What does Solzhenitsyn mean by "the thoughts of a prisoner--they're not free either"? How are the thoughts of the prisoner being held captive?

The thoughts of a prisoner aren't any freer than the prisoner himself because they keep going back to the same thing over and over again. A prisoner night think to keep his/her mind working and off their troubles; they might try to distract themselves from their present situaion and incarceration, but sooner or later their thoughts return to food, what will happen to them in prison, staying warm, and even the fate of their fellow prisoners.