Especially significant here is his insistence that the "Law of Science" will "no Mediocrity admit," for Horace discusses poetic practice rather than the rules which aid it. Secondly, the belabored inference drawn from the principle in the final couplet has no precedent at all in Horace. Gwynn has made every effort to place the rules outside the realm of human eccentricity and to give them the stature of "Nature's Laws."
Considering that the tenets of humanist architectural theory are traditionally classified very differently from those of literary criticism, as Gwynn acknowledges in his "Preface" (p. iii), he manages to accommodate them surprisingly well to the organization of the Ars Poetica. A good example is his treatment of Horace's discussion of the transitoriness of language, as of all things, and the necessary dominance of the rules of usage (11. 46-72). The most obvious parallel is the inevitable ruin of the pompous buildings which men erect (p. 11). But he develops none of the botanical analogies which Horace used to illustrate the rhythms of life and death (11. 60-69), for his purpose is to emphasize instead the parallel between usus and the architectural concept of "use." Horace insists, "multa renascentur quae iam cecidere, cadentque/ quae nunc sunt in honore vocabula, si volet usus" (11. 70-71). Gwynn elaborates on this:
But Use has rais'd the Greek and Roman Rules,
And banish'd Gothick Practice from the Schools.
Use is the Judge, the Law, the Rule of Things,
Whence Arts arose, and whence the Science springs.
(p. 11)[11]
Horace and Gwynn both think of "use" operating as a kind of historical necessity causing the resurrection of a rule or a form. Gwynn adds to this the concept, drawn from Vitruvius and his commentators, that the rules of architecture prescribe forms which satisfy particular uses and reflect directly the strengths and limitations of building materials and techniques. These are the major premises of Gwynn's assertion that "on Nature's perfect Plan,/ I form my System" (p. 31). To buttress this confidence in rules he develops the parallels between Horace's history of poetry (11. 73-98) and the history of architecture. Cecrops, the first king of Athens, is to architectural practice what Homer is to heroic poetry. Daedalus is to the theory of architecture what Archilochus is to the meter of dramatic poetry (pp. 11-12). The emphasis on the giving and systematizing of rules, although without precedent in Horace, reflects the same preoccupation with the authority of origins as John Wood's Origins of Building, Pope's Essay on Criticism, or even Locke's Two Treatises on Government for that matter.
Horace provides less precise correspondences for one of the most important rules. Vitruvius's rule of decor (De Architectura, I, ii, 5) is only generally parallel to the rules governing decorum of language, characterization, and genre. Gwynn nevertheless introduces all of its major implications. It dictates the observing of clear correspondences between a building's form and its use, inhabitant, and site (or "situation," to use the eighteenth-century term). It is based upon the notion that architectural styles have recognizable social, ethical, religious, and aesthetic attributes. The attributes, which evoke predictable psychological responses, express the uses which the styles were created to satisfy and the cultures in which they were developed. One brief passage beginning "If to adapt your Fabrick, you would choose,/ To suit the Builder's Genius, or his Use" (pp. 15-16) effectively summarizes the primary dictates of decor. The architect has to choose forms and ornaments having attributes in common with the social station and character traits of the builder of a residence,[12] with the deity to whom a temple is dedicated, with a building's function, or with the landscape in which it is placed. Thus, for example, the heavy, plain Tuscan order is appropriate for "A little Structure; built for Use alone," the "gaiety" of the middle Ionic order for a country villa, and the "delicacy" of the Corinthian order for an elegant church or palace (p. 25). An architect's skill is most often measured in the poem by his adherence to decor.
One violator of decor serves as a focus in an important passage wherein Gwynn tries to integrate Vitruvius and Horace while making a transition to one of his central concerns, the peculiarly English reinterpretation of decorum of situation. In discussing the difficulty of treating traditional subjects in novel ways, Horace compares an erring "scriptor cyclicus olim" with Homer, summarizes some general principles, and then turns to a consideration of how to win public applause (11. 119-178). Gwynn is more suspicious of originality than Horace (p. 17), and uses Ripley as an example of one who erred in trying to avoid customary forms. Ripley's Custom House (1718) and Admiralty building (1723-26) become the equivalent of the Cyclic poet's bad verse, while Morris, Flitcroft, Gibbs, Leoni, and Ware become the modern Homers of architecture (p. 18). Gwynn ends the verse paragraph with Horace's theme of suiting the parts to the whole. With Ripley's performance as a background Gwynn turns to architecture's most fundamental rules: