"Why invent Atlantis?" is a very good question - and one which is normally brushed off with some mumbling about a "moral tale" somehow designed to be edifying to - and instructive for - the Athenians of the "Social War" era in which Plato was putting quill to parchment.
This page covers some six possible explanations for the invention of Atlantis. These are, to an extent, mutually exclusive, as they depend on how one wishes to answer the question of whether Plato's character Critias - the narrator of the Atlantis tale - represents Plato's own ideas, or else is a representation of a position Plato seeks to refute.
The first two theories I wish to investigate centre upon the notion that Plato's Critias is the in-story originator of the story.
The first of these is explicit in the text: a thought experiment, the primary goal of which was to depict Plato's ideal city in a state of action. In the Timaeus [19c], Socrates is made to state the following: -
Tellingly, Socrates himself expresses his inability to carry out this experiment, handing over to Critias (and Hermocrates), which ties this theory in with the next I would like to cover.
Of the sophists, who are compared to poets, Socrates opines [Tim. 19e]: -
While pointedly removing Critias from this grouping, it soon becomes clear that Critias too is "wide of the mark": though it may be that the text of the Critias, which is incomplete, is wanting, Critias fails to provide a detailed account of the war, instead outlining only the general condition of the Athens of around 9,500 BC and - in much greater detail and with far more relish - that of its fearsome Atlantic enemy. Critias' blustering style bears the hallmark of the speeches of the sophists.
Nonetheless, it would be remiss not to note that Critias, in contrast to the roaming sophists in the quote above, was very much tied to Athens, with limited evidence for his activities elsewhere, albeit, as noted by Xenophon, he did spend rime in Thessaly, ostensibly "establishing a democracy," presumably on the Athenian model.
For the purposes of the remaining motives dealt with here, the identification of Critias as the "author" of the Atlantis myth is abandoned, with the myth representing an attempt by Plato to pass on specific teaching.
In this particular section, the 9,000 year datum is of paramount importance as a means of establishing Athens' great antiquity vis-à-vis Egypt, and the work stems from criticisms of Plato's perceived plagiarism of Egyptian prototypes in his Republic.
In the Saïte priest's speech, during which he shoves all manner of sunshine up the collective Athenian fundament [e.g. Tim. 23de, 24d], there appears to be a great deal of focus on Athens' temporal pre-eminence over his own civilisation: in her guise as Athena, the Saïte goddess Neith founds Athens a millennium before Saïs. Alan Cameron notes much disagreement in the centuries after Plato as to the precise nature of the relationship between Athens and Sais, which Plato notes at Tim. 21e, and cites Proclus Lycaeus, who mentions contemporary jibes about Plato's borrowing his political notions and hierarchical system from the Egyptians.
According to Proclus Lycaeus [On the Timaeus of Plato 1.75-76]: -
Plato's contemporary and rival Isocrates [Busiris 15] says, of the mythical pharaoh Busiris: -
Perhaps even more significantly, Plato's student Aristotle [Politics 7.1329ab] is convinced of the pre-eminence of Egypt with regards to the caste system: -
With regards to the fragment highlighted, the translator H. Rackham notes that it is "[p]erhaps to be read as denying the originality of Plato's Republic," which suggests that Proclus' statement that Plato's work being ridiculed as derivative of Egyptian custom was based on real criticisms.
Consequently, Plato's timeline for the development of the civilisations of Atlantis and Egypt by Athena-Neith can be read as a response to these criticisms, in that the Saïte priest is made to declare the primacy of Athens [Tim. 24c], whilst admitting similarities between the two systems [Tim. 24ab] and, indeed, the division of the Athenians by Ion into four distinct castes is similar. Herodotus also outlines the elaborate caste structure employed by the Egyptians [2.164]: -
Plato rightly points out that the Athenians too had ancient traditions of class distinctions (indeed, it could be argued that the tetrapartite system of the Athenians represents a closer approximation to Plato's schema than does the Egyptian model), and is ultimately citing Egyptian authority to refute claims that he derived his scheme from the ancient country on the Nile by providing a precedent for the Egyptian caste system located at Athens itself.
As such, Plato's riposte to his alleged critics represents a breathtakingly audacious piece of propaganda.
As noted elsewhere on this site, Plato's familiarity with the west stemmed from his interest in the political scene in Magna Graeca, and the Sicilian city of Syracuse in particular.
During the course of his career, he made several trips to Italy. During the first, as noted above, he befriended Dion, a key figure in Sicilian politics during the following decades. For his part, Plato was disapproving of the dissolute lifestyle of his hosts, which would be a bugbear as his efforts alongside Dion proceeded during the reign of Dionysius' son and namesake Dionysius the Younger.
These endeavours prompted a second journey, which took place in around 367 BC, which formed an attempt to mould Dionysius into a "philosopher king" along the lines formulated and set out in Plato's Republic, which he had likely recently completed. Dion had indicated that his young charge was interested in philosophy, but was often misled by his companions at court.
Plato was cynical about Dionysius' commitment and feared the worst, a judgement which proved to be well-founded.
Plato endured many trials in the west on Dion's behalf, and, while their mutual desire to mould Dionysius' character may have been genuine, Dionysius' allies were suspicious of Dion's motives, noting that he had some claim on the rule of the city, and suborned the tyrant. Both men were eventually forced into exile on the Greek mainland, where Dion joined the Academy, becoming close to Plato's nephew and eventual successor Speusippus.
He eventually returned to Syracuse with a Zacynthian force - his "guardians" no doubt - but proved to be a poor strategist. Nevertheless, he twice managed to enjoy some measure of power in Syracuse, though his Platonic agenda was poorly received by the populace, who demanded democratic reforms.
Dion was eventually assassinated by his Zacynthians in 354 BC at the behest of Calippus (or Callicrates), an Athenian, who was a fellow student of Plato's. He was no more successful a "philosopher king" than Dion and met the same end thirteen months into his rule.
Dionysius, meanwhile, had sought refuge on the mainland, tellingly in Timaeus' home city of Locri Epizephyrii (also the birthplace of his mother Doris), where he became tyrant for a decade. The reaction of the Locrians is best summed up by the following description by Schlomo Berger: -
During the events of Dion's rises and falls from power, Plato was back in Athens, writing his Timaeus, Critias and later his Laws. The weakness of any constitution which invests power in one man, in this case Dionysius, were by then self-evident, as the condition of the city was dependent upon the character of the leader. In Dionysius' case, this character was found wanting. He was a lover of luxury, capricious, and only superficially interested in philosophical learning. The same yardstick could be applied to the archons of the people of Atlantis: once the inherited higher nature began to wane, these began to exhibit the same traits as Dionysius.
It is surely no accident that the two dialogues feature characters from both Syracuse in the form of Hermocrates, the spiritual father of the tyranny there, and the city Dionysius imposed his terror upon once ousted from Syracuse.
It is also telling that, in the Laws, Plato outlined a new - though, significantly, still only "second-best" [5.739a] - city constitution. Perhaps this was intended to be more practical in the real world than the purely theoretical Kallipolis of the Republic, whose true existence would only be possible in the higher world of Forms.
Atlantis would then be a bitterly-disappointed old man's excoriation of people, places and events which had for him, at one time, held such high promise.
This fourth motive stems from ideas expressed by Plato in his Republic and again later in his Laws: the "Noble Lie." The scholar Kathryn Morgan has done detailed work on this in her survey of the Atlantis tale's place in mid-4th century Athenian history.
I shall quote from the Republic [3.414bc]: -
The "Phoenician tale" referred to here is almost certainly the myth of Cadmus, as suggested in the Laws [2.663e-664a]: -
In the Republic, he introduces a caste system (the one claimed as being a knock-off of the Egyptian system), along these lines: the ideal state is governed by phylakes ("rulers" or "guardians"), defended and administered by a warrior caste termed epikouroi ("auxillairies") and, under these and separated from them, are the polloi, the common people, broken down into husbandmen and artisans (two of the four classes which Strabo [8.7.1] credits Ion with having created), as well as merchants.
Basing this schema on myths such as that of Cadmus and the spartoi (his exemplar of the "Noble Lie") and Hesiod's five ages (gold, silver, bronze, heroic and iron [8.547a]), Plato suggests that those born with souls of gold are to serve as the philosopher aristocracy, whilst silver souls are given to those whose calling is warfare. The possibility of meritocratic movement between the classes based on the specific abilities of an individual - both upwards and downwards - is mooted [3.415c]. The city's working people possess souls of brass and iron.
The tale told by Critias is an extension of this argument, which depicts this ideal city in war against a powerful enemy. That enemy is Atlantis. Thus, for the purposes of this myth, Atlantis is the "Noble Lie."
Lastly, we must consider the most obvious - and most commonly cited - reason for Atlantis to "exist": Plato was seeking to caution his fellow Athenians as to the wages of tyranny.
This is often given by experts as a hand-wave as part of a dismissal of those promoting the reality of Atlantis, and with some reason.
For instance, Pierre Vidal-Nacquet has argued persuasively that many aspects of Plato's Atlantis find their real life counterparts in democratic Athens. Though is it all it's cracked up to be?
To my mind, when considering whether or not Plato wrote the myth of Atlantis in the Timaeus-Critias as a word of warning to the Athenians of the 360s, one must ask why he would need to concoct a fable from times so distant when a perfectly good example of an overbearing regime getting its comeuppance took place within Plato's and many of his contemporaries' lifetimes - and in Athens itself to boot.
The Delian League was founded in 478 BC with the purpose of facilitating Greek efforts to keep the Perians out in the wake of the defeat of the second Persian invasion of the Greek mainland the previous year. It was envisaged as an amphictyony centred upon the time-honoured temple of Apollo on the island of Delos, but, given Athens' ambitions (yes, my dear western compatriots, even "democracies" can be high-handed and imperialistic), it developed into something of an Athenian Empire, prompting attempts at secession by states such as Naxos (in about 471 BC) and Thasos (in 465 BC), wary of having traded the threat of a "barbarian" master for a Hellenic one. Both states were punished by the removal of fortifications and fleets, as well as a loss of voting rights in the League.
Athenian hegemony over their "partners" was cemented in 454 BC, when the pantomime of the centrality of the Delian sanctuary - and what remained of the pretence of equality - was ended and the League's treasury transferred to Athens. The subject states were burdened with demands for tribute, which only increased as the democratic Athenians persisted in a policy of "forever war," baiting the Spartans and kicking off the Peloponnesian Wars, which resulted in Athenian defeat in 404 BC and the dissolution of the League.
Thus, while starting with noble aims, and suitably featuring Athens as the leader of the Hellenes - echoing Plato's characterisation of his home state in the dialogues - the Delian League/Athenian Empire soon expanded its control over the Aegean islands and the "opposite continent" of western Anatolia (cf. Tim. 25a) in the manner of the Atlanteans, and - in the same vein - continued to deteriorate in character, until the Spartans (still led by their nobility, so a plus from Plato) put an end to these shenanigans.
This seems an overelaborate attempt to get people to listen, people who either remembered the end of the first Athenian Empire or people whose parents remembered it and, as is always the case with the next generation, knew better than their forebears and would do the same things again, only better this time.
No, far more likely to my mind is that Plato, in having Critias tell the tale, is indeed pointing to the end of the Delian League, but not necessarily as the handwaving academics would have us lesser mortals believe. Remember, it was none other than Critias who came to the fore in the post-Delian, Spartan-imposed oligarchy. He had his chance to install a better government than what had preceded it and failed, ephemerally and spectacularly.
In having Critias relate the tale in the unsatisfactory manner in which he does, Plato is perhaps pointing out that such men who would grapple the reigns of power out of the hands of the demagogues and strategoi may promise to improve matters, but very rarely do they succeed.
Despite his idealised state of Athens in the Timaeus-Critias and its forerunner in the Republic, Plato, it seems, was keenly aware of the flaws in human nature when power is presented on a silver platter.