Is there life out there ... are there intelligent or advanced civilisations existing or may have previously existed before our own? These are questions even for scientists to be "open" to .. like Dawkins , Kaku , and N.deGasse Tyson who says he is. Aliens is often put with the fairies and santa claus in certain debates.
The thing about science is that it's a methodology; So if you apply it anywhere, then you need to apply it everywhere.
The hypothesis "UFOs are aliens visiting Earth" entails a number of testable elements, ALL of which must remain unfalsified if we are to accept the hypothesis as plausible.
One such element is the question of whether it is plausible that intelligent life exists outside our solar system; Another (related) question is whether it is plausible that intelligent life exists elsewhere within our solar system.
The answer to the latter question, we can be confident is 'no' - it's plausible that simple life might exist on Mars, or Europa, or in the clouds of Jupiter or Venus; But if such life was present AND had advanced to the point of using technology, we would almost certainly have found good, hard, repeatable evidence for it by now.
The answer to the former question is almost certainly 'yes' - we now know that there are enough Earth-like planets in the universe - even just in our galaxy - to render the chances that ours was the only one to develop intelligent life pretty remote. From what we know of physics, chemistry and biology, and from what we know about the frequency with which planets are found, and the number of stars in the galaxy, intelligent life would need to be incredibly uncommon for it not to exist elsewhere.
But that's only part of the story. Intelligent life elsewhere is a far cry from intelligent life visiting us here. The Galaxy is a big place, and the speed of light is an absolute limit; So visiting even 'nearby' stars is extremely difficult. If we were to discover strong evidence of intelligent life at a nearby star - say within a dozen lightyears - and were to decide to put a massive effort into visiting them, we would need all our resources just to build and send one crewed spacecraft there; And that craft would be in no condition to land, and then re-launch and return to Earth, even without a kidnapped alien or some samples taken from one. We certainly wouldn't arrive with the ability to zip around in their atmosphere, frightening drunken alien yokels and turning inside out whatever their analog of a cow is.
In fact, an alien civilization a dozen lightyears away would be very hard to detect - the inverse square law is against us here, and we need very sensitive equipment to receive deliberately sent, highly directional, signals from our own space probes within our solar system. If an identical Earth, with an identical technology, existed 12 lightyears away, we would be hard pressed to detect it; About the only signals we would likely pick up would be from the DEW radars used in the cold war - and we only 'broadcast' signals from them for about three decades, from the 1960s to the 1990s.
In short, aliens almost certainly exist; And equally, they almost certainly are not here, and are not likely to even know that we are here. (Just as we don't know where they are). So anyone who claims to have seen one has probably been indulging too much in mind-altering activities.
