* Psychohistory really grinds my gears. No matter how much I try to just think Edward Gibbon! Plot devices! Rule of Cool! it just keeps nagging at me. For instance:
** Is there no such thing as free will in Foundation-world? What if one of the later Emperors had decided differently - to keep it all together, to revitalise science, to move stuff off Trantor?
*** Ultimately, "free will" is ''not'' free. The decisions people make are products of their environment; while an everyday Joe might have the "free will" to spontaneously commit murder in broad daylight, the odds of someone actually doing that are infinitesimally small. Furthermore, the actions of individuals are meaningless in the face of macro-scale human social dynamics. Even people in power are limited in their options and thought patterns due to social and political inertia, as demonstrated by General Riose. And, all that aside, psychoshitory is fundamentally a ''probabilistic'' science - it only determines the likelihood of events, not perfect prediction. The primary focus of the Seldon Plan, especially early on, is setting the starting conditions such that the odds get stacked towards people "choosing" the right courses of action.
** The whole idea of a Seldon Crisis is that the Foundation is put in a situation where there's only one option available to them - in this way they have to follow the optimal course and establish the Second Empire in the shortest amount of time. But every Seldon Crisis we've seen has been resolved by some awesome iconoclast going against what everyone else thinks and pulling some rabbit out of his ass. How does Asimov square that circle? He doesn't want to say 'psychohistory can predict what everyone is going to do' - he says it only works on large groups. So what then - did Seldon predict that Narritivium would step in and sort everything out?
*** In regards to the free will issue, the suggestion is one of inertia: yes, one of the later Emperors ''could'' have decided differently... but how well would he have been able to pull it off (especially 'to revitalise science' - that isn't something you can just decide to)? To make an example, moving stuff of Trantor could have been done... but that could easily be seen as a sign of weakness, inspiring coups that render what that Emperor decided not quite so relevant anymore. As for the second point... well, I can't argue about most of the ones we saw (although, we didn't actually get to see all that many, and the last Crisis we saw had another problem than the one you raise), but at least the First Crisis can be defended to a degree - Salvor Hardin's actions weren't ''necessary'' to make the other kingdoms realize what a threat atomic power under Anacreon control would mean, it just speed it up.
*** For the problem you have with the Seldon Crises, you slightly misunderstand the point Asimov was making. It also helps to know that when he wrote the original trilogy, he was operating by different rules than in his prequels, including that psychohistory worked on individuals to a limited extent. Seldon's plan was never meant to simply predict events, but assured that they would happen the way they did. Each crisis was supposed not only to restrict the Foundation's choices to one if they wanted to survive, but also to breed the next one and the social forces that would compel people to make the right choice then. The fact that we are presented with the events as the results of actions by individuals is a storytelling convenience and nothing more. Seldon didn't predict that Salvor Hardin or Hober Mallow would be the ones to do what they did, just that in those times, the Terminus-controlled religion and ambitious merchants would be the driving force behind the solutions to their respective crises. In fact, The Traders and later The General show that sometimes there wasn't even an individual face to the solution. Linmar Ponyets is ultimately just a cog in the machine that subjugates independent worlds by the force of religion, and the protagonists of The General fail to stop Bel Riose completely, whose threat is eventually ended when his Emperor decides he's gotten too powerful to control for much longer and has him executed.
** Psychohistory invalidates itself. It only works when there isn't one man with the power to change the course of the entire galaxy. One man like the Mule... or Hari Seldon himself. If another psychohistorian were looking at the numbers, he'd conclude that the galaxy would fall into chaos for 30,000 years if he weren't looking at the formation of Terminus. Surely Hari himself could have seen that paradox?
*** Not a paradox by psychohistorical terms. Feedback from psychohistorical methods can be accounted for by the equations. The Terminus element is accounted for and derived from psychohistory. One man like the Mule is not accounted for by psychohistorical methods because it is too small to be be accomodated for. Problem with psychohistory mainly that it cannot account for external non-human factors, like Gaia and the Solarian transhumans.
** Psychohistory only works most of the time. It's possible for things to go differently (The Mule and Seldon himself are examples of this). That's one of the reasons the Second Foundation exists: to get things back on track if things do go wrong.
** Actually, there is solid historical basis for this. It is fairly well-known that social conditions have to exist to allow for certain types of individuals to arise to prominence. For example, Gaius Julius Caesar was ''not'' the first person to try to overhaul the Roman Republic. He was merely the first to succeed to significant extent because generations of social problems had reached a critical boiling point. Likewise, being a scientific genius during the early Middle Ages in Europe wouldn't have scored you the same kudos it would have either during the previous Roman Empire or the later nation-states.
*** The European Middle Ages invented the scientific method. Your argument is invalid.
*** False. The scientific method had been practiced by civilizations long before the Middle Ages. The Ancient Greeks, Indians and Chinese all had variants, and much of Medieval European advancement came courtesy of ancient records of these. There is a reason we are writing using the Latin alphabet and doing math using the Hindu-Arabic numerical system.
*** All right, you are correct, but still : the European Middle Ages continued using the Ancients' proto-scientific method and perfected it into the modern scientific method (e.g. : creating the Universities : la Sorbonne, Bologne, Trantor). The original argument is still invalid :) To clarify, my argument was not "the European Middle Ages were the best or even the first good, pro-science civilization ever". I was answering the argument : "The European Middle Ages was a bad, anti-science civilization". I think it is pretty easy to demonstrate the European Middle Ages was not an anti-science civilization. It does not mean that other civilization weren't ALSO pro-science and, in that, in fact, the also pro-science European Middle Ages benefited from them.
*** And yet, even if someone's accomplishments are the product of social movements, those social movements should be able to be anticipated by psychohistory -- thus the Mule should have been anticipated. As the Empire in ''Foundation'' is roughly the Roman Empire during its collapse, mighty conquerors should be expected -- see Attila the Hun, Odoacer, etc. Additionally, the Crisis of the Third Century saw the apparently-imminent collapse of the Roman Empire averted, though the same external reasons would cause the collapse of the Western Roman Empire centuries later. In that case, reform should have been a possibility for the Empire. Furthermore, there has never been a case where the entirety of civilization has fallen into a dark age (even in the Bronze Age Collapse, it didn't apply to China), so the outcome of psychohistory lacks historical precedent.
** Even if you accept that psychohistory could (at least for story purposes) predict what crisis would arise and how it would be resolved, the idea that it could be timed well enough to correspond to Seldon's pre-scheduled appearances seems too precise. Anacreon's attack was timed to coincide with the prince's coming of age -- how could that be aligned (within a few days) to Seldon's appearance on a anniversary of the Foundation? Even if the Second Foundation was manipulating things behind the scenes, it seems hard to believe.
*** Simply, Seldon calculated that the external crisis would boil up to a point where one of the Four Kingdoms would think they had a chance to get away with toppling the Foundation, which would survive thanks to the religion of science, or rather, its hold in the minds of the people. The fact that it seemed to be timed with the prince's coming of age is just random chance - and, in fact, Hardin mentions that he believes the external crisis was accelerated when the Foundation found the Imperial cruiser. Plus, it is not just a few days passing between the crisis happening and Seldon's appearance, as it is obvious that, at least, a couple of weeks have gone by - Hardin has had to travel to the other Four Kingdoms to argue for the signing of the new treaties that give the Foundation greater power.
* Why are they still human? It would take billions of years to spread across the whole galaxy - why has evolution not occurred?
** Evolution requires natural selection. Death and birth rates for humans aren't influenced by external, environmental factors to the extent animals' are.
** OK, why haven't they used that technology to live for hundreds of years?
*** Why haven't modern-day humans used it to? Because external, enviromental factors are not the ''only'' factors.
*** Considering that Asimov's Robots Trilogy are connected in the same universe as the Foundation, they ''did'' in pre-Empire times as the Spacers. Humanity has seen how a long life-span can move entire worlds to stagnation and lack of scientific cooperation. As scientists start to live longer, they can decide to spend their long lives in a specific matter, and avoid needing constant successors so a new thing can be discovered[=/=]invented. It could be also [[spoiler: R. Daneel Olivaw saw to that not to happen.]]
** Also, there is FTL travel in the Foundation-verse, so it didn't take billions of years for humanity to colonize the Milky Way at all.
** Who says they haven't evolved? Granted, 20,000 or so years is not enough time for a massive evolutionary change to occur naturally. But, for example, Daneel managed to introduce telepathy into the ''entire'' Gaian population, rather than just a genetically-gifted minority! The Solarians re-engineered themselves into hermaphroditic psychokinetics who reproduce via parthenogenesis!
*** Not exactly evolution, mind you. As to the original post, FasterThanLightTravel has existed since the end of the 20th/beginning of the 21st century in this 'verse.
** Dogs, at least, had definitely evolved into something very different from what you or I would call a "dog". In ''Foundation and Earth'', the explorer who encountered the feral dog pack underestimated them because he couldn't even ''conceive'' of a dog being aggressive or dangerous, suggesting that they've been selectively bred (or discretely culled by human-protective robots) to be completely submissive and perhaps toothless.
* The way Galactic civilization declines outside the Foundation seems implausible, as every time knowledge of atomics is lost, the people for whom it's become LostTechnology seem to switch over to chemical fuels without much trouble. But these same declining societies are alleged to have forgotten pretty much everything about practical science and engineering of ''their'' time, let alone of technologies that became obsolete before recorded history. So who's figuring out how to power ground-cars and spaceships with chemical fuels, if there aren't any competent mechanics or physicists to reverse-engineer them? Sure, fossil fuels may be "primitive" by Galactic standards, but they're ''far'' from entry-level technologies that just anyone could whip up on the fly.