Humanist architectural theory was losing its authority even as it was being widely disseminated; it was also, as was only half clear to Gwynn, becoming increasingly unintelligible. Wealthy patrons of the art looked more and more upon exact knowledge of it as unbefitting the learning of a gentleman. Archaeological studies of antiquities, instead of helping to fix the rules of proportion, were contributing to aesthetic relativity by demonstrating the disparity between ancient practice and Vitruvius's rules. Claude Perrault attempted to resolve these disparities by a system of mathematical averages, but the result of his empirical method is only to substitute one source of relativity for another.[8] By the 1740's what Rudolf Wittkower has called the "break-away from the laws of harmonic proportion"[9] was well under way, and it represented but a part of the collapse of the several systems of arithmetic and geometric proportion which had dominated humanist theory. Developments in the history of thought made this collapse inevitable. The old aesthetics were based upon correspondences between divine and human artifacts. Thus in designing a building the architect emulated the Divine Architect who "ordered all things in measure and number and weight" (Wisdom 11:20). The geometric forms and the systems of mathematical and harmonic proportions of a building answered to those of the cosmos; likewise the aesthetic attributes of the cosmos—with their attendant moral ones—such as symmetry, uniformity, regularity, and fitness had their correspondences in architecture. Such assumptions provided immutable bases for the rules external to the individual work of art, but the breakdown of analogical reasoning in the seventeenth and eighteenth centuries necessarily undermined such philosophical-theological foundations. The new epistemology, further, shifted attention from the external world—the previous source of the rules—to the mind perceiving it.
By the 1740's consciousness was growing of threats against the ethical and aesthetic values of Renaissance humanism (best expressed in Pope's Dunciad in Four Books), and of the consequent need for new sources of authority for the rules of art. One highly eccentric quest for authority was published just a year before The Art of Architecture: John Wood's The Origins of Building; or, The Plagiarism of the Heathens Detected (1741). Wood reconstructs the history of architecture to make it conform to Old Testament chronology. Thus he attributes the major tenets of Vitruvius's architectural theory to various patriarchs and ancient Jewish heroes, or, when he finds any justification for doing so, directly to God. Gwynn's attempt to buttress the rules is far more mundane. He seeks support from contemporary philosophy; for example, he introduces the epistemological and ethical systems of Shaftesbury to account for some principles of decorum, but without perceiving the subjectivity he was imposing on them. He rationalizes some of Vitruvius's analogies between natural and architectural forms. But even more clearly indicative of the futility of his effort are his appeals to authority. He implores such aristocratic patrons as Pembroke, Chesterfield, and Burlington to "Be to my Muse a Friend; assist my Cause;/Be Friend to Science, fix'd on Nature's Laws" (p. 30). Perhaps most important, however, is the authority of Horace himself, who provides the model for the poem.
Although neoclassical critics generally accepted the reality of correspondences between architectural and literary criticism, Gwynn did not find the Ars Poetica an entirely manageable model.[10] Horace's figures of the mad painter and the mad poet which frame the poem at either end serve Gwynn well, for his imitations of them as the mad painter and the mad architect emphasize the personal, social, and artistic consequences of attempting to build without rules, talent, or even a clear need to build. But within the poem, allusions to Horace are often much more elusive. He usually succeeds best in keeping close to Horace when citing the most general principles. Thus Horace's attack on bombast and timidity ("professus grandia turget;/serpit humi tutus nimium timidusque procellae," 11. 27-28) occasions an attack on misunderstood magnificence and on stodginess:
Others affect Magnificence alone;
And rise in large enormous Heaps of Stone;
Swell the huge Dome, and Turrets bid to rise,
And Towers on Towers; attract the Gazer's Eyes.
Some dare not leave the old, the beaten Way,
To search new Methods, or in Science stray ...