Pillars hold things up, and salt keeps things clean, but it's a poor exchange for losing your self. People do go back, but they don't survive, because two realities are claiming them at the same time.
This quote takes place toward the end of the final chapter, Ruth, when the Jeanette is thinking about whether or not she should revisit her past by going home to visit her mother. Jeanette is thinking but speaks directly to the readers as she does so. Jeanette's discussion of pillars refers to Lot's wife. According to the Bible, Lot's wife turned into a pillar of salt when she turned to look back at burning Sodom and Gomorrah. God had previously told Lot not to have his family look back, but Lot's wife still did. Jeanette parallels the wife's act of looking back to the act of looking backward over one's past. Jeanette proposes that it is impossible to live in your past while you are in your present. Your life always is changing and going forwarded, if you cling excessively to your past identity it will destroy you. For Winterson, the self is malleable and always in a process of being created and reinvented. Jeanette has been able to change as she has grown. If she were to have clung only to her childhood identity, her liberation would not have been possible. Eventually, Jeanette develops a profound understanding of her self where she can see who she was before and what she is now. By not defining herself only by her past, Jeanette will not become one of the living dead. This quote crucially relates to one of the main themes in the book: the need to find your own identity. Jeanette's lengthy philosophical thinking on the matter helps to inform the reader for his or her own life.