Let's think about how we come to know things: we know that the world population has reached around 8 billion people, that the USA is around 28 times bigger than Germany and that human beings evolved out of monkeys. We know this because we've read it, learned it in school by studying long established course materials or because somebody told us so.
Transferring knowledge by having conversations, i.e. telling somebody something and getting things told by others, is one of the most natural thing mature users of language engage in every day. If you tell me that your mom lies in hospital for one week and I don't have a reason to expect you lying to me, I take myself to know where your mom spends her evening. If I tell you I'm still suffering from the break up from last year, you can count on knowing about my feelings towards my ex.
So as a hearer getting told something by a speaker makes the hearer having acquainted knowledge about something. From a philosophical point of view, there are countless questions regarding this hypothesis. Who is the hearer? The one who's addressed by the speaker or just any random person who listens to the conversation? When is something a telling? Is the same sentence expressed in one's sleep and fully conscious the same kind of speech act? Where is the difference, if any? What is it to know something, what are the conditions for knowledge and what to we don't know if somebody lies at us? The list could go on endlessly and it is for this reason, that philosophical stances on this specific speech act and the epistemology behind it express broadly different views on how is it to acquire knowledge through the exchange of words.
Richard Moran (2018) attended himself to this rather complicated and analytically demanding issue of the act of telling in his work The Exchange of Words: Speech, Testimony and Intersubjectivity. Broadly speaking, his work is a stretched argument for the specific speech act of telling being a fundamentally social act requiring two intelligent beings capable of language in order to be such, namely a meaningful conversation or utterances in the sense of testimony. In that, he is arguing against those for whom language and specifically telling is merely a vehicle in order to get to the stuff we are actually interested in, namely the state of minds of our opponents. If it was possible to know about the minds of others directly and without the potentially misleading way of language, we would be better off. But according to Moran, this view gets the whole sense of the act of telling completely wrong. In telling somebody something, one does so much more than just offering her opponent one's private state of mind. It is a different thing to let somebody observe what one thinks or holds for true than intentionally directing one's state of mind to someone.
As we've seen, there are countless questions to ask when talking about testimony or the exchange of words by people telling others something. What I am mostly concerned with thereby is the role of sincerity in this speech act which is so familiar to us. What does sincerity do in telling, why is it so important to us and what does our social nature have to do with all this?
The general philosophical view on the role of sincerity in speech acts like telling is that of guaranteeing to get access to the speakers state of mind. Sincerity is needed as we are not in the position to telepathically read of others minds what they think, like and know. So the reason why I am sincerely talking to you, is because I want to share the content of my thoughts with you. Seems kind of right, but doesn't really catch what telling somebody something really is, right?
At least Moran thinks so: On this picture, there would hardly be a difference between me showing you a photograph of my mother lying in the hospital bed and me telling you that my mom will spend the next week in hospital. But there is. And this is why:
The role of sincerity in telling is not only that of a guarantee for access to the speakers mind, but more and less than that at the same time.
In telling her hearer something, the speaker acts in some way, namely, as an informant. That is: she addresses her hearer with the words she directs to him, she frames her words as being true, she presents herself as being responsible for the truth of what she told and finally, she acts as a reason to believe her herself. Because she produces her telling intentional on so many levels and in that, suggests the hearer her words being true as well as herself responsible for their truth, the hearer can take the fact that she intentionally told him so or a reason to believe the speaker.
In that, speaker and hearer are engaging in a social act and more specifically, a social act of expression: the speaker may express the content of her telling in other ways (e.g. by producing a photograph, by painting a picture, by taking the hearer to a specific place and show him sth. etc.). But only in the case of telling him the information she wants him to know, the hearer is not let alone with drawing conclusions from what he perceived, but is led by the speakers responsibility she instantly produced when she told him the thing. The speaker does so, because she knows that the hearer knows that and what she is responsible for, when she tells him something.
But there is another side of sincere speech: one can lie about one's real state of mind and still can to that sincerely. Why is that? Imagine a person (I'm sure the hypothesis still works for everyone of us imagining oneself) responding to the question: "Are you a sexist?" "No! Absolutely not. I don't follow sexist opinions, values or anything like that. I consider myself an anti-sexist." The person really considers herself to be an anti-sexist, she talks sincerely when giving that answer. But the fact that we not know ourselves ever fully is the reason for sincerity's fallibility. Maybe, deep down the person does actually have sexist opinions, values or attitudes. Maybe her being sexist is suppressed or she is not even educated enough when it comes to sexism and doesn't even know she's living with quite a sexist mindset. This is why she could tell sincerely being an anti-sexist but at the same time being lying about her real state of mind. In this sense, sincerity does less than guaranteeing the hearer with a direct access to the speakers mind.
But: would we still say the sexist who thinks of herself being an anti-sexist is talking insincere? I think I wouldn't. In the end, all sincerity can do is warranting for the potentially limited self-knowledge we all have and by expressing what I take to be true, I speak as sincere as I can. So, expressing one's real state of mind is neither sufficient nor necessary for sincerity. Sometimes, what I take to be true even might be so, and sometimes I learn that it wasn't.