Near the end of a teaching evolution roundtable discussion at the recent SICB San Antonio meeting, someone asked the gathered speakers, "Why are you passionate about this?"
The response was immediate and united. But I think Randy (A Flock of Dodos) Olson was the first to respond, "Truth."
But Barbara (Creationism's Trojan Horse) Forrest wasn't far behind. She said that the first principle of philosophy was the ability to distinguish between true and false statements.
After that, because there were a slew of academics in the room, the conversation broke down a bit into the difference between well supported by evidence (scientific truth) versus eternal verities that always have been and always will be (what most people think of when they think about truth).
Which sort of supports Randy Olson's point that scientists don't do a great job of making simple points. They are highly trained to make qualified statements and deal in subtleties and nuance.
But maybe sometimes we should just say, "It's the truth."
(I'll give my own answer to the question in a later post.)