Antithesis

Setting opposing ideas or words side by side, so that one lights up the other through contrast.

In The Idiot

a surprising face! [...] The face is cheerful, and yet she suffered terribly, didn't she? [...] It is a proud face, terribly proud, and yet I don't know whether she is good. Ah, if only she were good! Everything would be saved!

The “cheerful” face is set beside the suffering, “she suffered terribly,” and the opposition shows what Myshkin sees: the look of a celebration covering a pain that only he notices.

Part One: The Arrival, Myshkin

if she now accepts the capital, it is by no means as payment for her dishonor as a girl, for which she is not to blame, but simply as compensation for a crippled destiny.

Part One: The Arrival, the narrator, about Nastasya

will you accept me as I am, with nothing! [...] I accept, Nastasya Filippovna... [...] I accept you as honorable, Nastasya Filippovna, and not as Rogozhin's woman.

Part One: The Arrival, Nastasya and Myshkin

I was Totsky's concubine... Prince! Now what you need is Aglaya Epanchina, not Nastasya Filippovna [...] That's how all of you are: either you deal with the dishonored women or with the honorable ones, only one choice!

Part One: The Arrival, Nastasya

I love her not with love, but with pity.

Part Two: The Painting, Myshkin

that the most hardened murderer, with no remorse, still knows that he is a criminal, that is, in conscience he judges that he acted wrongly, even without any remorse at all [...] but these people Evgeny Pavlovich began to speak of do not even want to consider themselves criminals, and think to themselves that they had the right and... that they even acted well.

Part Three: The Explanation, Myshkin

Aglaya Ivanovna loved as a woman, as a human being, and not as... an abstract spirit. Do you know what, my poor prince? The most likely thing is that you never loved either one!

Part Four: The Denouement, Radomsky