You are an intentional agent in the world. You have desires that motivate you to act so as to create or preserve states of affairs in which the things that you desire are realized.
You are surrounded by other intentional agents with their own desires.
Furthermore, your “desire that P” does not, in itself, motivate anybody but you to realize states in which P is true.
How can you get others to act so as to realize states in which P is true or, at least, refrain from acting so as to realize states in which P is false?
I have already discussed two options.
(1) Bargaining. “If you act so as to realize P, then I will act so as to realize Q.” You give the other person an instrumental reason to realize P – as a means to realizing Q.
(2) Threats. "Unless you act so as to realize P, I will act so as to realize not-Q."
In this post, I want to introduce another fact about intentional agents and discuss another way in which you might get them to realize P.
This fact us that, while desires motivate agents to realize states in which those desires are objectively fulfilled, their actions are mediated by their beliefs. If an agent is thirsty and believes that a pitcher contains clean, cool water, he will drink the water - perhaps discovering after the fact that his belief was mistaken.
So, as an intentional agent with a desire that P, one way you can get another intentional agent with a desire that Q to act to realize P is by altering his beliefs. If, given his beliefs, an act that realizes P will realize Q, he now has a motivating reason to perform the act that will realize P on his way to realizing Q.
This phrase, "reason to act so as to realize P" is intentionally ambiguous. He may be caused to intentionally realize P as a means to realizing Q. Or he may be caused to act in ways that realize P as an unintended side effect or realizing Q. Your desire that P is only concerned with the realization of P, not with how it is done (unless that is a part of P).
So, if you can convince another agent with a desire that Q to believe T, he will act so as to realize P. If you convince him to believe not-T, he will not act so as to realize P. Given these facts, in this simple model, your only motivation is to convince that agent to believe T. That is the only option that will realize P.
Is T true?
You have no motivation to even ask, let alone answer, that question in this simple model. The other agent's motivation to act depends only on believing T, not on whether T is true.
What is true of you in this case is true of every other agent out there giving you information. If convincing you to believe V will cause you to act so as to realize Q, then that other agent has a motivating reason to cause you to believe V. He has no reason at all to refrain from convincing you to believe V based on the fact that V is false - not unless this is built into Q or that agent has some other motivating reason to refrain from convincing others of falsehoods.
That other agent may have no motivating reason to consider the truth value of V in getting you to believe V, but you do. Your desire that P gives you a motivating reason to realize P. You use your beliefs to choose those actions (and inactions) most likely to realize P. You will act as if your beliefs are true. When they are not true, this will likely have an adverse effect on your ability to predict accurately. You might choose the action that realizes not-P.
You are thirsty. There is a glass of what you believe is clean, cool water free for the taking in the server tray. You drink from the glass. You are mistaken; it is not clean water. You end up being violently ill. If you had known that in advance, you would have never drank from the glass. Your lack of true and relevant beliefs would cause you to act in a way that you would not have acted if your relevant beliefs were true and complete. And, of the two actions, the one grounded in false beliefs was mistaken.
Your choice of actions that (you predict) will realize P will almost always (though, in important cases, not always) depend on having true beliefs. So, while that hypothetical other agent only has motivating reason to convince you of what will cause you to realize Q - whether true or false, you have a motivating reason to be convinced only of that which is true.
And while you only have a motivating reason to convince others of that which will cause them to realize P, they have motivating reasons to be convinced only of that which is true.
Now, while other people are motivated to tell you that which will help to objectively satisfy their desire that Q, what if their desires includes a particularly strong aversion to making false claims?
If such a person existed, that person would refrain from providing you with false information - or, at least, with information he thought to be false - even when he would otherwise benefit. Perhaps, when put up against something like his own aversion to death or the well-being of his child, he may be motivated to lie. However, where his aversion to lying is strong, it would take something like this to get him to lie.
Even here, a particularly strong aversion to lying would serve as a particularly strong motivation to find some other way - any way - to accomplish the same end without the lie. Here, too, the motivation is the same as that a person with a strong aversion to pain would have to find some option that promised not to involve pain before reluctantly settling on an option that does.
The instrumental value of identifying those with an aversion to lying would motivate agents to adopt methods that reliably identify agents as having or lacking this aversion to making false claims. This might include working with others to identify those who lack this aversion - perhaps identifying them as "liars".
And, if methods existed (e.g., by using the reward-learning system) to promote or strengthen this aversion to lying, your motivating reasons to acquire accurate information suggests that you employ these methods - and negotiate with others having the same interest - to promote this aversion to lying. A social institution for encouraging this aversion to lying, identifying those who do not have it, and labeling them publicly, could well be mutually beneficial.