Throughout the 1880s, in his notebooks, Nietzsche also developed an equally elusive theory of the "eternal recurrence of the same" and much speculation on the physical possibility of this idea and the mechanics of its actualization recur in his later notebooks.
However such a concept of eternal return was used metaphorically, and evidenced for not being taken as a literal theorem of Nietzsche for how in fact things are or aren't, by how he claimed it as a most "abysmal" of convictions amongst human values.