Most of what we know we don't really know first hand. I've never seen a cancer cell. But I trust this community of experts who have, so I believe that cancer exists. But we trust these experts, and we trust that the experts have a system of checks and balances and self-correction. And we have to insist that experts have certain certifications. They're not perfect. Every once in awhile there's an engine falls off the wing of a plane, or a tax audit happens and you find out your expert made a mistake. But it's a pretty good system. It's the best system we've got.