"Mind if I smoke?" The human held up a slim cigarette and a plain steel lighter.

"Your lungs are already full of tumors and you want to inhale yet more chemicals on your deathbed? Go ahead I guess, you are dying anyway. Just let me adjust the air return first." I got up from my chair and adjusted the air return controls so that the smoke for the human’s cigarette would get pulled out into the vents away from me.

"No problem. I understand you don’t like the things anyhow." The human sat up a tad straighter on its bed, propped a pillow behind its back, flicked the lighter and took a deep drag before blowing a perfect smoke ring right at the air return. "Developed the habit back in my younger days. Didn't think I'd survive The War anyhow. But that’s not what you are here to talk about is it? Cover letter said something about being an interviewer for the Council of Species?"

I sat back down in the chair and arranged my legs comfortably, settling the notepad across it. Old-fashioned plaspaper it may be, but I found it helpful. "Yes, that is my Title and Role for the council. They are confused as to why humans think as they do about some things. Atomic power for one. Fission and fusion have been in use for millennia, replacing carbon-fuel systems. So why are humans so deathly afraid of them? It is almost an instinctive revulsion to them that we have never seen on a species-wide level before."

The human took another drag on its cigarette. "I see you need to do a bit of digging into some human history. Try looking up the Hiroshima and Nagasaki events. The Chernobyl and Three Mile Island events too for that matter. But that’s only tangentially relevant. I was just a simple soldier in The War, but before that I was a college student, and one of the many things a professor said stuck in my head. He called it the Unthinkable. The thought or idea, that once it has been thought, cannot be un-thought. For humanity and atomics, that thought for a long time has been 'the world will die in fire."

"What?" I looked at the human askance. "That makes so little sense. How would atomic power cause the earth to die in fire?"

The human coughed, a deep racking sound that brought phlegm up from the depths of its diseased lungs. "Right, short version then. When humanity first got around to figuring out the practical mechanics of atomic power, we were in the middle of World War Two, and all of the factions needed a high-impact weapon to force some of their more fanatical opponents to admit defeat. Splitting the atom produces a ton of power in a really short timeframe, so the idea to use that mechanic to make a bomb occurred to someone or another on both side of the conflict. The Allies came up with a working design first, tested it, and used it to force the end of that War. That was the Hiroshima and Nagasaki events. The Chernobyl and Three Mile Island events came later, atomic power plants where things went wrong. But that’s beside the point."

Another cough racked the human and I took the opportunity to get in another question as I furiously scribbled notes. I was horrified, though I dared not show it. Atomic power could be weaponized directly? And humanity had already done so, used such weapons? "So the first use of atomic power on earth was military? What of it? Surely humanity had seen that level of destruction before?"

The human spat a long stream of black phlegm into a waste receptacle beside its bed. "Yes, but not from a single weapon. That same war had seen other cities leveled, but it took tens of thousands of bombs to do it. Hiroshima was simply gone, for all intents and purposes, in a single detonation. Nagasaki was the same. After that, the Great Powers of earth had seen the Atomic Bomb unleashed, and so had to have this weapon for themselves, if only to dissuade other Great Powers from using it. And they developed it further. The bombs used at Hiroshima and Nagasaki are nothing compared to newer designs. Someone somewhere in that time first gave voice to the thought that 'the world will die in fire', and it has been so ever since. When humanity saw the destruction those two bombs unleashed, saw the shadows burned into the walls and sidewalks that were all that was left of people, we saw the potential for humanity to destroy earth for the first time. And we saw leaders of Great Powers prepared to do just that if threatened. So yes humanity fears atomic power, because we still bear the scars of it unleashed."

I scribbled some more before looking up. The Hiroshima and Nagasaki Events were from the smallest atomic bombs humanity had produced? I was more horrified yet. What could such weapons do now? Shatter Ships of the Wall? Crack moons? Sunder planets into rubble at a single strike? "So humanity met its first weapon of mass destruction, and now considers the whole thing tainted by association?"

The Human gave a lopsided grin, the cigarette hanging off the side of its mouth. "Not quite. Before the Hiroshima and Nagasaki events, we humans had seen war and weapons of mass destruction. Mostly chemical ones though, and nothing that could end a city in one blow. Once we had seen that, we could not unthink that such a thing was possible. Now the Council and the Galaxy are discovering the same thing. What you have used to power you cities and your ships, we first unleashed to end two of ours. And now the seed has been planted. Through you and others like you, the Council and the Galaxy has been introduced to atomic weapons. Some egghead somewhere will design or reverse-engineer one. The thought has been expanded. It is no longer ’the world will die in fire’, but ‘the galaxy will burn’."

The human began to cough once more, and the life-sign monitoring systems began to chirp and wail. The nurse rushed in, shoving me aside so she could get to the human. As the nurse went to the med panel, the human calmly flicked the spent cigarette into the waste receptacle and grinned at me.

It was right, I could not unthink that horrible thought.