Perhaps S believes x and Q believes ~x. They wildly disagree over the dinner table. So Q writes an article in HP (the major competitor to HQ) arguing to the effect that S has a right to believe x but the law of the land should not reflect x but instead ~x. Q thinks that if the law according to x is applied then there would be lots of suffering to people who are P. Suffering is bad. Therefore, we should instead apply ~x to the law because, says she, the application of ~x would cause much less harm to people who are P.
What should we think of such an argument? Should S capitulate? Should she give up x? Or should Q give x another look?
Suppose S goes along with Q's suggestion. S believes that x but agrees to have the law of the land reflect ~x. She agrees to live in tension with what she thinks is true by living as if it isn't. Caught in such a tension she agrees to bite her tongue. It would be no good having agreed to have the law reflect ~x for her to carry on arguing with Q over dinner. Q is happy and gets her way. The argument had enough force to silence S. Of course, S can carry on believing anything she likes; it's a free country after all. She just has to keep shtum.
Perhaps you, like me, are not quite sure about what has just happened. What if you were S? Would you be okay with the outcome? No? Me neither. You probably think Q's argument seems to miss the point: x or ~x? Only one of them is true. Telling me about what might happen if x became law tells us nothing about x apart from its consequences. So what should S say when she gets together with Q for dinner?
The first thing to notice is that Q's argument in HP seems to avoid any discussion about whether or not x apart from its negative effect on people who are P. In fact her argument contains no reason to think x is false apart from any application of x being harmful to people who are P.
Consider the the principle that 'the truth of any p is determined by the harm it inflicts on people of X, Y, or Z should it be made law'. Call the principle y. What determines the truth of y? It cant't be y. y must be taken as basic, a fundamental self-evident truth. But is it?
Consider a concrete example. What if people who are P are people who like torture? They like to do it and have it done to them. What if x is something like, 'torture is morally wrong'. Passing a law that would forbid people who like torture from torturing each other would go against y. And if y is a fundamental, basic truth and y determines whether or not x, then there are no grounds for passing a law to prohibit torture. Perhaps Q would agree but say that in such a case there should be no law since people of P are carrying on without harming anyone else. S should point out (perhaps by penning an argument in HQ) that since y determines whether or not x, Q has admitted that torture is not morally wrong. But surely there are reasons for thinking torture is morally wrong. If Q thinks that there are reasons for thinking it is morally wrong to torture, then those reasons must be normative reasons and y does not determine whether or not x.
Consider another example. What if people who are Y are white supremacists. They believe that the color of one's body is an indication of intrinsic value. The more white, the more important. Now suppose people of P are not white and they agree - people with white bodies are more important than people with non-white bodies. Furthermore, they would be deeply offended by any law that told them that they were of equal value to people of Y. If y determines whether or not x, then there are no grounds for passing any law about the equal importance of people who are Y and P let alone enshrining it in any declaration.
It is not that y is irrelevant. It is that there is more to whether or not x. Q should return to the dinner table with S and ask why S thinks x. Then Q should go back to HP and make an argument that says more than y. Of course, S will probably have a reply and publish it in HQ.