There is a world of difference between believing that there is intelligent life elsewhere in the universe, and believing aliens have visited Earth.
The first is based on the simple fact that out of the so many trillions of stars out there, surely at least some must harbour intelligent life. Straightforward statistical reasoning. We have a fairly good idea of how life evolved in Earth, so why not elsewhere.
The second belief attracts the likes of Goh Meng Seng, conspiracy theorists, occultists, astrologers, fanatics, morons, and a bevy of pseudo-scientists more interested in fame than in science. Even if there is intelligent life in our galaxy what are the odds of such life travelling thousands of light years to earth. What are even the odds of two intelligent civilizations being in the same galaxy and at the same moment in time? You can't quantify these odds, which means it is beyond the bound of scientific inquiry at the moment. In other words, pseudo-science or scientology, or more appropriately, mysticism.
