I think this was a great idea for a post. If LessWrong rationality is worthwhile, then it ought to get lots of replies on concrete facts - not moral preferences, theology, or other unproveables.

I used to believe that embryos pass through periods of development representing earlier evolutionary stages - that there was a period when a human baby was basically a fish, then later an amphibian, and so on. I believed this because my father told me so; he was a doctor (though not an obstetrician), and the information he had given me about other subjects was highly reliable. Most knowledge is second hand - it was highly rational for me to believe him. I now know (also second hand!) that Haeckel's ideas were debunked a long time ago - although they might well have been in a textbook when my father was at medical school.

To me, the lesson is trust, but verify.