|Cupcakes in the shape of orange extra-terrestrials. These may not be a fully accurate representation of aliens. (Photo credit: Wikipedia)|
Do I believe that people exist who the U.S. government has classified as aliens because they entered the country illegally? Yes, but I'm guessing that isn't what the questioner had in mind. Do I believe that the U.S. government has been concealing the bodies and spacecraft of extra-terrestrials for decades at Area 51? No. I do not believe this because of insufficient evidence to support such an implausible theory. After all, this sort of conspiracy would require a level of competence that far exceeds the level of competence we can reasonably suspect from our government. But once again, I'm guessing that this wasn't the intent behind the question.
Getting closer to what I suspect the questioner was after, we might pose the following question:
Do I believe that sentient life forms exist somewhere in the cosmos outside of our planet?No. I do not believe this for the simple fact that I have no evidence whatsoever to support such a belief. Without evidence pointing to sentient life forms, it would not be rational to believe that such life forms do in fact exist. And no, the fact that we may have difficulty explaining some things is not evidence that we must have been visited by aliens at some point in time.
But do I believe that it is possible that sentient life might exist somewhere else? Yes, it strikes me as being possible that such life might exist somewhere. It is even kind of fun to think about what various life forms might be like, if they will ever find us, and the like. But recognizing this as a possibility is a far cry from believing that aliens do exist.
As an atheist, I don't believe that gods exist because I have insufficient evidence to suggest that they do. This does not mean that I am absolutely certain that no possible sort of god could exist somewhere. Atheism requires no such certainty.
In order to maximize true beliefs and minimize false beliefs, we look to evidence. In the case of both aliens and gods, we don't have sufficient evidence to support the belief that they exist. Of course, we also don't have sufficient evidence to conclude that they do not or could not exist in some form or another. Thus, it seems to me that the rational position is to believe that they probably do not exist until such time as we have evidence indicating that they probably do exist. As such evidence were to mount, we'd revise our beliefs accordingly. This is the hallmark of reason: we allow our beliefs to be influenced by - and ideally based upon - evidence.
Do I kind of wish there were aliens out there somewhere? Sure. Do I think it would be cool to find evidence of sentient life away from our planet, especially given the damage we have done to our planet? Absolutely. But it would be absurd to turn these desires into a belief that such life forms exist.