We can make claims that are true about justice. It isn't always easy to see whether the claim is true or false. That's what we're trying to discover when we reason. Why people get confused. This is incorrect.
People can have opinions about matters of fact.
This is an opinion about a matter of fact. Either there is something wrong with it or there is not. It is not a matter of personal preference as is one's preference for ice cream. This is an opinion about a matter of fact. Either people do live on other planets or they don't.