One kind of truth
Quakers have declined to swear oaths for more than three centuries, on the strength of 'let your yea be yea, and your nay, nay.' The reasoning is simple: swearing to tell the truth on special occasions implies a lower standard the rest of the time. So when I was a witness in a small claims case last month, I affirmed instead.
The clerk didn't blink. But I've been thinking about it since. Plain speech isn't about being blunt, or about thee and thou (I only use those with my sister, mostly to annoy her). It's about having one kind of truth, for the courtroom and the garden gate and the email to the plumber.
I fail at it daily. 'Fine, thanks' is my most frequent untruth. But I'm practising.