Continuity mistake: In one of the first family scenes, the youngest daughter shows she has lost a tooth. The next shot, the tooth is back in her mouth. (00:15:10)
Continuity mistake: The bowl of green jello that Doris throws at the warden lands the right way up, but in the next shot it is upside down. (00:21:00)
Continuity mistake: When the doctor shuts all the blinds in his office, and then interviews Prot, his left hand collar button is undone/done up in different shots. (00:25:00)
Continuity mistake: The taxi ride to the planetarium begins in a Ford Crown Victoria but ends in a Chevy Caprice. (00:36:02)
Continuity mistake: When Jeff Bridges is in front of Robert Porter's house, with the sheriff, you can see that the shadows of the background mountains are different to the people's shadows. (01:31:25)
Continuity mistake: When Prot takes his sunglasses off for his "return" (because he would supposedly no longer need them due to the lower light levels on K-Pax), he places them on the nightstand upside-down, with the earpiece flush to the table. Two subsequent shots (including an elaborate effects shot) show the sunglasses right-side-up. (01:48:45 - 01:53:30)
Continuity mistake: When the woman who never leaves her room is seated at the table with some other patients, she's talking to the doctor and closes her hand fan. The very next scene shows the fan open again.
Answer: He rejects it for two main reasons. First, each of the items you mention have possible, even if unlikely, explanations. Some people have strange or no reaction to certain drugs (for example I have almost no response to any painkillers). People who have had their corneas replaced with artificial lens can see near ultraviolet (though nowhere near 300-400 angstroms). The sheriff described Porter as being very bright, and he was in to astronomy, so while a great stretch, not impossible he somehow formulated the information he presented. The second reason, building upon these, is Occam's razor. As a person in the sciences, Dr. Powell is driven to believe things have a reasonable explanation, even if we don't currently know what it is, and thinking Prot is just a bright and unusual human is a more reasonable belief to him than believing Prot is an alien possessing a human's body.
jimba
Just remarking, there's no comparison of painkillers and psychiatric drugs. Thorazine and Haloperidol (Haldol) are both powerful anti-psychotic drugs with numerous side effects. Prot is immune to Thorazine and Haloperidol (as well as alcohol), which is more than extraordinary, it's otherworldly.
Charles Austin Miller