It goes like this:

1. We are advancing as a society, not just scientifically, economically, medically and technologically (which few would dispute), but also morally, politically, ethically and philosophically.
2. Therefore we know more than dead people about pretty much everything, especially those dead people who lived before the European Enlightenment. Relative to us, dead people were at best mildly benighted, and at worst downright stupid, and wicked to boot.
3. Some of those dead people—Plato, Aristotle, Augustine, Aquinas, and so on—were regarded as very clever in their day. They were experts, in their own lifetimes. Yet they were horribly misguided about virtually everything, nevertheless. They turned out to be on the Wrong Side of History.
4. But wait: that means that one day—unless we have suddenly reached perfection, which is far from obvious—our experts will turn out to have been horribly misguided about everything as well. They (and perhaps, perish the thought, we) will turn out to have been on the Wrong Side of History, not just about scientific and medical matters (which we can accept quite happily), but about moral, political, ethical and philosophical ones.
5. So our views on these things are no more likely to be correct than those of Plato, Aristotle, Augustine, Aquinas and co—and probably, given our lack of longevity and chronological stress-testing, a good deal less.
6. So why should anybody listen to a word we say about anything?