While Tess does not remain pure in the strictest sense–she does, after all, commit murder. She is pure in the natural sense, rather than abiding by social law.
Tess is not unsusceptible to the view of society; she certainly feels sorrow and alone when society outcasts after she becomes pregnant with Alec’s baby. However, she has realizations that reveals she is more aligned with nature’s law then most people are. When society rejects her and she feels alone, she feels the least alone in nature: “and it was then, when out in the woods, that she seemed least seem” (114). Once she undergoes her second tragedy (Alec’s rape), she gains more worldly knowledge and realizes that she was not the one who was misaligned with nature; everyone else who sunned her was. She realized, “It was they that were out of harmony with the actual world, not she […] She had been made to break an accepted social law, but no law known to the environment in which she fancied herself an anomaly” (115). She did not remain pure according to the laws of society; she did, however, remain pure according to the laws of nature: “But for the world’s opinion those experiences would have been simply a liberal education” (127).
We also see how purity relates to nature in Tess’s interaction with Angel. Angel is first drawn to her because of Tess’s withdrawal from society. He tells her, “Society is hopelessly snobbish, and this fact of your extraction may make an appreciable difference to its acceptance of you as my wife” (208), although we later learn that Angel holds on to society’s standards more than he lets on. When Angel does reveal his social values and tells Tess she is a peasant woman, Tess responds by saying “I am only a peasant by position, not by nature!” revealing that Tess believes she is not naturally a peasant (247).
We, of course, can’t discuss Tess’s purity without questioning whether or not she remains pure after she killed Alec. The murder itself is not natural. However, in a way this act set Tess free and allows her to become herself in her purest and most natural form. After she kills Alec, she runs to Angel, and this is one of the only times in the novel where she seems truly content and, ironically, the most innocent. Once she reunites with Angel, he notices, “Unable to realize the gravity of her conduct she seemed at last content; and he looked at her as she lay upon his shoulder, weeping with happiness” (385). Killing Alec may not have been the right thing to do socially, or even morally; however, it was what she felt she had to do, and in that sense, it was an act of nature. It did not last, and she had to answer for her decisions. While I wouldn’t say she was happy about being put to death, she did seem content. She said told Angel, “I am almost glad–yes, glad! […] I have had enough” (395). Despite it being the laws of society that killed her in the end, in a way she was set free from all of the pain society has given her.
People heavily critiqued this novel (because of the subtitle) because they believed purity should be judged by society’s standards, and those who depart are not pure. However, this subtitle was controversial because it caused people to question what they considered pure. This is why Hardy chose to highlight this quality in his title–because Tess was pure, not society’s standards, but by nature’s.