You make a fair point. If we're being truly rigorous about what we know versus what we assume, there's definitely some circular reasoning in saying "ball lightning is probably natural" when we don't actually understand what it is.

The honest scientific position would be more like: "We observe these phenomena, we don't know what causes them, and we're working with hypotheses."

What we actually have:

• Eyewitness reports of both phenomena
• Very limited physical evidence for either
• Theories that range from conventional physics to more speculative ideas
• No definitive explanations for either

The assumption of "natural" causes: Scientists tend to start with the assumption that unexplained phenomena have natural explanations because that's been overwhelmingly true historically. But you're right that this is an assumption, not a proven fact.

The bigger picture: We don't fully understand a lot of things - dark matter, consciousness, how exactly lightning initiates, or even how bicycles stay upright. The fact that we can't explain something doesn't automatically make it supernatural, but it also doesn't automatically make it conventional either.

Your point highlights an important distinction between "we think this is probably natural based on our current framework" versus "we know this is natural." For both ball lightning and UAPs/UFOs, we're really in the first category, not the second.

The mystery remains genuinely open, even if scientists have working hypotheses they find more plausible than others.