There's also bubbles of swamp gas, birds, weird atmospheric optical phenomena, miscalibrated radars, and hoaxes.

I'm not hugely interested in figuring out which one of a million different, mundane explanations is responsible for any particular UFO sighting. There's a never-ending stream of those sightings, all of them very low-quality, and they never amount to anything interesting. Maybe some spy wonk somewhere three stories underneath the Pentagon has looking at them as his OKR, but I don't get paid for it!

> There's also bubbles of swamp gas, birds, weird atmospheric optical phenomena, miscalibrated radars, and hoaxes.

Damn, some of my favorites aren't even here!

I think the never-amount-to perspective is interesting. It's very outcome-focused. IMO this psychology is not normally a great fit for UFO research, which is rather income-focused and outcome-distracted. So you get a diffusing effect and the most promising work is to be found in pockets and almost never in the big-picture view.

What third category could there be? God?

I don't want to share my list after reading that category. Please keep going! Maybe my list is more boring than I give it credit.

Animals Weather Airborne rubbish (in the form of plant or land material if you consider 'human made' already covered)

Etc.