Sources of Knowledge and Justification
Beliefs arise in people for a wide variety of causes including psychological factors such as desires, emotional needs, prejudice, biases of various kinds, etc. Obviously, if beliefs originate in sources like these, they don’t qualify as knowledge even if true. For true beliefs to count as knowledge it is necessary that they originate in sources we have good reason to consider reliable: these are perception, introspection, memory, reason, and testimony.
Perception
Our perceptual faculties are our five senses: sight, touch, hearing, smell, and taste. We must distinguish between an experience that can be classified as perceiving that p, which entails that p is true and a perceptual experience as perceptual seeming. The reason for making this distinction lies in the fact that perceptual experience is fallible, for the world may not always be as it appears to us in our perceptual experiences. We need therefore a way to referring to perceptual experiences in which p seems to be the case that allows for the possibility of p being false. That’s the role assigned to perceptual seemings. So, some perceptual seemings that p are cases of perceiving that p, others are not. When it looks to you that there is a cup of coffee on your desk and in fact there is then the two states coincide. If, however you hallucinate that there is a cup on the table, you have perceptual seeming that p without perceiving that p. One family of epistemological issues about perception arises when we concern ourselves with the psychological nature of the perceptual processes through which we acquire knowledge of external objects. According to direct realism, we can acquire such knowledge because we can directly perceive such objects so that what you perceive is the thing in itself. According to indirect realism, we acquire knowledge of external objects by virtue of perceiving something else, namely appearances of sense data. An indirect realist would say it is not the thing in itself that we perceive but a thing-like sense-datum or some such entity. Direct and Indirect Realists hold different views regarding the structure of perceptual knowledge. Indirect Realists would say that we acquire perceptual knowledge of external objects by virtue of perceiving sense-data that represent external objects. Sense-data, a species of mental states, enjoy a special status – we know directly what they are like. So, Indirect Realists think that when perceptual knowledge is foundational, it is knowledge of sense-data and other mental states. Knowledge of external objects is indirect and derived from our knowledge of sense data. The basic idea is that we have indirect knowledge of the external world because we can have foundational knowledge of our own mind.
Are Our Perceptual Faculties Reliable?
We take our perceptual faculties to be reliable. But how can we know that they are reliable? For externalists, this might not be much of a challenge. If the use of reliable faculties is sufficient for knowledge, and if by using reliable faculties we acquire the belief that our faculties are reliable, then we come to know that our faculties are reliable. But even externalists might wonder how they can, via argument, show that our perceptual faculties are reliable. The problem is this. It would seem the only way of acquiring knowledge about the reliability of our perceptual faculties is through memory, through remembering whether they served us well in the past. But should I trust my memory, and should I think that the episodes of perceptual success that I seem to recall were in fact episodes of perceptual success? If I am entitled to answer these questions with ‘yes', then I need to have, to begin with, reason to view my memory and my perceptual experiences as reliable. It would seem, therefore, that there is no non-circular way of arguing for the reliability of one's perceptual faculties.
Introspection
Introspection is the capacity to inspect the, metaphorically speaking, “inside” of one’s mind. Through introspection, one knows what mental states one is in. Compared with perception, introspection appears to have a special status. It is easy to see how a perceptual seeming can go wrong. Could it be possible that it introspectively seems to me that I have a headache when in fact I do not? It is not easy to see how it could be. Thus compared to perception, introspection seems to be privileged by virtue of it being less prone to error. How can we account for the special status of introspection?
First, it could be argued that when it comes to introspection, there is no difference between appearance and reality. Therefore, introspective seemings are necessarily successful introspections. According to this approach introspection is infallible.
Alternatively, one could view introspection as a source of certainty. Here the idea is that an introspective experience of p eliminates all possible doubt as to whether p is true.
Finally, one could attempt to explain the special status of introspection by examining the way we respond to first person reports: typically, we attribute a special authority to such reports. According to this approach, introspection is incorrigible. Others are not (or at least not typically) in a position to correct first person reports of one’s own mental states.
Introspection reveals how the world appears to us in our perceptual experiences. For that reason, introspection has been a special interest to foundationalists.
Perception is not immune to error. If certainty consists in the absence of all possible doubt, then perception fails to yield certainty. Hence beliefs based on perceptual experiences cannot be foundational. Introspection however might deliver what we need to find a firm foundation for our beliefs about external objects: at best outright immunity to error or all possible doubt, or perhaps more modestly an epistemic kind of directness that cannot be found in perception.
Memory
Memory is the capacity to retain knowledge acquired in the past. What one remembers though need not be a past event. Memory is of course fallible. Not every instance of taking oneself to remember that p is an instance of actually remembering that p. We should distinguish therefore between remembering that p (which entails the truth that p) and seeming to remember p (which does not entail the truth that p).
One issue about memory concerns the question of what distinguishes memorial seemings from perceptual seemings or mere imagination. The distinctively epistemological questions about memory are these: First, what makes memorial seemings a source of justification? Is it a necessary truth that, if one has a memorial seeming that p, one has thereby prima facie justification for p? Or is memory a source of justification only if, as coherentists might say, one has reason to think that one's memory is reliable? Or is memory a source of justification only if, as externalists would say, it is in fact reliable? Second, how can we respond to skepticism about knowledge of the past? Memorial seemings of the past do not guarantee that the past is what we take it to be. We think that we are a bit older than just five minutes, but it is logically possible that the world sprang into existence just five minutes ago, complete with our dispositions to have memorial seemings of a more distant past and items such as apparent fossils that suggest a past going back millions of years. Our seeming to remember that the world is older than a mere five minutes does not entail, therefore, that it really is. Why, then, should we think that memory is a source of knowledge about the past?
Reason
Some beliefs would appear to be justified solely by the use of reason. Justification of that kind is said to be a priori: prior to any kind of experience. A standard way of defining a priori justification goes as follows:
A Priori Justification
S is justified a priori in believing that p if and only if S's justification for believing that p does not depend on any experience. Beliefs that are true and justified in this way (and not somehow "Gettiered") would count as instances of a priori knowledge.
What exactly counts as experience? If by ‘experience’ we mean just perceptual experiences, justification deriving from introspective or memorial experiences would count as a priori. For example, I could then know a priori that I'm thirsty, or what I ate for breakfast this morning. While the term ‘a priori’ is sometimes used in this way, the strict use of the term restricts a priori justification to justification derived solely from the use of reason. According to this usage, the word ‘experiences' in the definition above includes perceptual, introspective, and memorial experiences alike. On this narrower understanding, paradigm examples of what I can know on the basis of a priori justification are conceptual truths (such as "All bachelors are unmarried"), and truths of mathematics, geometry and logic.
Justification and knowledge that is not a priori is called ‘a posteriori’ or ‘empirical’. For example, in the narrow sense of ‘a priori’, whether I'm thirsty or not is something I know empirically (on the basis of introspective experiences), whereas I know a priori that 12 divided by 3 is 4.
Several important issues arise about a priori knowledge. First, does it exist at all? Skeptics about apriority deny its existence. They don't mean to say that we have no knowledge of mathematics, geometry, logic, and conceptual truths. Rather, what they claim is that all such knowledge is empirical.
Second, if a priori justification is possible, exactly how does it come about? What makes a belief such as "All bachelors are unmarried" justified solely on the basis of reason? Is it an unmediated grasp of the truth of this proposition? Or does it consist of grasping that the proposition is necessarily true? Or is it the purely intellectual experience of "seeing" (with the "eye of reason") or "intuiting" that this proposition is true (or necessarily true)? Or is it, as externalists would suggest, the reliability of the cognitive process by which we come to recognize the truth of such a proposition?
Third, if a priori knowledge exists, what is its extent? Empiricists have argued that a priori knowledge is limited to the realm of the analytic, consisting of propositions of a somehow inferior status because they are not really "about the world". Propositions of a superior status, which convey genuine information about world, are labeled synthetic. a priori knowledge of synthetic propositions, empiricists would say, is not possible. Rationalists deny this. They would say that a proposition such as "If a ball is green all over, then it doesn't have black spots" is synthetic and knowable a priori.
A fourth question about the nature of a priori knowledge concerns the distinction between necessary and contingent truths. The received view is that whatever is known a priori is necessarily true, but there are epistemologists who disagree with that.
Testimony
Testimony differs from the sources we considered above because it isn't distinguished by having its own cognitive faculty. Rather, to acquire knowledge of p through testimony is to come to know that p on the basis of someone's saying that p. "Saying that p" must be understood broadly, as including ordinary utterances in daily life, postings by bloggers on their web-logs, articles by journalists, delivery of information on television, radio, tapes, books, and other media. So, when you ask the person next to you what time it is, and she tells you, and you thereby come to know what time it is, that's an example of coming to know something on the basis of testimony.
The epistemological puzzle testimony raises is this: Why is testimony a source of knowledge? An externalist might say that testimony is a source of knowledge if and only if it comes from a reliable source. But here, even more so than in the case of our faculties, internalists will not find that answer satisfactory. Suppose you hear someone saying ‘p’. Suppose further that person is in fact utterly reliable with regard to the question of whether p is the case or not. Finally, suppose you have no evidential clue whatever as to that person's reliability. Wouldn't it be plausible to conclude that, since that person's reliability is unknown to you, that person's saying ‘p’ does not put you in a position to know that p? But if the reliability of a testimonial source is not sufficient for making it a source of knowledge, what else is needed? Thomas Reid suggested that, by our very nature, we accept testimonial sources as reliable and tend to attribute credibility to them unless we encounter special contrary reasons. But that's merely a statement of the attitude we in fact take toward testimony. What is it that makes that attitude reasonable? It could be argued that, in one's own personal experiences with testimonial sources, one has accumulated a long track record that can be taken as a sign of reliability. However, when we think of the sheer breadth of the knowledge we derive from testimony, one wonders whether one's personal experiences constitute some evidence base rich enough to justify the attribution of reliability to the totality of the testimonial sources one tends to trust. An alternative to the track record approach would be to declare it a necessary truth that trust in testimonial sources is justified. This suggestion, alas, encounters the same difficulty as the externalist approach to testimony: it does not seem we can acquire knowledge from sources the reliability of which is utterly unknown to us.