Have you ever seen an old photo of yourself and been embarrassed at the way you looked? Did we actually dress like that? We did. And we had no idea how silly we looked. It’s the nature of fashion to be invisible, in the same way the movement of the earth is invisible to all of us riding on it.
What scares me is that there are moral fashions too. They’re just as arbitrary, and just as invisible to most people. But they’re much more dangerous. Fashion is mistaken for good design; moral fashion is mistaken for good. Dressing oddly gets you laughed at. Violating moral fashions can get you fired, ostracized, imprisoned, or even killed.
If you could travel back in a time machine, one thing would be true no matter where you went: you’d have to watch what you said. Opinions we consider harmless could have gotten you in big trouble. I’ve already said at least one thing that would have gotten me in big trouble in most of Europe in
the seventeenth century, and did get Galileo in big trouble when he said it – that the earth moves. 
It seems to be a constant throughout history: In every period, people believed things that were just ridiculous, and believed them so strongly that you would have gotten in terrible trouble for saying otherwise.
Is our time any different? To anyone who has read any amount of history, the answer is almost certainly no. It would be a remarkable coincidence if ours were the first era to get everything just right.
It’s tantalizing to think we believe things that people in the future will find ridiculous. What would someone coming back to visit us in a time machine have to be careful not to say? That’s what I want to study here. But I want to do more than just shock everyone with the heresy du jour. I want to find general recipes for discovering what you can’t say, in any era.
The Conformist Test
Let’s start with a test: Do you have any opinions that you would be reluctant to express in front of a group of your peers?
If the answer is no, you might want to stop and think about that. If everything you believe is something you’re supposed to believe, could that possibly be a coincidence? Odds are it isn’t. Odds are you just think whatever you’re told.
The other alternative would be that you independently considered every question and came up with the exact same answers that are now considered acceptable. That seems unlikely, because you’d also have to make the same mistakes. Mapmakers deliberately put slight mistakes in their maps so they can tell when someone copies them. If another map has the same mistake, that’s very convincing evidence.
Like every other era in history, our moral map almost certainly contains a few mistakes. And anyone who makes the same mistakes probably didn’t do it by accident. It would be like someone claiming they had independently decided in 1972 that bell-bottom jeans were a good idea.
If you believe everything you’re supposed to now, how can you be sure you wouldn’t also have believed everything you were supposed to if you had grown up among the plantation owners of the pre-Civil War South, or in Germany in the 1930s – or among the Mongols in 1200, for that matter? Odds are you would have.
Back in the era of terms like “well-adjusted,” the idea seemed to be that there was something wrong with you if you thought things you didn’t dare say out loud. This seems backward. Almost certainly, there is something wrong with you if you don’t think things you don’t dare say out loud.
What can’t we say?