For belief in a synthetic proposition to be apparently justified and apparently true, three elements must be present: (1) positive evidence, (2) consistency with already held apparently justified apparently true beliefs, and (3) either intersubjective confirmation—for testable claims in the present, or plausibility—for untestable claims, including nearly all claims about the past. These three elements do not arise independently of one another, but at the same time and in a mutually reinforcing manner.

POSITIVE EVIDENCE

Positive evidence refers to evidence based on experience—usually, but not necessarily, sensory experience or the readings and measurements of reliable machines and instruments. The opposite of positive evidence is negative evidence, which consists of the claim that belief in a proposition is justified because it has not been disproven. In logic, this is known as the argument from ignorance. While beliefs based on negative evidence are not necessarily false and, in principle, any proposition that cannot be confirmed or controverted by positive evidence could potentially be true, if this approach were taken with respect to every proposition there would be no limit to what could be believed. Put another way, there are a nearly infinite number of statements about the universe or about existence that could possibly be true, but only a handful of such statements are supported by positive evidence.

If positive evidence were not required for a belief to be considered apparently justified and apparently true, there would be no limit to what could be believed. There would also be no way to effectively distinguish between reliable and unreliable beliefs. Furthermore, many beliefs that have never been disproven conflict with each other. For example, Hinduism and Buddhism’s teaching that there is an endless cycle of rebirth driven by the law of karma conflicts with the Christian and Islamic teaching that there is only one lifetime and that heaven/paradise, hell, and, in some versions, purgatory, await all people in the next life or following a last judgment. These claims can’t be disproven, but they contradict each other and there is no positive evidence for their truth.

Despite the dangers posed by accepting claims that cannot be disproven as possibly valid, it could still be argued that suspending judgment and not forming a belief either way as to the truth of propositions that cannot be disproven is the best way to avoid holding erroneous beliefs. However, this approach undermines the pursuit of truth because it implies that beliefs that are not supported by any evidence are just as robust and worthy of consideration as beliefs that are supported by some evidence. (Whether there is a moral obligation to value truth or the pursuit of truth is a separate question. For now, I will straightforwardly assert that I value truth and its pursuit.)

Yet another reason to withhold belief in even the possibility that a proposition that has never been disproven is true is that erroneous beliefs are easy to form but difficult to change. The best way to avoid having to reject baseless beliefs is to avoiding forming them in the first place. Even a cursory examination of human history shows that humans have held countless beliefs that have been unsupported by evidence and later proven false. The more people who commit to only believing in propositions for which there is positive evidence, the less likely it is that erroneous beliefs will be propagated or accepted as valid on a large scale.

Experience is the easiest way to fulfill the requirement of positive evidence. While interpretations of experiences and the relevance of experiences to propositions that are allegedly confirmed or controverted by those experiences can always be called into question, a person who values the pursuit of truth must not categorically dismiss any type of experience, not even one that is purely subjective or even one that is the product of mental illness, self-delusion, or hallucinations.

However, that does not mean that an interpretation of a purely subjective experience which holds that the experience provides evidence of objective facts must be accepted as valid, or in my language, as apparently justified and apparently true. Indeed, purely subjective or “inner” experiences can only ever provide evidence for subjective or “inner” claims—claims about the contents of consciousness. They do not provide evidence for any objective claims—claims about the world “outside” consciousness. Nor does it mean that a purportedly objective experience must be accepted as evidence of any objective factual claim where there is no intersubjective confirmation of that experience. While interpretations of experiences can always be called into question and the significance of experiences can always be doubted, experiences themselves cannot be dismissed unless there is evidence that those who claim to have had them are lying.

The readings and measurements of machines and instruments are also a way to fulfill the requirement of positive evidence. However, such readings and measurements must always be interpreted by people, who must experience those readings and measurements by seeing, hearing, or otherwise sensing them, and then analyzing them, before they can become the basis of propositions. Thus, propositions based on readings and measurements are also based on experience—specifically, sensory-perceptive experience.

CONSISTENCY WITH ALREADY HELD APPARENTLY JUSTIFIED APPARENTLY TRUE BELIEFS

Beliefs are not formed in a vacuum or in a linear fashion. Every person has a network of interconnected beliefs, with each belief arising along with, supported by, and supporting many other beliefs. In forming beliefs, people do not start with one apparently established fact, proceed to another, and another, until they arrive at a network of interconnected beliefs. Rather, each person’s network of interconnected beliefs arises organically, with multiple beliefs arising at the same time and mutually reinforcing each other. When one’s beliefs change, one’s entire network of interconnected beliefs also changes, if only to a small degree.

It is of course possible to hold conflicting beliefs within that overall network. For this reason, the only way to ensure that one’s beliefs are apparently justified and apparently true is to continuously test them against one’s own experiences, the reported experiences of others, and one’s other apparently justified apparently true beliefs. There is some circularity in this approach, but unfortunately such circularity is inevitable precisely because beliefs do not form in a vacuum or in a linear fashion but within a mutually interconnected network. This circularity provides additional grounds for acknowledging that all of one’s beliefs, even those that seem obvious, could conceivably be wrong.

Nonetheless, checking prospective new beliefs against one’s already held apparently justified apparently true beliefs can help one avoid adopting erroneous beliefs. If one has concluded that one’s already held beliefs are apparently justified and apparently true, and one has sound reasons for continuing to regard those beliefs as apparently justified and apparently true, then one is apparently justified in rejecting a prospective new belief that is inconsistent with those already held apparently justified apparently true beliefs. And of course, the corollary of this is that if the prospective new belief is consistent with one’s already held apparently justified apparently true beliefs, one is apparently justified in adopting the prospective new belief.

Another way of stating this element is that belief is only apparently justified and apparently true if it accords with the apparent facts of existence. However, because it is only possible to become aware of the apparent facts of existence through experience, and because beliefs in these apparent facts are themselves formed based on experience, in practice accordance with the apparent facts of existence is synonymous with accordance with already held beliefs about the facts of existence based on experience.

INTERSUBJECTIVE CONFIRMATION

Intersubjective confirmation means that many people have had the same experience and have described it using similar language. However, intersubjective confirmation is not simply a headcount to determine how many people report having the same experience. Majority consensus is not enough to render a belief apparently justified and apparently true because groups of people, including majorities in a society, can and often do hold mistaken or erroneous beliefs.

Intersubjective confirmation, then, is not simply synonymous with majority consensus. Rather, it refers to confirmation based on careful observation and careful reporting of what has been observed or experienced in language that other people can understand. But even under this standard, there are occasions when individuals have experiences that do not conform to the seemingly careful observations and reports of others, but which nevertheless end up being intersubjectively confirmed at a later date. A good example of this is the theory of continental drift, which was rejected by most geologists in the first half of the twentieth century after Alfred Wegener proposed it but was confirmed by empirical observation and became one of the main bases of the now universally accepted theory of plate tectonics in the second half of the century. Wegener proposed the theory of continental drift based on positive evidence and consistency with (apparent) facts about the natural world.

The foregoing example shows that even if there is no intersubjective confirmation of a proposition, if the other two attributes (positive evidence and consistency with already held apparently justified apparently true beliefs) are present, belief in that proposition can be said to be possibly justified and possibly true at the very least. This means it cannot be dismissed but on the other hand cannot be said to have come to the point of being apparently justified and apparently true.

There are also circumstances in which it is uncertain whether there is intersubjective confirmation—i.e., where multiple people may have had the same experience, but it cannot be established that they did have the same experience with any degree of confidence. Here, there is also possibly justified possibly true belief as long as the other two attributes—positive evidence and consistency with other apparently justified apparently true beliefs—are present.

One problem with intersubjective confirmation and, indeed, with subjectivity in general is that all people have cognitive biases and personal idiosyncrasies that prevent them from being fully reliable when it comes to confirming or disconfirming other people’s experiences. This means that, unfortunately, false positives and false negatives are not only possible but frequently occur even among large groups of people. However, if one isolated person has an experience that contradicts that of large numbers of people, it is likely the isolated person is the one who has experienced the false positive or false negative. Yet this is not guaranteed, and there are plenty of examples from history of individuals who held beliefs that none of their contemporaries held but were later proven right (such as Wegener).

Another key factor in intersubjective confirmation is repeatability. Unless multiple people have the same experience and describe it in similar language, there is no intersubjective confirmation, but obviously multiple people cannot have the same experience unless the experience in question is capable of being repeated for multiple people in the first place. For this reason, experiences that cannot be repeated generally cannot be trusted. However, as discussed in the section on plausibility below, this standard cannot effectively be applied to claims about the past.

The notion of intersubjective confirmation articulated above, as well as the notion of subjectivity it is based on, arguably presupposes the existence of “subjects” who observe the world as entities apart from the observed (or “objective”) world, or that the basic empiricist framework of the foregoing arguments arguably imply this presupposition even if it is not explicitly articulated. It is not my intention to draw a sharp distinction between mind/consciousness and matter/body or subject and object in the manner of Descartes, but the presupposition of this dichotomy tends to underlie Western philosophical and popular discourse in the modern era and so cannot easily be avoided. I will address these issues more thoroughly when I discuss the philosophy of consciousness, but here I will simply state that unfortunately, every epistemology presupposes a certain metaphysic, but it is not possible to posit a metaphysic without first articulating an epistemology. For this reason, many thinkers treat metaphysics and epistemology as inseparable. I do not, but only for the purely pragmatic reason that treating them separately makes my arguments clearer.

PLAUSIBILITY

If intersubjective confirmation were applied universally and without qualification, virtually all claims about the past and all claims about the contents of consciousness that don’t interact with the outer world would have to be considered untrustworthy. Regarding the latter, claims about the contents of consciousness are plausible, and there is no reason to doubt them, if it is theoretically possible to experience them. However, there are no grounds for believing it is possible to form beliefs about the world outside consciousness based on them.

Regarding claims about the past, while it is rarely if ever possible to confirm any claim about the past through present experience, it would be absurd to claim that therefore it is never possible to be apparently justified in believing any claim about the past. To avoid this absurd result, a qualifier is necessary when it comes to historical claims: if a supposed experience of people in the past or a present experience that is similar enough to approximate the past experience could not, under any circumstances, be had today, then there is no valid basis for believing the past experience ever actually occurred or for forming beliefs based on that supposed past experience.

This is a standard of plausibility. If a purported historical event is plausible based on information currently available, that is, based on current apparently justified apparently true beliefs, then one is apparently justified in believing that historical event occurred and in forming beliefs based on that event. This is similar to the requirement of consistency with the apparent laws and attributes of existence but is necessary to apply to purported historical claims because so many such claims contain elements that do not accord with the world as experienced today.

An example of a purported historical event that is not plausible is the New Testament’s claim (found at 1 Corinthians 15:5-8) that 500 witnesses saw Jesus after he allegedly rose from the dead. This claim is implausible because there is no evidence for any circumstances under which this or a similar event, i.e., witnessing somebody who has come back from the dead, could be repeated today. Of course, this example also does not meet the other two requisite factors of apparently justified apparently true belief: (1) one person’s claim that 500 people witnessed something does not qualify as an account of a positive experience without the actual testimony of those alleged witnesses and detailed accounts of what they claim to have experienced and (2) a person coming back from the dead, while common in the New Testament (see, e.g., John 11 and Matthew 27:51-53), is not consistent with the apparent facts of existence.

A good example of a historical event that is plausible because a similar experience is possible today is the death of Socrates by consuming conium maculatum or poison hemlock. If a person consumed a sufficient quantity of poison hemlock today and did not afterwards receive medical treatment, that person would die as Socrates is alleged to have done. Socrates’ death by hemlock is also consistent with the apparent facts of existence—specifically, the fact that poison hemlock is fatal—and was witnessed by multiple individuals who provided actual eyewitness accounts. Even if it possible to call into question whether Socrates ever lived or was killed by poison hemlock based on the reliability of the sources which record this alleged incident (and as with most historical sources, it is), it is not possible to call the plausibility of these claims into question.

LIMITATIONS AND NECESSITY OF THE STANDARD

It is certainly possible that there are aspects of existence which totally escape all current understanding, or that “current understanding” is dead wrong about some things. However, without a standard that requires beliefs to be based on positive experience, consistency with the apparent laws and attributes of existence, and either intersubjective confirmation or plausibility, there is no limit to what it is possible to believe. Without such a limit, all ideas could conceivably be given an equal footing irrespective of how much evidence supports or doesn’t support them. This, in turn, would make it far more difficult to have any basis for forming beliefs about what is true or not true.