What, When, Why, Where, How?

"I understand HOW: I do not understand WHY."

I believe this would be juxtopposition. Winston is trying to understand why the Party erases all evidence of the past. He does not doubt how they do it, but he does not fully understand why they do it. What is the point? HOW and WHY are juxtapposed to each other in this sentence. He knows HOW but not WHY. The real question is WHY? It is becoming just an accepted that the past is erased, but no one questions it. Except Winston, who is bound and determined to challenge the Party.