[AppyPappy]

I only need one. There is no physical evidence they exist.

[Quix]

Imagine, if you are able,

that your most trusted relative asserted very forthrightly that he'd worked around UFO's for an extended period of time in the Nevada region often referred to as Area 51.

Given that you'd found him truthful on all other matters your whole life, you'd disbelieve him, of course?

I chose to believe him, in my case. Though I admit, I had plenty of other rather reliable evidence in my dozens of years of studying the topic.

[usadave]

I only need one. There is no physical evidence they exist.

There are many accepted facts and conclusions in the scientific world that are based simply on theory, and not on physical evidence.