The relationship between father and daughter, before it was destroyed by the father's unnatural passion, is like that between Godwin and Mary.

She herself called her love for him "excessive and romantic."[xiii] She may well have been recording, in Mathilda's sorrow over her alienation from her father and her loss of him by death, her own grief at a spiritual separation from Godwin through what could only seem to her his cruel lack of sympathy.

He had accused her of being cowardly and insincere in her grief over Clara's death[xiv] and later he belittled her loss of William.[xv] He had also called Shelley "a disgraceful and flagrant person" because of Shelley's refusal to send him more money.[xvi] No wonder if Mary felt that, like Mathilda, she had lost a beloved but cruel father.
Thus Mary took all the blame for the rift with Shelley upon herself and transferred the physical alienation to the break in sympathy with Godwin.

That she turned these facts into a story of incest is undoubtedly due to the interest which she and Shelley felt in the subject at this time.

They regarded it as a dramatic and effective theme.