You find yourself in the heat of an argument and your mulish interlocutor refuses to see the light. ‘Please,’ you implore, ‘be reasonable.’ But what exactly are you asking? Do you want him to be more rational? Or to act as a typical person might act in his shoes? Maybe the whole question is hopelessly subjective, as your paragon of reasonableness may be his idea of madness, and vice versa. Or else perhaps your request is a mere smokescreen – a sly piece of rhetoric to mask your will to power behind the language of decency?

In a short and deftly argued book, Krista Lawlor, a professor of philosophy at Stanford University, dives into the questions of how to be reasonable and why it matters – in law, in our relationships, in morality and in political life.

In her introduction, she outlines the crux of her thesis:

We often work together, and when we do, we want the people we’re working with to keep an eye on what matters. We want them to accurately track value. People can make mistakes, but being concerned to get it right about value and being reliable in tracking it – that, on my hypothesis, is the heart of reasonableness.

And the qualities of these virtuous, reasonable souls?

Reasonable people are flexible; they are open-minded and ready to listen; they may have firm beliefs, and argue for them, but if they have made a mistake, they will admit it.

Lawlor interrogates her subject across a range of angles. She explores the role it plays in British and American law: ‘In the United States, laws invoking a reasonable person standard govern everything from alcohol sales to traffic law, from breach of contract to stalking.’ She compares the ways in which reasonableness contrasts, despite the surface similarities, with rationality; and how the famed ‘reasonable’ person who so frequently crops up in Anglo-American law overlaps with that other character, Homo economicus, the flattened figure of economics, who is so very keen on, and so astonishingly good at, pursuing his own material interests. And she discusses how beliefs as well as emotions may be reasonable or not (some, such as Schadenfreude, are, we read, always unreasonable).

In an engaging couple of chapters Lawlor considers how a society collectively seeks to map ‘the landscape of value’. This is not just a matter of individuals asserting brute, stubborn likes and dislikes, but of really communicating what, by their lights, is worthwhile and what isn’t, of trying to persuade, and of joining in a collective human project of working out what matters, even amid profound disagreement. De gustibus non est disputandum may make for a catchy slogan, but when it comes to the things you actually care about – whether that means the relative talent of one bassist over another, or whether religious piety is really worth the bother – it does seem a touch defeatist.

The register of the book is unavoidably academic, and occasionally the prose takes on a quality that borders on the comic.  A not untypical example:

Given that there can be reasonable disagreement about value and about the interpretation of our actions, we cannot say that I must feel guilt or compunction for doing what angers you. Your being miffed creates at most a defeasible normative pressure on me to respond with a corresponding reactive emotion.

But it is enlivened by its use of example and anecdote. We have references to Breaking Bad and The Good Place and the fiction of Ursula K. Le Guin. Lawlor discusses the Prisoner’s Dilemma (what should two accomplices do, if separated for questioning and uncertain of the other’s answers?) and the Dictator Game (if player A is given a sum of money which she can split with player B, what ratio is rational and what is reasonable?).

She also outlines a number of legal cases, including Vaughan vs. Menlove from 1837, in which a farmer set fire to his neighbour’s property by failing to stack his hay with sufficient prudence. And the book is bolstered throughout by the ideas of various philosophers, ranging from Seneca to David Hume to Peter Strawson and Judith Jarvis Thomson, with interesting passages on Thomas Scanlon’s ‘contractualism’ and John Rawls’s account of liberalism.

Lawlor finishes with a rather downbeat account of the state of the US – the polarisation, the breakdown in dialogue, the deluge of money into politics. Yet, reflecting on the evolution of our species, she offers solace with the idea that reasonableness may be a uniquely and distinctively human trait:

Getting it right about what matters, caring about others’ perspectives on what’s valuable, helping others steer toward the good things and away from the bad things – engaging in these human endeavours is a special experience.

Which, as a source of hope, seems hardly unreasonable.