Ishtar, the goddess of love, goes to visit her sister Ereshkigal, the goddess of death. This is not a simple jaunt across town, though. Ishtar has to go through a series of gates; to pass through each one, she's got to give up one of her garments--the last being her crown. Figures that a myth about a sex goddess would incorporate a striptease... Anyway, it turns out that Ereshikigal is kind of a bitch (who'da thunk it?), and since Ishtar's nekkid, she's also powerless.
Everyone on Earth stops having sex.
This is where it gets a little confusing. In some versions of the story, she's there to rescue her dead husband, Tammuz; in others, Tammuz comes to pry her from Ereshkigal's evil clutches. Either way, Ishtar manages to get out, but either she or Tammuz wind up having to spend half the year with Death from that point on. Hence the seasons, and later on we get Ceres quasi-rescuing Persephone from Hades, and Orpheus' failure to rescue Eurydice.
The important part of this myth, of course, is the striptease. (No, really.) As Ishtar goes further and further into the realm of Death, she gradually loses all of her power. In other words, Love is powerless over Death. (A new religion later developed which had some verses about God being love, and the gates of death being broken. Same themes, radically different message.)
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Fast forward a thousand years or so to a guy named Parmenides.
Parmenides lived right at the hairy edge when explanatory myth was beginning to give way to natural philosophy. People started investigating the causes of natural events, instead of just writing stories about gods to explain where winter came from. Sometimes these philosophers had rather weird explanations, but they were kind of the first scientists, in that they wanted to explain things as natural, rather than supernatural.
Parmenides wrote a rather strange poem in which he
Love may have no power over Death, but Reason trumps Ignorance every time.
Night tells Parmenides that there are two ways to go about gaining knowledge of things. You can study the things which exist, or you can study the things which do not exist. Unfortunately, knowledge of that which does not exist is reserved for the gods. But humans can study what does exist, and gain knowledge that way.
In essence, Parmenides is using a myth to knock over mythology, by having Night (who hides all knowledge) show him how to gain understanding. It's a primer for a philosophical method, buried in mythological verse. The most important thing this method said is that appearances can be deceiving. The whole thing about "what exists" and "what does not exist" is just as deceiving, because the idea of existence he's talking about isn't what we normally think of. When Night says that humans can only study what exists, she means that humans can only study what we perceive to exist.
Parmenides was a monist who declared that time, death, change, and everything around us--including us--are all illusion. Many of the natural philosophers who followed him scrambled to either prove him right or prove him wrong. Except Empedocles, who got to have it both ways.
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Fast forward another two and a half thousand years to a gentleman named Immanuel Kant. Kant distinguished between phenomena and numena--our sensory experiences of things, and things as they exist in themselves. He was basically saying the same thing as Parmenides: We can study what we can perceive, but there is an existence beyond what we can perceive which we will never reach--not even through Parmenides' much-vaunted Reason.
Because we can't experience the numenal world, we essentially take it on faith that our sensory world mirrors reality. All our science, all our reason, all our physical understanding, is based on a leap of faith. We've come back full circle to Ishtar at the gates of Death.
Parmenides' monism was based on faith. Everyone's philosophy is based on faith of some sort. We all start at the gates of Death.
