Here's the phrase in context
What he does is start with "seeing it", but then debunks "seeing it" with "human perception is subject to many errors and weaknesses", which is true, but does not mean seeing it can be dismissed. If somebody claims there is a spaceship in their backyard, and you go look, and there is something that might be a spaceship there, you damn well know something is there. What it is, what actually is there is another matter. But you don't discount what you are seeing by claiming seeing it is not proof that something is there.Everyone has heard the age-old dismissal of “I’ll believe it when I see it.” If you are or ever were a budding skeptic, this may have been your motto at one time. Unfortunately, seeing something or experiencing it is not proof enough to believe in it. Personal experience is known in scientific terms as “anecdotal evidence.” Personal experience can be very convincing. Unfortunately, human perception is subject to many errors and weaknesses