Friedrich Nietzsche defines eternal return in his work The Gay Science, and is it closely linked to his notion of amor fati. As Heidegger points out in his lectures on Nietzsche, Nietzsche's mention of eternal presents this concept as a hypothetical question, rather than postulating it as a fact. According to Heidegger, it is the burden imposed by the question of eternal recurrence - whether or not such a thing can be true - that is so significant in modern thought: "The way Nietzsche here standardizes the first communication of 'greatest burden' thought [of eternal recurrence] makes it clear that this 'thought of thoughts' is at the same time 'the most onerous thought'."
The thought of eternal recurrence appears in some of Nietzsche's works, in particular in paragraphs 285 and 341 of The Gay Science and later in Thus Spoke Zarathustra. The fullest treatment of the subject appears in the work entitled Notes on the Eternal Recurrence , a work published in 2007, alongside the version of Søren Kierkegaard 's eternal return , which he calls Repetition. Nietzsche sums up his thought more succinctly when addressing the reader with: "All has returned. Sirius, and the spider, and your thoughts at this moment, and this last thought of yours, that all things will return."