Having been one of Terry Tempest Williams' readers over many years, I don't think that Diane Tempest left the books blank because she felt silenced or powerless. But in Terry's remark during the interview about going beyond words, that is how I would choose to interpret her mother's actions, and I am okay with the uncomfortable mystery of the blank books. I'd say she did not want to be bound by whatever words she would write, which would more likely reflect that moment than her overall being. I am a journal writer of many years, but would be horrified to be understood by some of my worst journal entries, particularly because journals sometimes allow venting of thoughts one would not wish to have live forever in others' memories - perhaps particularly in the case of a cancer survivor who later succumbed, as Terry's mother did.
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