You saw an apparent moving light in the sky and just assumed it was an alien spacecraft because of the way it apparently moved. But the fact is, it was just as likely to have been a flying witch, a fairy, a will 'o' the wisp, a portal to Hell or anything else that has never been proven to exist. Or it might have been some strange, unidentified atmospheric phenomenon (perhaps some kind of plasma effect, "ball-lightning" or whatnot). But most likely, it was simply a mundane earthly object, phenomenon, and/or optical illusion that you mistook for a flying object and assumed to be something extraterrestrial.
Seeing something which you cannot identify does not constitute evidence that you've seen an alien spacecraft, no matter how much obtuse wordplay you try to perform. Without any physical evidence for scientists to examine for clues as to its origin, you simply have no way of knowing what the "thing" you saw was. Your insistence that it was "alien" all comes down to your own credulity and nothing else.
So you don't know you've seen an alien craft. You just think you know, in the same way that a Christian might think he knows that Jesus is his "Personal Lord and Savior." In other words, you don't really know; you simply believe. Your belief is unshakable, and is not supported by a single shred of evidence, therefore it's taken on faith. You have faith that what you saw was extraterrestrial craft, based solely on personal experience, the word of popular folklore, and your own subjective feelings on the matter.