May I add this, eldar do not see the future I believe. From what I have read they can see possibilities of the future. They interprete these visions how the see fit. They decide what they beleive will come true atleast I think thats how it works. otherwise there really would be no beating the eldar. to know what your enemy is going to do before they do it.... ahhh that is scary.
That's kind of how it works, but not quite. As TheMightyPikachu says it's more a probability kind of thing than the idea that the future is set. Rather, the Farseers' (and arguably other Seers!) view of the future is
dendritic showing the many possibilities. The job of the Farseer is to see the nexus points and guide the future to what they wish.
It should be noted, however, that the vision of a Farseer is just one vision. The 'fluff' indicates (quite reasonably) that every individual sees the future differently, thus the actions of another Farseer can shift the strands of probability making the probable become the improbable as they shift their own preference from improbably to probable and, as the time of the next point and the desired future,
If you want an interesting example of how Farseer/precognitive vision might work then check out
Maze of Moonlight,
Shroud of Shadow and
Strands of Starlight books. While the 'future vision' is taken into extreme in that trilogy it is a good place to start, I would suggest, for visualising precognitive abilities. (Incidentally the books are fair enough; nothing too brilliant although some of the imagery and concepts behind it are quite excellent.)
I would hasten to add that ascribing the Emperor with 'definite future sight' would be a horrendous mistake. With the definition of the 'shadow point' from the novel of the same name (along with the premise behind both the Avatars and the description of the awakening of an exarch this is the only good thing about the entire novel) it is clear that even the Emperor's "vision" was subject to the same constraints as that of the Farseers, i.e. the Horus Heresy was itself a shadow point in his vision.
So to summarise, the Farseer sees the dendritic paths of the future and through dint of will (and more appropriately physical intervention of themselves or others) attempt to select which Path the future will take. It is not certain that the future will happen since there are competing visions of the future and there is always personal choice, but with this ability the Farseers can change the very 'fate' of the universe.
The Eldar are kinda in a snag in the fact that they are all alone in a universe full of enemies, enemies who dwarf their resources.
The eldar have a number of things on their side, though. First off, they are very hard to find (I hesitate to say 'impossible') unless they
wish to be found. In this the wargame is totally biased since it is only concerned on fighting little skirmishes and, seemingly, people interpret this as the eldar "constantly being at war".
Secondly the eldar are probably the single most advanced race that we know about in the 40k universe. Yes, people are going to say the Necrons but while the Necrons have been slumbering away - if you are to believe the official timeline of the 40k universe - the eldar have been kicking around for a further 60 million years! While the C'tan supposedly maintain an almost perfect control over the physical universe (this is arguable based more out of statement than representation), they also for the most part do not count.
Thirdly, if you are to believe the 'fluff' (and on this point I do not), they are able to conjure matter out of nothing (well, the warp). They use this to create their technology, power their technology, etc. It is the magic make-all that gives them, in essence, infinite resources.
One of the major things keeping the eldar in their place is, and this is an arguable point, their birth/mortality rate which is about the only thing that can explain it. Otherwise the only other answer is that they remain their because "GW says so" in a very bad form of storytelling.
The Imperial guard are an inexaustable supply of man power.
Well, not
inexhaustible...
They are a race that expects that at sometime in their life they follow a warrior's path.
An eldar does not
expect to walk any specific Path.
They are skilled warriors, not to mention artists, and engineers, etc. They are probably the most resourceful race in the 40k universe. But yes they are slowly dying.
Ah, I new there would be that 'dying' comment somewhere.
Kage