Well, to understand the eternal return really you have to understand the two ideologies that Nietzsche is opposing: first, Judeo-Christianity, and second, the Enlightenment, especially as it is espoused by Hegel.

Nietzsche is also notable for being in some ways an archetypal romantic of the period: the individual, not society; the will, not reason; the pagan past, not the christian present; nature, not civilisation. In short, it's important to remember that this is a man who Shelley and Blake especially would have, perhaps in horror, recognised as "one of their own".

Now, when we look at the enlightenment we often take it at its word (i.e. it is fundamentally opposed to and a contradiction of Christianity). In terms of ideological structure, the enlightenment has certainly come a long way from roman catholicism, at least. But Nietzsche saw that the most fundamental doctrine that broke the pagan past from the christian present was still in place: the linearity of time. A cursory glance at the poetry of hesiod will reveal a conception of time based on the seasons: everything has its time, and everything comes to dust, but these things will have their time again someday. It underlies even a lot of greek philosophical talk. if you were looking for a single principle that united the disparate doctrines of Pythagoras' transmigration of souls, aristotle's eternal universe, Heraclitus' world of unending becoming, I suggest to you that it is a conception of history as recurring or reiterating. The doctrine only begins to decline at the time of Epicurus (who felt the need to answer arguments based on the mortality of man), being finally vanquished by the triumph of monotheism in the Roman empire. Even Jewish history, with its conception of a single God involved in one act of creation and in relationship with individuals who were to be held accountable for their moral actions, implied one life for men, one course to history. But the belief in the final and irrevocable revelation of God to man in the coming of the Messiah to Israel in the early first century absolutely situated the Christian system within an ordered, non-eternal, non-repeating timeline.

Now, the enlightenment may have challenged many of the dogmas and principles of christianity, but this principle it not only protected but exaggerated. In the histories written in England by men like Macaulay, it gets an outing as "whig history", the belief that history is a process of refinement and improvement, moving inexhorably towards better and happier days (and more whiggish days, naturally). In the idealism of Hegel, it gets its outing as the belief that Reason is working itself out in the minds and affairs of men historically, so that great men like Julius Caesar and Napoleon are prompted by the rationalist's version of the Holy Spirit, and that we are moving up and up into Absolute Spirit, an unfolding of history that reveals itself as the coming into existence of God. Of course, it's this conception of history, as you, dear reader, might have guessed, that provides the ideological backdrop for the musings of Erasmus Darwin and his nephew Charles, and of a certain Mr Marx. I need hardly be so patronising as to spell out how their ideas might be related to this zeitgeist of historical progression.

And this leads us slowly towards to the great paradox and irony of Nietzsche. Nietzsche wants nothing more than to escape individuation without escaping individualism. That is to say, Nietzsche sees nothing good in the world but that it belongs to the individual. Society leaves him sneering with contempt. He's not so much a prophet of selfishness as some people think, and he could probably see the heroism of saving an old lady from a monster in a way that Ayn Rand absolutely couldn't. What he couldn't see was how there could be any heroism at all if the hero had any mind for what society expected or would think of him. In other words, all kindness rooted in our caring as a group, all kindness rooted in our concerns for our own respectability, all kindness that came from somewhere other than our own strictly individual desires was something to be sneered at and opposed.

These societal pressures, this urge to prompt us towards a guilt complex, is finally, in Nietzsche's mind, to do with the linear conception of history. For if all men are strictly judged by their actions, and live but once, then their instincts towards equality, the rule of law and dispassionate universalised kindness make sense. This because either God or History are judging them. if you like, there's an objective order to be on the "right" side of, and we are all brought to reconcile, or in Nietzsche's view, compromise with it.

Nietzsche finds his medicine for the modern maladies of christianity and the enlightenment in amor fati and the eternal recurrence of all things. "Live Bravely!" Zarathustra cries. At their heart, Christianity and the Enlightenment say that there is a correct way of going about things and that you ought to adopt it. Eternal recurrence allows Nietzsche to say "live so that you will not regret anything: and regret nothing by adopting no dispassionate values. Be kind if kindness is in you, be cruel if cruelness is in you. But no more of this nonsense of Oughts and Musts. Those days are over. We have killed God and we have killed History."

Another way of demonstrating it would be to talk about Christianity, Buddhism and Nietzsche. Christianity at its heart says men in history are essentially good and accidentally bad, and consequently do bad things, and they must cease to do bad things if, when once they step outside of history, they are to be reconciled with that ultimate Good which is God. Buddhism says that men are bad insofar as they are . They must cease to desire because desire leads to suffering. Self is shaped by desire and continues to exist as long as there is desire. To step out of samsara, the endless cycles of birth and rebirth, which is hell, we must cease to desire, cease to be self, attain nirvana, which is non-desire and non-existence. Nietzsche says, in effect, that we must learn to love samsara. He's too pagan to say that desire and selfhood and ultimately existence are bad, but not christian enough to say that there's an absolute good in them that must be separated from the bad. Consequently, where the buddhist says "Desire is false. Leave this plane of false desire for the quietness of non-desire" and the Christian says "Desire is true. Leave this plane of badly ordered desire for what you really want.", Nietzsche says "Learn to really want this. Again and again and again forever. Then you will be the superman."

You will have to search hard to find one disciple of Nietzsche who does not talk of his philosophy as being the one of the future, of his superman as evolving out of the men of the past, of the transvaluation of all values as being the next stage of historical development, of the brave radicalism of the socialist project. This is because they all imagined that Nietzsche's hatred of Christianity marked him as a fellow-traveller of the enlightenment. But Nietzsche hated the enlightenment. He had as much contempt for Voltaire as he had for a parish priest. And this is the central irony of Nietzsche's thought. Another is that if there was one concept Nietzsche would have found it hard to grasp, it was irony. If he was anything, he was always sincere.