"Post-truth doesn't mean that no one cares about truth, it doesn't mean that there isn't any such thing as truth, it just means that there's a critical mass of people who no longer think that they have to form their beliefs based on what's true," says philosopher Lee McIntyre. This is not new; it probably goes back to Galileo and science denial. But today post-truth is more virulent than ever, from Trump to Brexit. What can we do about it?