This is not an easily read book. It assumes quite some previous knowledge about political philosophy on the part of the reader, and it occasionally formalizes its arguments that perhaps makes them more strict, but also harder to follow.
That said, its investigation and critique of "ideal theory", the notion that political and moral philosophy should attempt to find a blueprint or a set of criteria for a just society, is convincing. The main target of criticism is John Rawls and his theory about Justice as Fairness. Starting from an argument by Amartya Sen, Gaus claims that an ideal theory must contain some kind of orientation aspect, i.e. a statement on in which direction, so to speak, a just society lies. This statement must be something other than just saying that we given our starting state should move to the next available better state. Gaus argues that any ideal theory faces what he calls "The Choice", that is, there must be occasions where the ideal theory concludes that we should not move to the next available better state, because that would bring us further from the ideal state that is has previously identified. Gaus doesn't say so, but this actually boils down to the statement by Lenin that one must break eggs to make an omelet. Which highlights the problem with "The Choice".
Gaus goes on to argue that ideal theory has an additional difficult problem if the state that it identifies as ideal is very different from anything we know. If there is something that has been learnt over the last 200 years or so, it is that attempts at achieving some perfect ideal society inevitably goes wrong.
Gaus discusses the problems with the Open Society, drawing on arguments by Karl Popper and Friedrich Hayek, among others. He states that we should welcome the different views on justice that an Open Society brings. Different perspectives can search, share, debate and dismiss each other's insights while engaging in cooperative social relations. My concern here is the difficulty in maintaining those cooperative social relations. The last 10 years or so have seen an erosion in social relations in several Western democracies. This book, written before 2016, is, I fear, a little too optimistic.
In conclusion, there are many interesting arguments in this book, but it does not really add up to a standalone philosophy. His critique of political philosophy in general is at times ascerbic, but I must say I agree with him. Maybe Gerald Gaus last book, The Open Society and Its Complexities, will bring more to the table. I will read it next.