Ever since the time of Plato, the traditional "Tri-Partite" or "Justified True Belief" (JTB) theory has been the standard starting point for
almost all discussions of knowledge. The traditional model posits that --
(JTB) S knows that P iff (a) P is true; and [Truth Condition]
(b) S believes that P; and [Belief Condition]
(c) S is justified in believing that P. [Justification Condition]
I will therefore assume this traditional model as the background for the essay's title question. Given this context, the question is inquiring whether the truth condition of the JTB model of knowledge is really as necessary as is traditionally assumed.
In order to answer this question we need
to consider the ramifications of a theory of knowledge that omits this truth
condition. For this hypothetical model of knowledge we would then have --
(K1) S knows that P iff (a) S believes that P; and
(b) S is justified in believing that P.
Given the context of an investigation into the necessity of the truth condition, we can safely ignore, at least for the present, any questions relating to S believing that P, or the detailed nature of the "justification" involved. However, because we are ignoring the details, it does need to be made clear that the following discussion will assume that those details of justification that we are passing over are not so stringent that it is not logically possible for a claim to be suitably justified and yet false. There are some approaches to justification that demand just such infallibility. If one does choose to adopt an "infalliblist" model of justification, then the JTB and the K1 models are logically equivalent, since "proper" justification for the belief that P would guarantee the truth of P. For the purposes of this essay, therefore, I will assume that the justification involved in both the JTB model and the K1 model are "fallibilist". Otherwise, the essay's title question becomes moot.
I also need to add a brief introductory comment on the impact of truth anti-realism to this discussion. The JTB model's truth condition is usually understood from a truth-realist perspective, where the truth of P is evidence transcendent in an absolute sense. However, the position of truth-anti-realism is that there is no such thing as evidence transcendent truth. The anti-realist truth-status of P is determined by the evidence. If P is beyond any evidence then its truth-status is undefined. This might initially appear to make the truth-condition of the JTB model superfluous, with the presumption that the evidence that dictates the anti-realist truth-status of P would be properly contained within the details of the justification condition. But this is generally not so. Both the K1 and the JTB models of knowledge are specific to a single individual's claim to knowledge. Only the most subjectivist (solipsistic?) of the truth-anti-realist theories would maintain that it is each individual's evidence that dictates the truth-status of P. The rest of the truth-anti-realist theories stipulate that it is the coherence of the evidence available (in practice, or in principle, or at some limit) to some relevant population that dictates the truth-status of P. So only for the most subjectivist of truth-anti-realists would K1 be logically equivalent to JTB. Therefore, for the purposes of this essay, I will treat the truth condition in a manner neutral between truth-realism and truth-anti-realism, while specifically excluding extreme subjectivist truth-anti-realism. Otherwise, as above, the essay's title question becomes moot.
Given that clarification of context, the first thing notable about K1 as a theory of knowledge is that it perfectly captures the point of view of S when S makes a claim to know that P. It is generally assumed within the JTB theory of knowledge that whether P is true or not is evidence transcendent (at least with respect to the evidence available to S then and there). In other words, in the JTB model of knowledge, the epistemological status of S with regards to P does not determine the truth status of P. Therefore, from the perspective of S, all that S has epistemologically available is the belief that P, and any justifying rationale that P. On that basis alone is based any claim of S to know that P. The actual truth-status of P is not available to S. If this is the case from the perspective of S, what purpose does the truth-condition serve? What function, if any, does the truth condition contribute to S's understanding of "knowledge" that is not satisfied by K1?
Suppose Alice tells me she knows that Bob is in the kitchen. From that and K1, I can infer that Alice believes that Bob is in the kitchen, and the Alice has judged that she has adequate justification for that belief to qualify as knowledge. From the JTB model I could also infer that it is true that Bob is in the kitchen. But what additional information have I gained from that last inference? Especially when I must consider the fact that whether or not Bob is in truth in the kitchen is an evidence transcendent fact for me as well as Alice. Alice could believe Bob is in the kitchen, and (in her judgement) have thoroughly adequate justification for that belief, and yet never-the-less be wrong about it. And so could I, even if I went and looked. So by either model of knowledge, if she tells me Bob is in the kitchen, I am in exactly the same predicament. I can only claim to know that Bob is in the kitchen if I trust Alice's judgement about her own justification. In a single person scenario, therefore, from the perspective of S, the truth condition adds nothing meaningful to our understanding of knowledge. I can properly claim to know only what I believe, and judge that I have adequate justification for believing. I, myself have no access to the truth-status of P. Likewise, in a two person scenario, the truth condition also adds nothing meaningful to our understanding. I am fully aware that Alice can properly claim to know only what she believes, and judges that she has adequate justification for believing. Neither of us have access to the truth-status of P.
Which brings us to the second thing notable about K1 as a theory of knowledge. It is a purely internalist theory. Only Alice can tell whether or not she believes that Bob is in the kitchen. All I can do is observe her behaviour and speech. I can infer from these the likelihood that she believes what she tells me, but it remains possible that she is intentionally misleading me. I have no basis of authority to challenge her claim to believe that Bob is in the kitchen. Similarly, only Alice can judge whether or not her justification for a claim to knowledge is sufficient or not. I am not (under normal circumstances) party to the details that she uses to justify her belief. I cannot, for example, see Bob in the kitchen from my location, and have only her claim to justify a belief that she can and does see Bob in the kitchen. So I am forced to take any claim by Alice to know something at face value. I have no basis of authority from which to challenge the adequacy of her justification. By K1, therefore, any claim of S to know that P is necessarily infallible and unchallengeable. Now, of course, this is not completely absolute. Depending on those passed over details on the nature of justification, it remains quite possible that for a K1 claim to knowledge to be "properly" justified, Alice must be able to offer supporting rationales that I can examine and challenge. But again, depending on those details, not necessarily. If her supporting justification is perceptual evidence, for example, not being able to share her perceptions I cannot challenge her perceptual judgements. Barring a lengthy (and energy expensive) conversational exploration of her supporting rationales and criteria of judgement, therefore, I am likely going to be forced to take her word that she has the necessary sufficiency of justification. I am not normally going to be in a position to challenge her claim. Which means that, in general, any claim of S to know that P must be considered infallible and unchallengeable -- in practice at least, if not in principle.
In order to address the (more or less) infallible nature of K1 claims to knowledge, what the JTB model adds over the K1 model is a thoroughly "externalist" condition on S's claim to know that P. By design, it is not a condition that S has any access to. But it is a condition that the rest of the population can impose on S. This externalist nature of the truth condition only starts to make a contribution to matters when claims to knowledge are considered in multi-party over-time scenarios. The truth-status of P, while evidence-transcendent for any one party, is asymptotically approachable by a multitude over time (in either truth-realist or truth-anti-realist terms). The truth-condition of the JTB model is intended to capture this asymptotically approachable limit. The difference between Alice's claim to know that Bob is in the kitchen therefore takes on greater significance when considered under JTB compared to K1. Under JTB, Alice's claim is not just that she has judged her justification sufficient for a claim to knowledge, but she has also warranted that as the asymptotic limit of evidence is approached by a relevant population over time, she will not be proved wrong. This is the key additional feature that the JTB model's truth condition adds over the K1 model -- a warrant (an assurance) that despite the fallibility of justification, the claim is none the less true, and will never be proved wrong.
While this additional warrant contributes little of meaning in one or two party single interactions, it becomes significant under more populous and interaction-iterative scenarios. Because under these extended conditions, it is not just the justification of the knowledge claimant that matters, it is the coherence of the entire accumulated body of knowledge of the population involved. In other words, in the case of the JTB model, not only must S believe that P, and have personally adequate justification to qualify that belief as knowledge, but that belief must properly cohere with the entire body of knowledge of the relevant population. This is a significant addition because it expands the scope of justification from the claimant's personal judgement, to the entire population's collective judgement. By thus raising the bar of justification, it makes more likely that S is not ultimately proved wrong in his claim that P. And that is where the truth-condition pays its way.
In the normal course of events, it might not make much difference whether Alice is ultimately proved wrong in her claim to know that Bob is in the kitchen. But if Alice is claiming to know behind which bush lurks the tiger, it becomes of vital importance to the tiger's intended lunch whether she is ultimately proved wrong in her claim or not. Under such evolutionary survival conditions, the extra warrant that a JTB claim to knowledge offers, over a less assured K1 claim, is significant and worth having. Under a K1 claim to knowledge, no one else who might have some input into the position of the tiger can properly raise any timely objections to Alice's claim to know where it lurks. Under a K1 model of knowledge, Alice believes the tiger is over there, and she judges that she has sufficient justification to qualify that belief as knowledge. No one else can challenge her claim without a time and energy expensive investigation of her rationales and judgement criteria. But under a JTB claim to knowledge, if anyone within influence of her claim has any contradictory information, they can immediately (and much less expensively) challenge her claim to know where the tiger lurks. If my survival depends on the validity of that claim, if I am the tiger's intended lunch, then I want the additional assurance that the JTB model of knowledge offers. I want the expectation that others will feel free to judge the accuracy of Alice's claim to know where the tiger lurks.
The difference between the K1 and JTB models of knowledge is sufficiently meaningful that the English language has provided separate words for what they define. The JTB model defines what we call "knowledge", and the K1 model defines what we call "opinion". Without the truth-condition, K1 claims are reduced to opinions. One's opinions are one's opinions, infallible and unchallengeable, and whether they are true or not is largely irrelevant. One cannot properly challenge another's opinions as invalid, improperly formed, or inadequately justified. One can only challenge them as being right (true) or wrong (not-true). And that, of course, is an appeal to the truth-condition. If our opinions are right (i.e. true), and suitably justified, then we elevate their status to that of "knowledge". We can conclude therefore, that (with the exception of extreme subjectivist truth-anti-realists) the truth of P (in either truth-realist or truth-anti-realist terms) is indeed necessary for S to know that P. A claim to know that P is a freely challengeable warrant that P will not ultimately be proved not-true.