After learning that he had terminal cancer early this year, Oliver Sacks (b. 1933), a neurologist and the author of books like ‘‘Awakenings’’ and ‘‘An Anthropologist on Mars,’’ published an essay about it in The Times. ‘‘When people die, they cannot be replaced,’’ he wrote. ‘‘They leave holes that cannot be filled, for it is the fate — the genetic and neural fate — of every human being to be a unique individual, to find his own path, to live his own life, to die his own death.”