The authors argue that the main purpose of reasoning is for coming up with reasons. Yep, the reason we reason is to produce reasons.
"Why do you think this? Why do you do that. We answer such questions by giving reasons, as if it went without saying that reasons guide our thoughts and actions and hence explain them." (p.109)The authors point out, though again this is not news, that in fact most of our reasons are after-the-fact rationalisations. We decide first, based on criteria we're mostly not even aware of, and then we come up with reasons that we hope make that decision seem reasonable. Reasons are how we explain things to ourselves and others. But on the whole, our reasons are fictions that we make up to explain ourselves to ourselves and the world.
Simplistically, in a court of law, reasons are sought and given and then tested and weighed for veracity. A reason has to be consistent with the physical facts. But it also has to be consistent with the psychological facts, i.e. how the jury think they might act in similar circumstances (for which they ask themselves how the reason feels). If we the jury find the defendant's reasons plausible then they are not guilty. If not then they are guilty and owe us and/or society a debt.
Ask yourself... Why do I believe the things I believe? You've probably got reasons already. But now ask, Why is that reason a justification for believing anything? What is it about the reason that makes your belief reasonable.
For instance, I believe that the UK is probably better off in Europe so I voted to remain in it. The reasons are actually a little vague. I don't like the Tories. I think the world is safer if we work together more closely. But those who voted to leave also had reasons. Maybe their reasons were less vague - the EU is an inefficient bureaucracy, with too many unelected officials making decisions, it costs us too much, it's run by foreigners, it allows too much immigration, and so on.
If reasoning was anything like the classical view of it, then this kind of divided opinion couldn't happen. We'd all weigh up the evidence and decide the most rational course to take, and most of the time there would be broad agreement. But we don't do this.
What we do is have a feeling about it, and then fish about for reasons, which the media provide for us. Or we're confused, then we hear a reason that resonates and stick with that. Which is why when people give reasons for political decisions, they often unconsciously repeat, word for word, a political slogan, like, "I want my country back" (a line uttered in a TV program around the time - but ironically uttered by a spy who was helping the Nazi's subjugate his country).
We're all doing this. Deciding on what feels right, then producing reasons ourselves, or reproducing reasons we've heard from third parties. And since we also accept the myth that reasoning and "being rational" are the highest faculty of humans, we assume that our reasoning must be the best. We think that our reasons are good. And why? Well for reasons. And the criteria for judging those reasons? Well they are also reasons. And so on down into the unconscious functioning of our minds that we cannot yet fathom.
Things happen for a reason. Yeah, right!