It was so unlike him to yield to such an irrational impulse! Did he really mean to ask her to marry him?
She had once shown him the impossibility of such a hope, and his subsequent behaviour seemed to prove that he had accepted the situation with a reasonableness somewhat mortifying to her vanity.

It was all the more agreeable to find that this reasonableness was maintained only at the cost of not seeing her; but, though nothing in life was as sweet as the sense of her power over him, she saw the danger of allowing the episode of the previous night to have a sequel.

Since she could not marry him, it would be kinder to him, as well as easier for herself, to write a line amicably evading his request to see her: he was not the man to mistake such a hint, and when next they met it would be on their usual friendly footing.
Lily sprang out of bed, and went straight to her desk.

She wanted to write at once, while she could trust to the strength of her resolve.

She was still languid from her brief sleep and the exhilaration of the evening, and the sight of Selden's writing brought back the culminating moment of her triumph: the moment when she had read in his eyes that no philosophy was proof against her power.