This is certainly a theme that is explored within the text through the life of Tess. It is important to remember that the laws and customs that we accept as being so integral to our lives as humans are actually man-made inventions. Sociologists in particular focus on the ways in which the concepts of what constitutes criminal behaviour actually change over time as a result of context. However, this is something that most humans are blind to, and Tess herself certainly fits into this category. Note the following quote that comes after Tess has began her relationship with Alec:
Walking among the sleeping birds in the hedges, watching the skipping rabbits on a moonlit warren, or standing under a pheasant-laden bough, she looked upon herself as a figure of Guilt intruding into the haunts of Innocence. But all the while she was making a distinction where there was no difference. Feeling herself in antagonism she was quite in accord. She had been made to break a necessary social law, but no law known to the environment in which she fancied herself such an anomaly.
It is man-made custom that causes Tess to consider herself "a figure of Guilt" because of her pre-marital sex. It is only because in her time and day this was so frowned upon that she thinks of herself in this way. The narrator points out her error by saying she was "making a distinction where there was no difference." Although she may have broken a "necessary social law," she certainly did not break any law "known to the environment." Quite simply, in the eyes of nature, Tess is innocent. This conflict between the laws of man and the values of nature is one that continues throughout the novel.