- Technically, the Mule's conquests didn't spread modern technology throughout most of the Galaxy - it is clearly established that he not yet conquered half the Galaxy when he was stopped. The Encyclopedia Galactica's entries, on the other hand, is not quite implication to the contrary of this WMG - yes, they imply that the Foundation continues on a path roughly similar to that shown in Foundation's Edge, but the Encyclopedia is a Foundation project...
- The Mule's conquest was transitory (he started in his 20's and died in his 30's), so there would not have been time for technological knowledge to spread throughout the galaxy. There is a difference between merely possessing samples of advanced technology and understanding it, as was amply demonstrated by the post-Empire kingdoms in Foundation. What tech had been spread around would mostly have been incomprehensible to those who got their hands on it. Meanwhile, the First Foundation continued to advance. In Foundation and Earth the people of Comporellon want to seize Golan Trevize's ship because it represents advanced Foundation technology far beyond anything they have and they aspire to rebel against the rule of Terminus, which would be impossible with their own inferior technology. They were hoping to reverse-engineer the ship so that they could develop comparable ones.
But Daneel is the chessmaster playing the game across the galaxy. Gaia is a success, but has limits. Most notably, it cannot just assimilate anyone. Children possibly. But adults like Janov Pelorat cannot join the superorganism even if they really want to. Hence the Seldon Plan. A second Empire unifying the galaxy is a crucial stepping stone to Galaxia. Just as Daneel taught the Gaians the secrets of mentalics centuries earlier in order to create the group mind, it would be much easier to do the same to the entire galactic population if it were already unified under one government.
Thus, at the end of Foundation's Edge, Gaia wipes Mayor Branno of her ambitions of making a premature attempt to build the new Empire centuries early. Stor Gendibel is likewise mind-wiped of the events at Gaia, and returned to the task of advancing the Seldon Plan, and not coincidentally working to build a greater mental fusion than they have now. Gaia has set the Plan back on track.
Had Seldon lived long enough to run his calculations far beyond the establishment of the Second Empire he would have probably discovered Gaia/Galaxia. The Second Foundation almost found it when Stor Gendibel identified the "anti-Mules" that were seemingly doing even more to keep the Plan on track than the Second Foundation.
The Second Galactic Empire will be born, and will run for hundreds, perhaps thousands, of years while the Gaians gradually introduce mentalics to the population and construct the galaxy-spanning superorganism.
- 1 - Balance of Power: The Foundation plays off the local barbarian kingdoms of the Four Kingdoms against each other.
- 2 - Religion: Because science and technology are declining in the barbarian kingdoms, they come to revere Foundation tech with religious awe, actively encouraged into a mystical cult by Foundation leadership. Thus even when one of the Four Kingdoms becomes so mighty that balance of power doesn't matter anymore, the cult of science encourages a revolt by the general population against their leaders.
- 3 - Trade Alone: Eventually, the infiltration-through-science-as-religion gambit would play itself out. This is due to a combination of farther away kingdoms wising up to it (hearing about what happened to prior conquests), and due to the rise of regionalism/nationalism in farther-away barbarian kingdoms, given how much time has passed since they gained independence (the science religion couldn't make people outright betray their kingdoms anymore). Instead, the Foundation has gained enough power that it shifts into becoming a mercantile empire, applying economic pressures to win without a fight.
- 4 - Foundation & Empire: eventually the Foundation finally comes into conflict with the remnants of the Galactic Empire, mighty even in decay. Yet it is inherently impossible to stop the decline of the Empire at this point, so all the Foundation needs to survive is tenacity. A weak general would never be a threat to them, and if there was a strong general under a weak emperor, he'd rather seize the throne. The two can't be the same person: if a strong emperor tried to lead a conquest in person, corrupt royal intrigues would sprout up back in the capital. The only scenario dangerous to the Foundation would be a strong general under a strong emperor, but by the same token, the strong emperor would eventually feel threatened by the strong general and have him removed.
- 5 - Independent Traders: With no major external rivals anymore, the Foundation encounters the same corruptions of over-centralization that the Galactic Empire did on Trantor. Corrupt politicians make the office of Mayor hereditary, until the position is held by their incompetent grandsons. Conversely, the merchant-princes going out into the barbarian kingdoms to build mercantile empires amass considerable wealth and power on their own, and start chafing against the corruption of the central government. This will lead to a civil war, but while the traders will lose, it will result in the Foundation enacting democratic reforms to fix the systemic corruption that caused the crisis in the first place. This crisis was interrupted by the Mule, whose mentallic powers Seldon did not predict.
Foundation's Edge then skips ahead two centuries, to the eighth crisis. With the Foundation Federation now dominating about a third of the galaxy, there are arguments to move the capital closer to the center of its territory, not on far-away Terminus at the edge. This measure is opposed and fails: moving the capital closer to the center of the galaxy would make the Foundation over-eager, biting off more than it can chew by trying to conquer the rich and powerful provinces of the Interior sectors too quickly.
So...going by the analogy that it's "The Fall of the Roman Empire - In Space!", what could the sixth and seventh crises have been? Given that it's stated there are no major enemies facing the Foundation anymore? They're more powerful than any barbarian kingdom, and their last major external threat on a strategic scale was the Galactic Empire.
- Going by the analogy of Rome's early expansion, a major problem encountered by any expansionist empire if it succeeds far enough and long enough is the question of extending full citizenship to new conquests. The Foundation goes from controlling only a tiny fraction of the galaxy around the Four Kingdoms, to a third of it 500 years after its founding.
- That was already an issue but never rose to crisis-level. Hober Mallow wasn't from Terminus, but after a few centuries they started extending Foundation citizenship to people from the Four Kingdoms and beyond.
- One possible sign of some form of extension of rights to new conquest crisis: by Foundation's Edge, the political entity of the (First) Foundation is no longer referred to as simply "the Foundation", but as "the Foundation Federation". Perhaps the sixth or seventh crisis was not a crisis of the individual rights of citizens, but of increasingly unworkable centralisation around Terminus (in other words, the "rights" of other star systems in the Foundation)?
The eighth crisis was an argument to move the capital closer to the interior of the galaxy, a debate which failed, because this would lead to reckless over-expansion, instead of gradually incorporating more territory at a stable rate (having already gained control over a third of the galaxy). What were the final two crises?
This is a multi-faceted question, because it challenges the assumptions set up at the beginning of the story, due to the revelation of Gaia/Galaxia as an alternate plan for bringing about a Second Empire.
It's explained - on literally the last page of Foundation & Earth - that Galaxia would be the best option compared to the First or Second Foundations, due to the inherent limits of Psychohistory: it only works on a galaxy populated by humans, whose psychology and behavior is comprehensible and thus predictable. Even robots technically count as a human "action" and can be predicted by Psychohistory. There are two challenges to it:
- 1 - Sapient races apparently develop very rarely, frequently only one in an entire galaxy - the Milky Way only contained humanity. Statistically, however, there should be some galaxy out there in which at least two major sapient races developed. They would be locked in inevitable conflict to the point of extinction, giving the victor an aggressive and expansionistic mindset. This would drive them to spread out into inter-galactic conquests. In contrast, due to the vast distances involved, humanity never felt a need to conquer other galaxies, instead content to fight over control of the Milky Way. Such an alien race would eventually invade, and be inherently more powerful than other races that only control one galaxy - and its actions would be unpredictable by Psychohistory.
- 2 - Transhumans. Subsets of humanity might eventually modify themselves so drastically from baseline humanity - through genetic engineering, cybernetics, etc. - that they functionally become a different species, whose mindset and actions are incomprehensible to psychohistory. Such as the Solarians, a relict population of the original Spacers who kept genetically modifying themselves for thousands of years.
An interesting consideration, however, is that Seldon was aware of Gaia/Galaxia, but disagreed with it. He did intend for it to be a fallback plan or alternative should Psychohistory fail, but he gently chided R. Daneel Olivaw that it might not be necessary. Logically, this leads to several possibilities:
- 1 - The original "Seldon Plan" assumes it will be chosen instead of Gaia/Galaxia, or that Galaxia will fail somehow. In which case the final two crises don't really take it into consideration. This version of the Plan also strictly assumes that no Aliens or Transhumans appear to mess it up.
- 2 - Given that Seldon knew about Gaia, it's possible that the final two "Crises" actually anticipated and incorporated Gaia into its chain of events. Leading to two possibilities:
- 2A - Seldon intended Gaia to complement his Plan.
- 2B - Seldon opposed Gaia. He was willing to keep it around as a fallback option, but if the Foundation succeeded in creating a Second Empire without Gaia's intervention, he'd want a plan in place to defeat it.
- 3 - It's possible that the final two crises actually do deal with Aliens or Transhumans. Psychohistory might not be able to predict their actions - but by the same logic, Psychohistory didn't predict the Mule either. A mutant with mentallic powers (later revealed to be a renegade from Gaia). Are not the Second Foundationers, with their mentallic powers, essentially Transhumans already? Only a matter of degree removed from Solarians? Yet while Psychohistory couldn't "predict" the Mule, as such, Seldon's "Plan" went beyond mere predictions, with anticipated fallbacks and safeguards specifically against unpredictable events. In short, it's possible that Hari Seldon himself already predicted everything that the characters discuss in the final chapter of Foundation & Earth, and had already incorporated all of these possibilities into his plan!