Vico does not spend much time over the historical events of the royal period. Here in fact lay one of the chief differences between his criticism and that which had already been originated and was continued after his time dealing with the first centuries of Roman history. Vico aims not at substituting historical for legendary anecdotes but at understanding the essence of institutions and the ways in which they change. He uses two guiding principles, as we have seen in considering the royal period: first, that it was a period not of monarchy but of aristocracy and that therefore the type of heroic society or the patriarchal republic is applicable to it: secondly, that the names of the kings are symbols or "poetic characters" for the institutions of this society. In Vico's judgment, as we have had occasion to observe, the constitution of Servius Tullius should not be considered the basis of popular liberty, as the later Romans considered it; it was really the basis of the liberty of the feudal lords, since by it the patricians granted to the plebeians the bonitary tenure of their land together with the duty of paying rent to and serving at their own expense in war themselves, the patricians. And Junius Brutus, in driving out the Tarquins and replacing them by two consuls or annual aristocratic kings, restored to the Roman republic its primitive form; that is to say, he delivered the lords from the domination of their tyrants but left the people under the domination of their lords.

The patricians' oppression of the plebeians after the restoration of Junius Brutus and the struggles and resistance caused by it constitute the soul of the new development and contain the secret of the greatness of Rome, the "key to universal Roman history," clavis historiae Romanae universae. Polybius's explanation of this greatness is too vague. He describes it as due to the virtue or the religion of the patricians and relates the facts of this virtue rather than their cause. Vico also criticises Machiavelli, at one time because he adduces certain civil and military institutions as the cause of Rome's greatness without investigating the cause of those institutions, that is to say the character of Roman society: at another time for adducing what was only a partial cause, the high spirit of the plebeians. He thinks Plutarch worst of all, since envy of the virtue and wisdom of Rome leads him to ascribe her greatness to fortune. The fact was that Rome subjugated the other cities of Latium and then Italy and the world because her heroism was still young, while among the other Latin peoples it had begun to decay. Thanks to this youthful vigour the patricians were strong enough to preserve their order and the religion which formed its foundation and safeguard (the nobles, Vico observes at this point, were always and everywhere religious, so that the first sign of contempt for religion among them is a symptom of national decadence); the plebeians were spirited enough to demand a share in religion, auspices and all civil rights; the lawyers, lastly, were wise enough to interpret the old laws and apply them to any new case that might arise, and strove with all their might to alter the text of these laws as little and as slowly as possible. These were the chief causes of the growth and permanence of the Roman empire; for in all its political changes it contrived to remain faithful to its principles. Prowess in war was another result of the rivalry of the orders; since the nobles were naturally consecrated to the safety of their country, as the only means of preserving the civil privileges of their order, and the plebeians accomplished brilliant deeds in order to prove themselves worthy of patrician honours. And when the Romans extended their conquests and their victories over the whole world, they made use of four rules which they had already applied to the plebeians within Rome itself. They reduced barbarian provinces to the position of clients by planting colonies in them: they granted civilised provinces bonitary tenure of their land: to Italy they gave the quiritary tenure: and to the municipia, the towns which had earned better treatment, they accorded the same equality with themselves which the plebs had finally won.

The result of the first struggles, in which the point at issue was according to Vico the bonitary possession of land (a right already recognised in the constitution of Servius, but cancelled by the nobles in return for arrears of rent), is seen in the tribunate, and later, when the plebeians claimed the right of quiritary tenure, in the laws of the Twelve Tables, ratifying this plebeian victory. But the law of the Twelve Tables represented at the same time the victory of written law, the end of the secrecy with which the laws had been fenced round by the patricians, who alone knew, understood, interpreted and therefore applied them as they thought fit. This publication and codification of a written law cannot have been benevolently granted by the patricians out of that anxiety "not to despise the wishes of the plebs" of which Livy speaks; rather they must have resisted it with all the stubbornness which Dionysius of Halicarnassus describes and expresses in the phrase "mores patrios servandos, leges ferri non oportere" (our fathers' customs must be preserved, and laws must not be passed).

Later historians decorated the origin of the Twelve Tables with various legends. They told, among other things, of the mission sent by the decemvirs to Athens to bring back new laws: a tale given by Livy and Dionysius, but unknown to Polybius and discredited by Cicero. How, in the savage aloofness of primitive nations, between whom oral communication could only have been instituted by the necessities of warfare, alliances and commerce, could the fame of Solon's wisdom have crossed the seas from distant Attica to Rome? How could the Romans of that time have possessed such accurate knowledge of the quality of Athenian law as to believe it capable of setting at rest the strife between their plebeians and their nobles? How could ambassadors have travelled between Greece and those Romans whom seventy-two years later the Greeks of Tarentum could still maltreat as strangers? And what shall we say of ambassadors who returned carrying with them the Greek laws from Athens but without knowing what they meant; so that but for the coincidence by which Hermodorus the pupil of Heraclitus, an exile from his country, happened to be in Rome, the Romans would have been unable to make any use of this unintelligible and inaccessible treasure? Again, how could Hermodorus have translated the laws into Latin of such purity that Diodorus Siculus pronounced it devoid of the slightest taint of Hellenism, and with a perfection unattained by any subsequent writer of any period in a translation from the Greek? How did he contrive to clothe Greek ideas in Latin words so appropriate (for instance, auctoritas) that Greeks, Dio Cassius among them, declare that their own language has no corresponding words by which to explain them? Heraclitus's letter to Hermodorus must have been conveyed by the same mail that served Pythagoras in his distant voyages up and down the world: it is, in fact, an imposture of the first quality, and the whole story of the Athenian origin of these laws is due to the arrogance of scholars, who derived them first from the other Latin peoples (such as the Aequi), then from the Greek cities of Italy, then from Sparta and finally from Athens, with whose name, thanks to the renown of the Athenian philosophers, they were at last satisfied. No doubt, the laws of the Twelve Tables present resemblances not only to Athenian or Spartan laws but to those of various nations, the Mosaic code among others; but this is due to the uniformity of national history. No doubt, the decemvirs were in antiquity supposed to have originated laws bearing clear traces of Greek influence, such as that prohibiting the Greek style of mourning at funerals: but this is because as we have seen the decemviral legislation, like the names of the various kings, became a "poetic character," and to it were referred all laws later recorded in the public archives which tended to the equalisation of liberty. But the original law of the Twelve Tables, with its primitive rudeness, inhumanity, cruelty and ferocity, which agrees so ill with the period of highly-developed civilisation at Athens, is a document of the greatest value for the ancient natural law of the Latin peoples, and the customs which had existed among them from the age of Saturn.

Quiritary tenure of land and a written code of law once gained, the struggle recommenced over the question of the right of marriage. The true meaning of this contest has been lost among the absurdities written on the subject by the ancient historians themselves, in the belief that its basis was the desire on the part of the plebeians (who were little more than wretched and common slaves) to be allowed to form connexions with the nobles. This error has made Roman history even less credible than the legendary history of Greece; for if we do not know the meaning of the latter, the former is in opposition to the true order of human desires. It shows us a plebs aspiring first to nobility, secondly to offices and magistracies, and finally to wealth: whereas men desire first of all wealth, then offices in the state, and lastly nobility. What the Roman plebs really claimed was not "connubio, cum patribus" but "connubio, patrum": not the right of connexion by marriage with the nobles—a claim which they would not have wished to make, and was at bottom unimportant—but the right of contracting solemn marriages as the nobles did. For without such solemn marriages, without privilege of the auspices, the plebeians were in fact unable to enjoy the quiritary tenure of land and to transmit it to their families, deprived as they were of descent, kindred and relatives. The demand for connubio was, in a word, simply equivalent to a demand for the rights of citizens, and it was satisfied by the Canuleian law.

The next demand of the plebeians was for privileges depending on public rights. Of these they gained first the imperium together with the consulship, and lastly the offices of priest and pontifex, which carried with them knowledge of the law. In this way the system of seigneurial liberty planned by Servius Tullius grew into a system of popular liberty, and the census, which was originally paid to the patricians, was paid hereafter into the public treasury, out of which the expenses of the plebeians in war were paid. The tribunes now proceeded to demand the power of legislation; for the previous laws, the Horatian and Hortensian, had not made plebiscites binding on the whole people, except upon the two special occasions which led to the secession of the plebs to the Aventine and Janiculum respectively. This new victory, which established the superiority of the plebs and transformed the aristocratic into a popular republic, was the Publilian law due to the Dictator Publilius Philo and decreeing that plebiscites should "be binding on all the Quirites" (omnes quirites tenerent). The authority of the senate came out of the struggle somewhat impaired, for while formerly the fathers had acted as "auctores" for the deliberations of the people, they were now the proposers of law to the people, which the latter then approved according to the formula submitted to them by the senate, or else "antiquated" the proposal (antiquo, to vote against a measure) and decided to make no innovation. Besides this, the plebs won the last office to be conceded to them, that of censor. The Petelian law, a few years later, abolished the last remnant of feudalism, the bond (nexus) which made the plebeians the bondmen of the nobles for debt and often compelled them to spend their lives working in their private prisons.

Some time later, when the division between patriciate and plebs with the corresponding comitia curiata and tributa was replaced by Fabius Maximus's division according to the property of citizens, who were now grouped into three classes of senators, knights and plebeians, the order of the nobles disappeared entirely: "senator" and "knight" were no longer synonymous with "patrician," nor "plebeian" with "base-born." The Senate however preserved sovereign dominion over the finances of the Roman Empire, though the Empire itself had passed to the plebeians; and thanks to the so-called "senatusconsultum ultimum" it maintained this dominion by force of arms as long as Rome remained a popular republic. Whenever the people attempted to take it into their own hands, the Senate armed the Consuls, who forthwith declared traitors and put to death plebeian tribunes who had originated these attempts. This may be explained as a right of feudal sovereignty subject to a higher sovereign, a view confirmed by the language of Scipio Nasica when he armed the people against Tiberius Gracchus: "whoever wishes for the safety of the republic, let him follow the consul" (qui rempublicam salvam velit, consulem sequatur). And indeed, once the road to office was opened by law to the multitude which rules in a popular republic, there was nothing left in time of peace but to contest its rule not by laws but by force of arms, and for those in power to pass laws for self-enrichment like the Gracchan agrarian measures, resulting at once in civil wars at home and unjust wars abroad.

With the triumph of the plebs and the change of constitution from aristocratic to popular, the whole face of society changed. In the first place, the aspect of the family changed. Here, during the rule of the patriciate, testamentary succession was admitted only at a late date and was easily cancelled, in order to keep wealth in patrician hands: kindred even in the seventh degree excluded the emancipated son from the paternal heritage: emancipation had the effect of a penalty: legitimising was not allowed: and it is doubtful whether a woman could inherit. But in the democratic society, since for the plebs wealth, strength and power all depended on the number of their children, family feeling began to grow up, and the praetors began to consider its claims and to satisfy them by means of the "honorum possessiones," thus remedying the faults or shortcomings of wills and facilitating the diffusion of wealth, the only thing desired by the common people.

A change took place, again, in the meaning of the institutions of property. The civil tenure was no longer a matter of public right, but was dispersed among the various private tenures of the citizens now forming the body of the popular state. "Eminent" tenure no longer signifies the strongest kind of tenure, unencumbered by any actual charge, even a public charge, but applies simply to an estate free from any private charge. Quiritary tenure is no longer that of which the noble was feudal lord and under the obligation to aid his client, the plebeian, if ousted from it: it has become a private civil tenure, capable of being defended by a civil suit as opposed to the bonitary which could be maintained by possession only.

The forms of legal process were pruned of the luxurious growth of fictions, solemn formulae and symbolic acts, simplified and rationalised: the intellect, the thought of the legislator was brought into play and the citizens conformed to an idea of a common rational utility, understood as spiritual in value. Causes, which were originally formulae safeguarded by accurate and precise language, became affairs or negotiations solemnised by agreement and, in the case of transference of tenure, by natural tradition; and it was only in contracts said to be completed by word of mouth, that is to say in stipulations, that the safeguards remained "causes" in the strict ancient meaning of the word. Thus the certitude of the law, when the human reason was fully developed, passed into the truth of ideas determined by the circumstances of fact, a "formula devoid of any particular form" (formula naturae, as Varro calls it) which, like a light, informs in all the minutest details of their surface the details of fact over which it extends. In popular republics the ruling principle is the aequum bonum, natural equity.