This time around it's K-PAX, the Kevin Spacey/Jeff Bridges "alien" movie. And, you know, the premise doesn't sound all that appealing to me, and the execution of it? It is the lamest possible ambiguous ending of all: the filmmaker fudged in as much ambiguity as he could so that he could play an ending that didn't resolve the ambiguity--so much ambiguity that neither answer makes any sense.
Properly done, both answers would be compatible with all the evidence but there would be one or two bits that pull you the other way. Instead, we get a situation whether neither answer is all that compatible with the evidence. Now, you have to grant me a little license here; anybody can always retcon a whole bunch of stuff to explain any inconsistency away. So I mean inconsistent using only stuff that's basically in the movie and that doesn't require insane leaps.
The solution that the movie seems constructed to prefer is that Prot really was from K-PAX. Bess disappears; Prot can see in ultraviolet; Prot knows all this stuff about the solar system where K-PAX is supposed to be; Prot disappears for a few days; Prot helps cure Howie and Ernie. None of these are easy to explain if Prot is just an alternative personality of Robert Porter. In fact, they are sufficiently strong that the movie is incredibly lame if Prot is just an alternative personality of Robert Porter. Sure, Bess could have snuck out somehow. Robert Porter could be a savant astronomer who happens to be able to see into the ultraviolet. But if so, it's a really dumb movie.
The alternative is to take all that evidence as being evidence that Prot must really be from K-PAX. But then we run into more problems. If he is from K-PAX, and he takes over Robert Porter when he arrives, because he doesn't physically travel literally--he uses some kind of host body--then how did he travel to Greenland? Why didn't Bess leave behind an empty body? Why did he push the kids as if he was Robert Porter recovering? Why was he hynotizable? Why if on Earth he was human (and on K-PAX he was K-PAXian) did he not have human vision (why was he still able to see ultraviolet)?
Why, if Prot was really from K-PAX and could really bring one human being back with him to K-PAX... why didn't he take back Robert Porter?
Lame. So, either way you resolve the ambiguity is lame; therefore, making it ambiguous between two lame endings? Even lamer.
The movie is based on a book; I wonder if the book is also lame.