The thing is, you don't know that you've seen an alien craft; you just believe you did. You saw what you saw—whatever it was—and assumed it was an alien spacecraft, when the simple fact is you have no possible way of knowing that whatever you saw was alien, or even a "craft" as such.
For all you know, it only appeared to be the size of two Volkswagens, and only appeared to perform the "extraordinary maneuvers" that you think you saw it perform, on the scale you believe it was. In reality, it might have actually been much smaller and closer than you think it was, and it might have even been a number of different objects that you unconsciously conflated into a single one. Your perception of the experience might have been erroneous, and of course your memory might also be completely distorted by countless re-tellings and mental revisions of the story over the past 40-some odd years.
Lots of posters in this thread are content to forward the (gently mocking) assumption that whatever you saw was a firefly, but I won't even go that far. While a firefly might certainly be a reasonable explanation under some circumstances, we have no way of knowing it was a firefly, any more than any other mundane phenomenon. There's simply not enough information to know for sure, and there certainly is no evidence to support the conclusion of anything extraordinary.
Even if indeed you saw exactly what you claim to have seen, then the best opinion you can offer about its identity with any degree of certainty is that it is unknown. And I don't mean "unknown" as in "something alien from another planet," nor "unknown" as in "something of non-human manufacture." I'm saying "unknown" in the classic English definition that you quite simply "don't know what the hell it was." You cannot make any such determination with any confidence because there's absolutely no evidence to support that determination. The total and complete lack of evidence that your "sighting" was an alien craft is roughly equivalent to the lack of evidence that it was a flying witch on a broomstick, a will 'o' the wisp, an angel, a portal to Hell, or anything else that has likewise never been shown to exist.
Of course, nothing's stopping you from making the assumption, or proverbial WAG ("wild-ass guess") that what you saw was an alien craft (or a witch, will 'o' the wisp, etc. for that matter), but you should at least have the forthrightness to admit that your conclusion is a "wild-ass guess" and not a certain fact. You should at the very least be able to acknowledge that neither you nor anyone else has any actual evidence on which to claim the existence of extraterrestrial life (let alone ET intelligence or technology). You should at least be honest enough to refrain from asserting this nonsense about "firsthand knowledge," and "scientific evidence" versus "anecdotal evidence." Evidence is evidence. Anecdotes do not constitute evidence. Anecdotes are claims, and claims do not represent evidence for themselves, no matter who's telling them. Evidence is the stuff that is required to validate claims.
The kind of analysis, logical weighing of evidence, and intellectual honesty that I just described is the fundamental procedure of critical thinking. It is the exact opposite of making determinations of unproven things without any evidence. It's also the basis of Dr. Sagan's ECREE quote, which is not, as you claim, a biased assumption, but the logical extent of critical analysis when applied to so-called "extraordinary" claims, ie. claims of the existence of things which have never been proven to exist.
