cell theory (Cell theory refers to the idea that cells are the basic unit of structure in every living thing)
and that all living things have certain characteristics but those characteristics are part of the definition
so any "life" that violated this theory would not actually BE life
its like theorizing that all dogs bark and then redefine the word dog to mean (a 4 legged animal that barks)
if there was a dog that was injured and could no longer bark it would cease to be a dog by this (redefined) definition.