Seems, madam! nay it is; I know not 'seems.'
Hamlet is calling his mother superficial because of her lack of grief over her husband’s death and the speed with which she went into bed with his brother. He shuns her contrived image throughout the play (“frailty, thy name is woman”), and especially in the closet scene (Act 4). As far as we can tell, he never believes that his mother mourns his father’s loss in any genuine manner.
Also, notice how quick Hamlet is to jump on his mother’s words: she says “common, and he immediately uses the word sarcastically; she says "seems,” and he quibbles with the details of that word. And yet, he displays a bit of passive aggression when he ostensibly agrees to comply with her wishes.
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