena by the physical senses has been anything other than purely anecdotal, and in no case have such experiences been successfully recorded or repeated. However, there have been cases in which multiple people reported perceiving paranormal entities with the physical senses, such as the so-called “Mothman” in the U.S. state of West Virginia. If it were possible to hear or see the objects of such experiences with the eyes and ears, it would presumably also be possible to make video and audio recordings thereof. Because there are no such recordings, at least none that can be demonstrated to be reliable, there is no basis for believing anyone’s claim to have experienced paranormal phenomena with the physical senses.

INTUITION AND COMMON SENSE

Intuition refers to purported immediate understanding without the use of reason. It is thinking a belief is true without being able to articulate why. It includes both gut feelings and assumptions based on prior experiences. Beliefs formed based on intuition typically feel reliable and doubting them typically feels wrong. For this reason, it is easy to become overconfident in the reliability of beliefs formed based on intuition. This is not to say that such beliefs always end up being wrong, but they often do.

Common sense refers to the faculty of the mind which applies the beliefs and values that one believes are common to one’s community or society to make sense of one’s experiences. Like intuition, beliefs formed on its basis typically feel right and doubting them feels wrong, at least to most people. Also like intuition, it is easy to be overconfident in the reliability of beliefs formed based on common sense. In addition, common sense is culturally relative, meaning what is considered common sense in one culture won’t necessarily be understood the same way in other cultures, though it is easy to overstate this point, as there is often significant overlap between different cultures about what is considered common sense. Nevertheless, what is considered common sense in a particular community or society is not universal, and those who value truth and the pursuit of truth therefore must never presume that it is a reliable basis on which to form factual beliefs. In my language, beliefs formed based on common sense alone are neither apparently justified nor apparently true.  

Intuition and common sense too often lead to erroneous beliefs to be considered reliable. A good example of this is the very unobvious fact that seemingly solid objects never actually touch each other at the atomic level. Based on intuition and common sense, it seems obvious and clearly true that two seemingly solid objects would touch each other at all levels when they are pressed together. However, this is not what happens at the atomic level because the atoms that make up objects that are perceived as solid at the everyday level have no hard boundaries that could “touch” each other according to the conventional meaning of that word.  

A more straightforward example is the well-known fact that the earth is nearly spherical and not flat. A surprising number of people still believe the earth is flat even though its roughly spherical shape has been repeatedly confirmed empirically. However, if there were not overwhelming evidence that the earth is nearly spherical, intuition and common sense would suggest that the earth is flat. Indeed, the vast majority of people in human history appear to have held this belief. Pythagoras was the first major thinker in the Western World, and possibly the entire world, to argue that the earth was spherical, in the sixth century BCE.

A mundane example of an unreliable belief based on intuition is the gambler’s fallacy, or the belief that one’s future luck in games of chance depends on prior outcomes. This belief may feel true for many gamblers, but there are no sound reasons for believing it is true. After all, if it were actually possible for people to reliably predict whether they would win money gambling, casinos would not be able to stay in business because they would not be able to turn a profit.  

Despite the foregoing examples of the unreliability of intuition and common sense, there are arguably limited cases in which they can lead to apparently justified apparently true beliefs. For example, two people who have been married for many years can sometimes accurately discern each other’s emotional states intuitively. This ability is based on many years of interaction between the two people and many past perceptions and inferences of each other’s behavior. But even in these situations, beliefs based on intuition can be wrong and it is wise to remain skeptical of their veracity. In the given example, even people who have been married for many years can have mistaken intuitions about each other’s emotional states and should not automatically treat their intuitions as reliable if they value truth and the pursuit of truth.

Regardless of their overall reliability for arriving at apparently justified apparently true beliefs, intuition and common sense can provide a point of departure for further inquiry that ultimately leads to the formation of apparently justified apparently true beliefs. It is possible for empirically testable hypotheses formed based on intuition or common sense to turn out to be apparently justified and apparently true, though whether they are apparently justified and apparently true can only be determined by means of sensory perception, induction, and other reliable means previously discussed, not by means of intuition or common sense.

Whether they are reliable or not, it is difficult to avoid relying on intuition and common sense some of the time. However, this does not mean that intuition and common sense are, in fact, reliable. Rather, it means that those who care about truth and the pursuit of truth must question the beliefs they hold based on intuition and common sense and determine whether they have more reliable grounds for holding those beliefs. While this requires the rigorous use of reason and relentless introspection, those who value truth and the pursuit of truth have no choice but to try to do this if they want to live consistently with their values.

INTROSPECTION

Introspection is the examination of the contents of one’s own conscious experiences using the “mind’s eye”—that is, the faculty of experience that looks “within” at the “inner” world of one’s own consciousness rather than “without” at the seemingly “external” world of physical objects and fellow conscious subjects.[1] Based on introspection, one can learn how one responds to stimuli, such as sensory perceptions and thoughts. For example, if one notices that thinking about a given subject causes one to feel anger, one is then apparently justified in believing that thinking about that subject causes one to feel anger. However, beyond such straightforward examples involving surface-level mental phenomena in the person practicing introspection, the reliability of introspection in forming apparently justified beliefs, particularly about mental processes of which people are not normally consciously aware, is questionable.

It might be possible to examine the contents of one’s consciousness at far deeper levels than one’s responses to stimuli, but it is difficult to determine how reliable examinations of this sort are, as there is no neutral standard against which to check them. It is difficult to even say for sure that there are “deeper” levels of consciousness, though I believe there are. In addition, while the belief that some mental processes are unconscious is apparently justified based on the best available evidence, this does not mean that those unconscious processes are deeper, in either the sense of more fundamental or more important, than what one experiences in one’s ordinary waking state. It is also not clear to what extent and how often introspection is overlain with presumptions and presuppositions that prevent the examination of the phenomena of consciousness in an unbiased way.

To be clear, I don’t dismiss the possibility that introspection can be unbiased. Unbiased introspection could even make people aware of their presumptions and biases rather than being beholden to them. Still, the possibility that the introspection is laden with presumptions and biases cannot be dismissed out of hand, and it is never possible to determine how biased or unbiased any other person’s practice of introspection is from the outside or for others to determine how biased or unbiased one’s own practice of introspection is. Furthermore, beliefs formed based on introspection are generally difficult to intersubjectively confirm or refute. Beyond the straightforward examples cited above such as which stimuli cause one to feel anger, which concern surface-level mental phenomena in the person practicing introspection, beliefs based on introspection are neither apparently justified nor apparently true.   

DIRECT EXPERIENCE

 As I use the term, direct experience refers to experience as a particular mode or state of consciousness rather than experience of an object of consciousness. Beliefs based on direct experience are beliefs about consciousness and its various modes and states as present lived realities. While one must still interpret direct experiences linguistically to form beliefs based on them, everything that one is apparently justified in believing based on direct experiences comes solely from the experiences themselves.

An example of a belief formed based on direct experience is the belief that the self or ego is a sense akin to hearing and sight, and that, like those, it can be either present or absent in consciousness. A more commonplace example of a belief formed based on direct experience is the belief that present consciousness is the consciousness of a distinct and separate personality—that is, a distinct and separate conscious self or ego. The latter belief accords with the reported everyday experiences of most people, though some people, including me, hold that the former belief is true while the latter is false. The metaphysical significance, if any, of such experiences and what they tell us about existence will be discussed in the following chapter. For present purposes it suffices to note that the only possible basis for formulating any non-hypothetical understanding of what different states of consciousness are or what they mean is direct experience.

IMAGINATION

Imagination is the faculty or process of mind by which the mind generates ideas of what could be, whether these ideas accord with or conform to the laws and features of the natural world or not. Beliefs rooted in imagination which do not accord with or conform to the laws and features of the natural world are only apparently justified and apparently true in the context of worlds that have been invented. Such beliefs can, however, be rooted in rules or features that are internally consistent within the invented world. For example, in the context of the invented world imagined by J.R.R. Tolkien known as Middle Earth, one is apparently justified in believing that Minas Tirith is the capital of Gondor during the fictional events of The Lord of the Rings, even though Gondor and Minas Tirith both only exist in the context of an invented world and not in the natural world.

It is necessary to distinguish between products of the imagination that are publicly shared and those that are only experienced privately. It makes no sense to speak of apparently justified apparently true belief in a product of the imagination that is only experienced by the imaginer, as the imaginer can change any aspect of his or her imagined world at any time, whether it is consistent with the rules of logic and laws of thought or with his or her previously imagined world or not. However, as with Minas Tirith, it makes sense to speak of apparently justified belief in the context of an imagined world that has been made public. Even if J.R.R. Tolkien had retracted everything he wrote in The Lord of the Rings and claimed that the events of Middle Earth actually differed greatly from those described therein, this would not change the attributes of Minas Tirith within the context of The Lord of the Rings because it contains products of Tolkien’s imagination that were made public.

It could perhaps be argued that it is possible to have truly justified true belief in products of the imagination that have been made public. However, it is possible to be mistaken about the attributes of a product of the imagination. Indeed, there are several well-documented examples of people being mistaken about products of the imagination that have been made public, such as the so-called “Mandela Effect.” Examples of the Mandela Effect include the belief that the fictitious monkey Curious George has a tail and that the Monopoly Man has a monocle. Neither of those beliefs is correct, though many people believe they are.

[1] While the belief that there is a real world full of real objects and fellow conscious subjects that can be known to truly exist by any unbiased experiencing subject who comes along is a fallacious belief known as naïve realism, in practice almost people are naïve realists at least some of the time.