I have been told that knowledge is usually analyzed as being justified true belief (although this conception has been criticized - namely after Gettier published his famous article - it seems to be widely accepted, at least as a good approximation of what knowledge really is). However, I don't see why the truth condition is necessary. Indeed, by definition, we can only evaluate the truth value of a proposition by making appeal to our justification for this proposition. Moreover, believing some proposition is believing this proposition to be true. When I say "I know x", it seems therefore to me that I don't say anything else than "I believe x and I am justified in believing x", with the truth condition of the tripartite definition of knowledge encompassed by the belief and justification conditions. Can someone explain me then why knowledge is not defined as just "justified belief" ? Many thanks in advance.
Truth is required because you cannot know falsehoods. If Bob is justified in believing that his package was lost in the mail, because it's been 2 months already and it hasn't arrived, but the package is in fact now waiting outside his door, would you say that he knows that his package was lost? I think not. Bob may think that he knows that the package was lost, but he is mistaken.
If you have justified but false belief that p, you may think you know p, but you don't. The truth condition filters out such cases as not genuine instances of knowledge
Here's another way to see this. The sentence "Alice believes that today is Sunday, and Bob believes that today is Monday" can be true. But "Alice knows that today is Sunday and Bob knows that today is Monday" seems contradictory. That's because "S knows that p" entails that p is true. So it's not possible for someone to know p, and for someone else to know not-p.
The standard response has it that justified belief falls short of knowledge because you could be wrong anyway, and you cannot be said to know what is in fact false (even if the latter is behind the veil of ignorance) without stretch. (Your self-ascription is predicated upon JTB, by the way, to the extent that "I believe x and I am justified in believing x" precludes "but x is false".)
We want to distinguish between our use of "know" as when people say, "Oh, I just KNOW it's true," when a missing person is feared for (for example) from when they use the concept of knowledge as dependent on actual truth instead of desired (or perhaps ideological) truth, and use this to actually reach out publicly to either share knowledge or apply it. There is a generic strength of belief common to both uses of the word/concept, because some beliefs are strongly held on feeling, others due to epistemic reasons. These latter are at least, then, epistemic ideals. The case where someone says they know, when not only is their belief false but not even sufficiently justified, is like the empty case of the strength conditions.
For example, strong ideologies are grounded in the pure strength of deductive inference, in the wish to unite all explanations in one ultimate rule, often from some incredible premise (e.g. the Nazi idea of being hereditarily destined to make the greatest scientific and technological progress, a belief that they "knew" was true). But our questions are not asked like that; there is no magic moment when one answer is literally that ultimate. Our knowledge is beholden to truth enough for us to emphasize the aim for truth in statements of knowledge, even in the wishful cases (we do wish we knew, and that what we wish overall is true...).
Truth is what is real. If something affects everyone in some predictable way, then it's real. A statement that is consistent with reality is objectively true. Otherwise it is objectively false.
A person may (often does) hold many beliefs that are objectively true, and others that are objectively false.
Rationality means ability to explain your beliefs based on the first paragraph. Being able to explain a belief is what makes it justifiable.
Being fully rational, only holding the explainable beliefs, their explanation based on everyone sharing the same objective reality, the same universal truth, makes for self-actualization. For a sagehood, being enlightened human we all It makes for a sagehood, it makes for a human that everyone of us should have been -- kind, understanding, and treating everyone with compassion, dignity and respect.