ahh Fuck it, These are the times that try men's souls.

DANGER—HUMAN!

NOT FROM BOOK

"Here's what I personally remember from the story:- a group of aliens(from different species) capture a human on Earth and put him in an escape-proof  prison of some sort( I think it has a moat with acid or boiling flames and it definitely had some sort of impassable death-dealing force-field/energy-field  around the perimeter). Anyway, the idea is that humans have existed in the galaxy for uncounted years, and would periodically build up great interstellar empires, as they were much more intelligent and much more aggressive than any other alien species, which, as a result of all that aggression, would then inevitably collapse into civil war, with whole planets destroyed - but there would always be at least one human-colonised planet, somewhere, which would just sink back into barbarism, rather than be completely destroyed, and the humans on this planet would sortie out eventually and start yet another interstellar empire. So, the result is that these current aliens in the story  have a dim ancestral memory, derived from near-annihilated civilisations in the past,  which states that humans are extremely dangerous and to be avoided at all costs.Anyway, they therefore decided to capture a human from Earth and put him into an escape-proof prison to see if the legend re humans is true - the idea is that if humans really are so dangerous, then he will sooner or later escape, no matter how impossible it might be. They then render him biologically immortal so that the experiment doesn't end with the human's death, and they also mess around with his mind so that he becomes incapable of either madness(and I think,also incapable of  suicide?). Anyway, they wait for a couple of years or so, and suddenly find that he has escaped - I don't remember the saliva bit, but I do know that he escapes from his cage, by waiting until some sort of robot-arm deposits food into an opening into the cage, and then he uses the robot-arm to swing out of the confinement over the deadly moat and then proceeds to escape in one of the aliens' spaceships. One of the aliens then points  out that, technically speaking, it wasn't impossible for the human to escape, given the circumstances and conditions, so that the experiment was flawed - but then one of the other aliens states that they don't know how he could possibly have gone past the supposedly-impassable death-field/energy-barrier, and then they all look at each other in stunned silence, confirming that humans are the most dangerous species around - and they also know that once the human arrives back on Earth, the whole cycle of human conquest and destruction will begin anew."