45 The good person out of the good treasure of his heart produces good, and the evil person out of his evil treasure produces evil, for out of the abundance of the heart his mouth speaks.
I had a young man ask in Sunday School how could it be true that "out of the abundance of the heart his mouth speaks" when often times people do not say what is on their heart. Often times people say what would gain the right response from someone else. In other words, the speaker a certain response from a listener then seeks to calculate what would be the best way to get that response.
I have been slowly listening to the book, How the Irish Saved Civilization by by Thomas Cahill. One of the chapters is on Augustine. One the ideas he puts forth is that Augustine is the first writers to say "I" and mean what we mean today by that word. He speaks from the heart. Writers and orators before Augustine calculated and then used the art of rhetoric to sway their listeners. In fact he says in the anceint Greek world, it was the obligation of each man to contribute to the democratic process.
Many people do seek to calculate much more than be authentic. But even so, a wise person can read their tone and selection of words to determine whether they are sincere. A speaker usually feels compelled at some level to be genuine even when they seek to sway instead of share.