Some schools of literary analysis contend that it is not possible or necessary to determine an author's purpose or intent. Yet other schools put much credence on relating theme and author background to author purpose. In this tutorial format, we'll restrict our discussion of purpose to the most important social theme in Tess of the d'Urbervilles.
Hardy criticizes the social value that allowed men who were promiscuous to go unchastened by society or family while punishing women who were promiscuous by social out-casting and family rejection (though Tess's family did not reject her). For a woman, it did not matter whether she was seduced and impregnated against her will--as Tess was--or not; she suffered the same punishments regardless.
Let's examine the cases of Tess and Angel Clare to illustrate Hardy's purpose in exposing this double and unjust social standard. Recall that Tess refused Clare's proposals of marriage because she was not worthy and that her mother--after Tess broke down and yielded to Clare's importuning--advised her to keep silent about Alex Stokes-d'Urbervilles' seduction. Tess is terrified to tell Clare her reasons but knows she must refuse him. He wears her down, gains her consent and they marry.
On their wedding night--spent ironically and tragically in an old, half-ruined d'Urberville mansion in Wellbridge--Clare confesses to Tess that in an angry depression railing against his father's edicts for him, he spent forty-eight hours in drunken promiscuity in London:
[narrator]: tossed about by doubts and difficulties in London, like a cork on the waves, [Angel Clare] plunged into eight-and-forty hours' dissipation with a stranger.
To this, Tess replies that she is now free to tell her sad tale because now their falls from religious and social graces are identical, they are "just the same":
"O, Angel—I am almost glad—because now you can forgive me! I have not made my confession. .... Perhaps, although you smile, it is as serious as yours, or more so."
"It can hardly be more serious, dearest."
"It cannot—O no, it cannot!" She jumped up joyfully at the hope. "No, it cannot be more serious, certainly," she cried, "because 'tis just the same! I will tell you now."
While Tess instantly forgives Clare and loves him more than before, Angel Clare (no angel he) rejects Tess. He estranges himself from her from that moment on until after a punishing year in Brazil when he relents and seeks her out only to find he is in time to try to help her escape the hangman's noose for murder.
Hardy's purpose is to make it clear that society and the favored status of men over women is to blame for Tess's downfall and tragedy, a tragedy Angel Clare learns to regret too late to save her from.
the d'Urberville knights and dames slept on in their tombs unknowing. The two speechless gazers bent themselves down to the earth, as if in prayer, and remained thus a long time, absolutely motionless: the [execution] flag continued to wave silently.