7.2.3 Fixing Plato’s Traditional Account of Knowledge

Gettier cases demonstrate that Plato’s traditional account of knowledge as justified true belief is wrong. Specifically, Gettier cases show that a belief being true and justified is not sufficient for that belief to count as knowledge. In all the cases discussed, the subject seems to have a justified true belief but not knowledge. Notice that this does not mean that belief, truth, or justification is not necessary for knowledge. Indeed, when speaking of propositional knowledge, all philosophers grant that belief and truth are necessary conditions for knowledge. A person cannot be said to know a proposition if they do not believe that proposition. And clearly, if a belief is to count as knowledge, then that believe simply cannot be false. Accordingly, attempts to solve the Gettier problem do one of two things: either they replace the justification condition with something more robust, or they add a fourth condition to JTB to make the account sufficient.

No False Premises

In Dharmakīrti’s case, the nomad believes there is water in the valley based on the false belief that a mirage is an oasis. And in Russell’s case, the man bases his true belief about the time on the false belief that the clock he’s looking at is working. In both cases, the inference that leads to the true belief passes through false premises. In response to this fact, American philosopher Gilbert Harman (1928–2021) suggested adding a condition to the JTB account that he termed “no false lemmas” (Harman 1973). A false lemma is a false premise, or step in the reasoning process. Harman’s fourth condition is that a person’s belief cannot be based on an inference that uses false premises. According to Harman, S knows P if and only if (1) P is true, (2) S believes P, (3) S is justified in believing P, and (4) S did not infer P from any falsehoods.

Harman theorized that many counterexamples to the traditional account share a similar feature: the truth of the belief is not appropriately connected to the evidence used to deduce that belief. Going back to Dharmakīrti’s case, what makes the statement “There’s water in the valley” true is the fact that there is water below the surface. However, the nomad comes to believe that there is water based on the mistaken belief that a mirage is an oasis, so what makes the belief true is not connected to the reason the nomad believes it. If Harman’s condition that the reasoning that leads to belief cannot pass through false steps is added, then the nomad’s belief no longer counts as knowledge.

Harman’s emendation explains why the nomad does not have knowledge and accounts for the intuition that the man in Russell’s case does not actually know what time it is. However, this cannot take care of all Gettier cases. Consider the case of Henry in fake barn country. Henry comes to believe he is looking at a barn based on his perceptual experience of the barn in front of him. And Henry does look at a real barn. He does not reason through any false premises, such as “All the structures on my drive are barns.” His inference flows directly from his perceptual experience of a real barn. Yet it is a matter of luck that Henry isn’t looking at one of the many barn facades in the area, so his belief still does not seem to count as knowledge. Because Harman’s account is vulnerable to the barn counterexample, it does not solve the Gettier problem.

Ruling Out Defeaters and Alternatives

While driving through fake barn country, Henry happens to form the belief “That is a barn” when looking at the only real barn in the area. While Henry’s belief is not based on false premises, there still seems to be something wrong with it. Why? The problem is that certain facts about Henry’s environment (that it is filled with barn facades), if known, would undermine his confidence in the belief. That the area is predominantly filled with barn facades is what is known as a defeater because it serves to defeat the justification for his belief. Contemporary American philosophers Keith Lehrer and Thomas Paxson Jr. suggest that justified true belief is knowledge as long as there are no existing defeaters of the belief (Lehrer and Paxson 1969). S has knowledge that P if and only if (1) P is true, (2) S believes P, (3) S is justified in believing P, and (4) there exist no defeaters for P. The added fourth condition means that there cannot exist evidence that, if believed by S, would undermine S’s justification.

The “no defeaters” condition solves all three Gettier cases discussed so far because in each case, there exists evidence that, if possessed by the subject, would undermine their justification. Henry cannot be said to know he’s looking at a barn because of the evidence that most of the barns in the area are fake, and Russell’s man doesn’t know the time because the clock is stopped. The “no defeaters” condition thus helps solve many Gettier cases. However, we now need a thorough account of when evidence counts as a defeater. We are told that a defeater is evidence that would undermine a person’s justification but not how it does this. It cannot be that all evidence that weakens a belief is a defeater because this would make knowledge attainment much more difficult. For many of our justified true beliefs, there exists some evidence that we are unaware of that could weaken our justification. For example, we get many beliefs from other people. Research indicates that people tell an average of one lie per day (DePaulo et al. 1996; Serota, Levine, and Boster 2010). So when someone tells you something in conversation, often it is true that the person has lied once today. Is the evidence that a person has lied once today enough evidence to undermine your justification for believing what they tell you?

Notice that because a defeater is evidence that would undermine a person’s justification, what counts as a defeater depends on what justification is. Of the theories of knowledge examined so far, all of them treat justification as basic. They state that a belief must be justified but not how to measure or determine justification.

The Problem with Justification

The traditional analysis of knowledge explains that knowledge is justified true belief. But even if we accept this definition, we could still wonder whether a true belief is knowledge because we may wonder if it is justified. What counts as justification? Justification is a rather broad concept. Instead of simply stating that justification is necessary for knowledge, perhaps a thorough account of knowledge ought to instead spell out what this means. The next section looks more deeply at how to understand justification and how some theorists suggest replacing the justification condition in order to solve the Gettier problem.