I had heard that the Bible advises keeping one's utterances to "yea" or "nay". I thought the advice was about not going on and on, about limiting one's words, about being taciturn. I looked up Matthew 5:37, KJV, and found that it was actually about attempts to counter fake news and lying.
When I tell you that my barn was struck by lightning, you may doubt that I am telling the truth. Of course, philosophically, the truth, the whole truth and nothing but the truth can be tricky to recognize. So, in an attempt to assure you of my truth-telling, I might say that what I have stated is true and that I swear to its truth by my life, by my mother's life, by something I value and hold dear. The passage in Matthew advises instead to just say my truth and not try to be more persuasive. Just state the facts and don't try buttressing the statement with an oath.
I think this passage could be referred to as a Biblical version of fake-news-fighting. The advice is to simply state one's message and let others deal with the truthiness of one's statement. I guess if you doubt my statement, you had better take a quick look at my face and see if I look truthful. You could drive out to my place and see if my barn looks like it has recently been struck by lightning. You could call my wife or my neighbor and try to get verification. You could just accept my statement and try to sympathize with a guy whose barn was struck by lightning. You could feel gratitude that your own barn is in such good shape.
