Exactly! But the fact remains that it can be done! Whether by alien, whether by some other weird half alive race or even by another walking talking building from a world we don't know (which admittedly falls into the category of alien), the point remains that the possibility that it can be done is definitely non-zero.
If you argue that a building is not alive, then decomposing it to it's elements makes both buildings and humans just a bunch of gathered atoms. The compositoin of the buildings atomic structure may be different from yours or mine but they are atoms, protons, neutrons and electrons regardless. The consiousness that drives these atoms is more pronounced in Humans than in Buildings. Just because we construct a building and just because we haven't seen it show any activity of what we define as "alive" we can't claim it isn't alive. There's not enough proof against it. Sure it doesn't walk talk or debate, but that just means it doesn't conform to what we as a race claim as "alive".
My point in all of this is to state that the death note cannot conform to our rules. It isn't bound by what human society has thought up as the concept of "life". The only ground fact in the entire series so far is that it was a mulitdimentional object that was sent from the shinigami world to the human world which humans could use against other humans. The fact that humans didn't use it against inanimate entities or even animals for that matter does not in anyway limit the deathnote's capabilities.
True neither light nor mello nor anyone else used the deathnote to say they toppled a building, but that's because none of them could. My emphasis on this phenomenon is that it does not reflect the incapacity of the deathnote but rather that of light, mello or anyone else who tried to use it. Neither light not mello ever considered a building to be "alive". They were brought up in human society bound tight by it's paranoid chains of the definitions of "alive" and "not alive".
My theory is that IF there existed a planet where buildings were alive, and IF the deathnote fell into that planet in the hands of such an "alive" building, then could it be used against other bulidings?...Yes and No...
I'll tell you why I think the answer is no. It's because the deathnote cannot work on one persons belief alone. For a deathnote to kill a person not only should the attacker know the targets name, the target must also know it to be their name. When light tried to kill ray penbar's fiance he didn't succeed at first since she used an alias. Light believed that the alias was her real name but it wasn't enough for the deathnote to claim her life. Not until Naomi revealed the name that she believed to be hers, could light use it against her.
Consequently, the deathnote is a kind of deathly connection between the predator and the prey. It wont work until all the dots are connected, namely that:
1. The predator knows the prey to be alive.
2. The prey knows that it's alive.
3. The predator knows the preys name.
4. The prey belives the said name to be it's own as well.
5. The predator writes that name in the deathnote.
The reason why a human can't kill a building is because point 1 is directly broken.
My reasoning then was that if there were such a human that could see beyond point 1 then would the rest of them apply. The biggest hurdle to that is point 2. Even if I were to know a buliding was alive, unless the building knew it as well it wouldn't work. Not just that, I'd also need a name and the buliding has to also believe it has the same name. Without these atleast one of the dots will remain unconnected and the deathnote cant claim anything.
All that aside however my only argument is that, hypothetically, if points 1 through 4 could be satisfied, the deathnote would damn sure work! The only inference from such a hypothesis though is that the real limitations for points 1 through 4 do not lie in the deathnote but rather in the predator and/or prey.
As a mathematician, I therefore have to state that a statment like "The Deathnote cannot be used to kill a building" is false. The real statement is "We cannot use the deathnote to kill a building" and that statement reflects our abilities with the deathnote rather than the note's inherent capabilities.
In summary, the act of one human killing another with a deathnote, is but a subset of the sum total of all possible actions allowed through the use of a deathnote and does not depict an entirety in itself.
**erm.. did I branch out too much ... *scratches head* O_O**
Edit: I have stated here that I am a mathematician, but I don't want to state that anymore given the probable ridicule I might face as hinted at two posts below, so I'd like everyone to consider that I didn't state it
