I would say that the greatest invention would be electricity. Although that's more of a discovery. Then in that case, the greatest invention would be the machine, because books, computers, and pretty much everything else can be made by machine. Then again, there are tons of different machines. Then in
that case... the greatest invention would be writing. Because everything really sprouted from the ability to write. Such as reading, and it also makes the internet possible. And computers, and electricity. So people could take notes on science-ie stuff.

