I've spent a lot of time on this blog using the tools of scepticism to go after religion, I haven't spent as much time as I should have applying it to other areas. After all, scepticism is such a valuable tool so using it only to attack the sacred cow is not going to do much more than preach to the choir. On a forum I frequent, the question was put forward whether if you believed you were abducted by aliens would you tell others? And this topic I think is great for showing the difference between plausibility and parsimony.
Is it plausible?
Does life exist on other planets? If so, is it complex life? If so, is it sentient life? If so, is it sapient life? If so, is it sufficiently advanced in technology to achieve interstellar travel? Barring actual contact life on other worlds, these questions cannot be answered definitely. But this doesn't mean that we can't be at least make informed speculation on the matter.
Firstly we know of a species that answers all but the last criteria: us. This world is teaming with life, and we are complex, sentient, and sapient. While interstellar travel is not yet possible, interplanetary technology (at least robotically) has been developed. It sounds tautological, but it is significant - the universe has the capacity to host intelligent life. So far it is looking plausible, but...
... our understanding of physics is beginning to show up the limitations of what can be achieved. The nearest stars are light years away and there's a finite speed that one can move to. Light travelling from alpha centauri take 4.3 years to reach earth, moving at a mere 300,000km per second. And even with constant acceleration, one could never get near that speed as the closer one gets to the speed of light the heavier an object gets needing more and more acceleration to move faster. Travelling the great distance just doesn't seem possible. However...
...there are ways within our current model of physics that aliens could come to earth. A journey could be made over hundreds / thousands of years and many generations. It could also be that the journey could be made with the aliens in a stasis of sorts, preserving them until they approached earth. And this is in our current understanding, it could be that current theoretical physics or even something new could throw open the possibilities of what is possible.
So while there are some concerns, it is still plausible that alien beings could visit this world even within our current model of reality. I would say that alien abduction is plausible, but is it a good explanation?
Is it parsimonious?
In order for it to be valid, two criteria pop up. Firstly whether the evidence itself is sufficient to substantiate the claims, and secondly whether there is a better explanation for the evidence. Positive claims require positive evidence, extraordinary claims require extraordinary evidence. Alien abduction is an extraordinary claim, so does the evidence fit the claim?
There is certainly some evidence to support the claims. Personal abduction stories are very compelling, and provide a powerful testimonial. Slightly diffuse from that is hypnotic regression therapy. Then there are other phenomena that while not supporting directly claims of alien abduction, would count as evidence. These include photos of alien craft, crop circles, and classified government documents surrounding alleged events.
Before talking about the strength of these lines of evidence, it must be mentioned is what is lacking. No alien artefacts, no alien organisms or human / alien hybrids, no alien technology, i.e. no hard evidence. Any of these would be a smoking gun, the extraordinary evidence needed is just not there. Without such evidences, there's just no way to overcome the extreme implausibility that is associated with the vast distances of space travel.
But what of the evidences that are out there now? Surely they deserve an explanation and at least add up to a tentative proposal that alien abduction is the best hypothesis to explain them. Is the evidence out of this world? Sadly no it isn't, and what is proposed can be explained without leaving this world.
The brain is a very imperfect memory device: it can hallucinate, see patterns where there are none, have false memories implanted and the imperfect memories that are there can be twisted and distorted upon future reflection. Testimony of aliens is as reliable as testimony of gods, fairies, ghosts, or any other eyewitness accounts. Science doesn't accept anecdotal evidence. Hypnotic regression therapy has no basis under which its accuracy can be accounted for, and much to show that it doesn't work (see: satanic cults)
Crop circles are an interesting case, they did seem genuinely paranormal until the guys who started the craze owned up to it. Likewise photographs of UFOs can be easily made, there's nothing special about the photos that show a need for further explanation. As for classified government documents concerning what was seen in the sky over the last 60 years or so? There was a cold war going on and a technological arms race.
There's just no need at all to suspect aliens at all, there are far more likely explanations out there without the need for appealing to the unsupported. Aliens just aren't a parsimonious explanation even if there is the slightest hint of plausibility to the concept.
I want to believe
One thing that comes up a lot is that if one works to dismiss a phenomenon (as I have done above), it must mean they are rejecting it because they don't want it to be true. i.e. one rejects God because they don't want to accept the consequences of the truth of God. I can say emphatically that I would very much like for there to be intelligent life found elsewhere in the cosmos. Indeed the potential for finding any life beyond our earth would be fantastic.
In that respect, scepticism is a tool against disappointment. It separates what we desire or wish to be true with what is most likely to be true. It eliminates the dangerous notion of credulity of happiness and aspires to seek as accurate knowledge as possible about the reality we reside in.