A negative definition = not
to have certain properties.
An antagonist defines emself completely through negative definitions. I've heard some people who call themselves antagonists saying that it's the only reasonable way to call their ideas in post-modern epoch.
Obviously there are disadvantages in negative definitions. Many people may get depressed if you don't have anything but criticism to offer. It seems like you're justifying the current state of affairs.
On the other hand negative definitions avoid the pitfalls of utopian thinking but I have to note that even the extensive use of negative definitions per se don't make anyone unidealistic.