Slavoj Žižek has spent decades writing and lecturing about ideology, so his name has become shorthand for the concept itself, and short spoken snippets of his talks circulate widely as reaction sounds. The phrase pure ideology distils his signature idea that belief is sustained by practice rather than argument, which is why the clip gets reused as a compact, knowing comment on political theatre and internet arguments. Its brevity keeps it flexible, and the oddly cheerful delivery stops it from sounding like a lecture.