Within a relativistic context though, how can liberals know (with any degree of certainty) that any political filter should be applied, or that they have applied the correct political filter?
Again, they would have to relativize even their own subjective political filter.
“This, thought Winston, was the most frightening aspect of the party regime-that it could obliterate memory, turn lies into Truth and alter the Past. The Party slogan was “Who controls the past controls the future; who controls the present controls the past.” This was where “doublethink” came into play, minds were trained to hold contradictory positions simultaneously and unquestioningly- for example you had to believe at one and the same time that Democracy was impossible and that the Party was the guardian of democracy. Winston could remember a time when the Party did not rule, when Big Brother had not become all-powerful; but according to the Party they had always existed and this lie was repeated ad infinitum until it “became” the truth. This, Winston thought was a far more terrible weapon in the hands of the Party than torture or execution.” 1984 P1 Ch3
“’Don't you see that the whole aim of Newspeak is to narrow the range of thought? In the end we shall make thoughtcrime literally impossible, because there will be no words in which to express it. Every concept that can ever be needed, will be expressed by exactly one word, with its meaning rigidly defined and all its subsidiary meanings rubbed out and forgotten.” 1984 P1, Ch5
With all relativists/post modernists, YOUR viewpoint can be “relativised” out of meaning, but THEIR viewpoint is “CORRECT”.