With his writing of The New Science, Giambattista Vico attempted to resist the validity of the new mathematical tool which he believed would put doubt into the 'truths' that united men and thereby create individuals which in turn would destroy civil society. He wished to keep philosophy within the realm of the probable and not reduced to mere sanitized mathematical perfection, for this would lead ultimately to skepticism and the decline of nations. This process was anathema to a man brought up on Greco-Roman conceptions of civil society (specifically the ideas of Aristotle and Cicero). Due to these strongly held convictions, Vico developed his New Science, founded upon a profoundly poetic version of civil society, which he called the 'sensus communis'. It was based on underlying principles of the beginnings of language and 'imaginative universals' grounded in metaphor. To understand Vico's 'sensus communis' one must first examine his concepts of 'imaginative universals' and language.
'Imaginative universals' derive from a belief in a common mental language  that manifests itself as 'vulgar' wisdom (maxims, proverbs, et cetera) which, although different across the world, express many of the same underlying concepts and views. To Vico, these came ultimately through the use of primal and primary metaphors which were universally recognized and preceded language, which is in itself divisive. This began a system of classification and therefore led to the mind's ability to objectify and operate critically.
The first sensation  led to the first thought through metaphor. Experience was emotional and expressions therefore were poetic metaphors. 'Imaginative universals' thus came into being as they were needed to deal with pressing emotions. These 'imaginative universals' created self-consciousness by fixing emotional sensations in time and projecting them onto an external reality removed from, but derived from, the body. By this process the 'part' became the 'whole'  (ie. Achilles is the 'imaginative universal' of 'brave'; therefore a 'brave' man is Achilles and not just merely 'like' Achilles).
Another key to the 'sensus communis' is religion. The first 'imaginative universal' was that of Jove, therefore religion actually precedes language and poetry and is grown from 'fear' (of the thunder) leading to a kind of religion, spirituality or superstition . This created a metaphor, which was, according to Vico, a fable in brief that united emotional force with practical judgement which then found its way into language. Thus the recognition of similarities between the body and nature led to a word and the fable of its origin simultaneously. These words were not spoken however, they were written. "All nations began to speak by writing since they were all originally mute". Vico reasoned that since all humans make sounds the same way, if speaking came first we would all speak the same language, therefore writing must have preceded speaking then the sounds were created to go with the symbols.
The first nations thought in poetic characters, spoke in fables and wrote in hieroglyphs creating a theologic language with metaphorical identification with the divine. The second nations, which followed, spoke in symbols, metaphors and images and wrote in signs (heraldic symbols) creating a heroic language which led to cause and effect and metonymic reduction. This was centred on aristocrats as opposed to the divine of the first nations. The third nations spoke in abstract terms and wrote a phonetic alphabet which led to synecdochic construction and was centred on 'vulgar' people. The last nations spoke and wrote in ironic discourse and therefore devolved into lawlessness presumably to later rise again as first nations. Each subsequent nation and language draws upon the previous age until the last which, although not explicitly stated, presumably begins the cycle again.
Speech, therefore, is the basis of culture, and not reason. It is the poetic heroes and not the philosopher kings which create society. Language must precede reason so that reason can be told (through the eloquent use of language). For example: To Vico, a normative legal text is utterly meaningless without living speech to clarify it. "Such manuals foster a habit of abiding by general maxims whereas in real life nothing is more useless"(Mooney:Pri.of Lang.p.209). It was better in his view to use the heroic Roman method of a minimum of laws where equity came with the skill of an eloquent lawyer.
Poetic wisdom was the synthesis of wisdom and eloquence, of res and verba. Poetry was not merely a product of the mind, but actually the logic of the mind's development. What counted for Vico was "the exchange between language on the one side and the reality it seeks to contain on the other"(Mooney:Pri.of Lang.p.202) which came togther in metaphors which were excellent for describing fluid realities. All language derived from these metaphors that reflected the primal community-founding judgements of the first men.
The 'imaginative universals' constituted the origin of the 'sensus communis' underlying agreements and 'mental dictionary', and made certain the decisions and the fable as the two were formed simultaneously, each ratifying the other. Thus the 'sensus communis' was based upon experience (the sensations leading to the formation of the 'imaginative universals'), not upon conceptions of a 'natural law'. Universal law was established by custom and was a reflection of the 'sensus communis'. It was the form of society itself and therefore the basis from which law was subsequently formed.
The 'sensus communis' was not based upon some mathematical, 'rational' ideal (such as Hobbes' Leviathan) but instead was the basis by which practical decisions were reached and made the choices that were reached certain (verum certum). It was "judgement without reflection, shared by an entire class, an entire people, an entire nation or the entire human race".
These were practical judgements based upon the needs and utilities around which a community had formed a consensus or agreement. These uniform ideas were communally based, and not centered on the individual (as in Descartes writings).
To Vico, there was a natural progression from religion (with the first metaphor starting the process) to language; language then to institutions which all people of the world practice (religion, marriage and burial rites); institutions led to property rights which then finally led to broader legal rights in society. There was a danger however, the further we progressed the more skeptical we seemed to have become.
This skepticism, captured in Cartesian thought, worked contrary to the 'sensus communis' and the skills required for effective public life and action. Vico feared the 'descent into multiplicity' which led to specialization and the growth of bureaucrats and technocrats. He believed in the certain failure of logic/reason, as represented by the Cartesian method, to come up with solutions for the civitas as it was too centred upon the individual.
Only rhetoric with substance (eloquence) gained any true insights on the true in the civitas. These truths were created by experience and exposed through eloquence. Society would fall apart when the philosophers forgot how to communicate and the rhetoricians became merely clever and not true, with the advent of the last nation's language of irony.
To stave this off, the citizens had to be trained to understand the conceptions of community and not just individuality, as this would later manifest itself in the political system. This individuality went entirely against Vico's wishes for a functioning Republic in which all men acted as citizens (civitas). For him, "keen perception and vivid language [were] sources of all freshness in a culture as well as the guarantee of its future" (Mooney:Vico Trad.of Rhet.p105)
There are some areas where Vico's logic is questionable however, and most derive from his deconstruction. He only goes so far and fails to take the next logical steps which might undermine his arguments. For example: Vico shows that the ancient gods are merely man-made constructs (metaphors) but he does not come to the obvious conclusion that his god is merely a construct like Jove. He prefers to leave his system as a subjective reality stuck within an objective framework created by his god and therefore unknowable, vague and not worthy of examination. The creation of Jove is interesting in itself for it seems to imply an inherent spirituality or else religion is a spontaneous construction, but no real proof is offered up either way. If it was just a 'leap of faith' why were the giants alone able to do it and not the other animals?
The origin of language deriving from metaphors is also disputed by modern linguists. How can there exist a mythical/poetic/metaphoric language without already possessing a language to describe the metaphors? What of syntax and grammar? What could be the metaphor/fable of 'the' or 'of'? The argument being made today is that language is innate because to learn language one must already possess a representational medium- it is not spontaneous.
In spite of these and other difficulties, Vico still managed to seem somewhat prophetic in his description of the decline of civil society and the rise of individuality, bureaucrats, the isolation of modern society and the decline of language. For this alone, if nothing else, Vico and his ideas of 'sensus communis' are worth examining.
Bergin, Thomas & Fisch, Max (transl) The New Science of Giambattista Vico, (Cornell:1984).
Jacobitti, Edmund "Political Thought and Rhetoric in Vico", New Vico Studies, vol IV (1986),
Chomsky, Noam "Language and Problems of Knowledge", The Philosophy of Language 2nd ed.
Mooney, Michael "The Primacy of Language in Vico", Vico and Contemporary Thought,
(Humanities Press:1979), p.191-210.
Mooney, Michael Vico in the Tradition of Rhetoric, (Princeton:1985), p.100-106.
Schaeffer, John "sensus communis: Vico, Rhetoric and the Limits of Relativism", New Science,