It's a mangled quote from Marx: "Freedom is the consciousness of necessity". In the Soviet Union this phrase was a staple of state ideology, and a popular folk mockery of it went like "freedom of speech is the conscious necessity to stay silent".
Interesting, I wasn't aware of that. Nevertheless I didn't detect any purpose of social commentary in the way the Strugatskys used it. Instead, I posted this as a comment because it gels well with Eliezer's take on free will.
It looks like, this month, I get to be the one to start the quotes thread.