Phlogiston is an absolutely reasonable theory given what was known at the time. It's a really interesting example. Phlogiston is basically negative oxygen. It's very similar to how our traditional notion of electrical current moves in the opposite direction from the actual electrons, because people had no idea whether positive or negative charge was the one that moved around, and they just arbitrarily assigned the positive charges as the ones that moved.

It doesn't quite work that way, because charges can be positive and negative, whereas oxygen can only be positive or zero. But until you're able to do some pretty careful measurements, or create a good vacuum, it's hard to tell.

It's a great analogy. Phlogiston theorized that combustion involved an undetectable (with the technology of the day) substance. That substance was investigated over time and the theory was refined until eventually people figured out that the sign was inverted. If you think of "the opposite of oxygen" as still being a pretty solid theory, not absurd, it comes out looking pretty good.