It doesn't exist until I've seen it!
In my favourite film, 'Down by Law', Jack and Zac share a prision cell and really dislike each other. At one point they have another argument and Jack says to Zac (or maybe it was Zac to Jack), "from now on as far as I'm concerned, you don't exist". Zac (or maybe it was Jack) wants to sound tough so he says "yeh, well you don't exist either".
I was out surveying on Tuesday on a remote bit of blanket bog in Kielder with Julia who is from Quatemala. We found this lizard and Julia was very pleased as she had never seen a reptile in the wild in Britain before. She said "now I believe that reptiles exist in Britain". That struck a chord with me because there are many species that I didn't believe existed until I saw them for the first time.
In my twenties I was (still am) a very keen birdwatcher, but for some strange reason it took me years to see my first little grebe. I did lots of birdwatching and went to lots of the right places to see little grebes in the Lee Valley near where I lived at the time. Even though I knew very well what they looked like from looking at pictures it was really difficult for me to believe that they really existed until I actually saw one. Since then I've seen them loads of times but I always get a great kick out of seeing them every time now!
Other species I still don't believe exist include: scaup, wood warbler, garden warbler (probably have seen or heard these but not realised it), pine marten, Carex aquatilis, Bryum caespiticum and of course many many millions more!