"What if they could see the faerys while someone else could not?"

this would show that fairies were one of those issues that belonged to the untestable realm, and therefore SUPERnatural. And, as always, the burden of proof is on the assertor --that is, see all the fairies you wish, but if you make a claim about them and their 'reality', those who don't see them can only shrug and ask YOU to explain why we should do more than shrug.