Full Idea
Cantor's diagonal argument showed that all the infinite decimals between 0 and 1 cannot be written down even in a single never-ending list.
Gist of Idea
Cantor's diagonal argument proved you can't list all decimal numbers between 0 and 1
Source
report of George Cantor (works [1880]) by Stephen Read - Thinking About Logic Ch.6
Book Reference
Read,Stephen: 'Thinking About Logic' [OUP 1995], p.163