I really hate that logic, by the way. All the fantasy setting does is introduce magic, which bends the laws of nature. The laws of nature are still there. They just get bent. It's HOW they get bent that would decide these things.
I had a guy on here say that in order for a fireball to make steam, you'd have to cast two spells, one with magical fire, and one with magical water, because magical fire doesn't mix with regular water the way magical water would, so if you cast both of those spells together, it'd make steam, but if you cast a fireball under normal water, it would just be negated. Oh. My. Goodness. I couldn't even respond to him.
Fire makes water evaporate, water makes fire go away (by taking the heat and becoming steam). It's simple logic.
Darkvision is a racial thing, not a magical thing. It lets people see in darkness without light. In older editions, it was infrared, which I liked. It made sense. Those creatures were bred to see in total darkness, so they developed other ways of seeing.
Now it's this ambiguous "Darkvision". I still prefer to think of it like infrared (which isn't "red", btw, more like the night vision on a camera).
Devil's Sight is different. It's magical. It lets you see through magical darkness as well. It bends the rules of reality to make it so you can see perfectly fine, in color, without light at all.
These things can make sense when you draw them out logically, using Suspended Disbelief to add in Magic, defining Magic as something that breaks or bends the laws of physics. BUT THE LAWS OF PHYSICS DON'T JUST DISAPPEAR!! The spell COULD be comprehensive enough to negate all laws of physics and let magical fire burn underwater. Or it could just create a hot spot hot enough to ignite the atmosphere, creating a fireball. DM's choice, as both works fine. All you have to decide is: what does the spell actually do? Create the effect in its entirety, or simply manipulate the laws of nature so the effect happens? Does it create a ball of fire, flames and all? Or does it create a hot spot that ignites the atmosphere, thus creating a ball of fire? Once the DM decides between those two choices, the rest makes sense.
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