In the past I've been absolutely one hundred percent sure that there were aliens, and that I'd been experimented on by them. (quite what value that could have had escaped me at the time). Nowadays I'm not a hundred percent sure, I'm pretty sure they're "sideways" beings... interdimensional travellers rather than star travellers, but that's just me. I can't be a hundred percent sure there are aliens out there. By the same token, I can't be sure that they're not.

On the other hand I'm pretty sure I don't have a tracking device implanted in the boney bit behind my right ear, and it's been over sixteen years since I made my belly button bleed by trying to pick out the surgical bug I thought was inserted there.

But... I wouldn't think someone was mad for believing in aliens.

Talking to them now, thinking they've stolen your eggs to breed a super race... now that I'd raise an eyebrow at. But believing in the possibility, even the near certitude that they can exist? That seems pretty sane to me.