“Before returning to Leonardo, we must mention two facts which demonstrate better than any description the extent of fascination with the problem of perspective during the latter part of the fifteenth century when perspective becomes virtually normative (as in Ghiberti’s modification of Vitruvius). In his -=Divina Proporzione=-, Luca Pacioli—the learned mathematician, translator of Euclid, co-worker with Piero della Francesca, and friend of Leonardo—celebrated perspective as the eighth art; and when Antonio del Pollaiuolo built a memorial to perspective on one of his papal tombs in St. Peter’s some ten years later (in the 1490’s), he boldly added perspective as the eighth free art to the other seven.

At the risk of exasperating many readers, we would venture to point out that this supercession of the number seven, the -=hepataos=-, can be interpreted as an indication of the symbolic conquest of the cavernous and vaulted heaven of unperspectivity. With the arrival of the eighth “art”,which can also be considered an eighth muse, the world of the ancient seven-planet heaven collapses; the “n-“, the negation retained in the (n)ight-sky [Nacht] of the unperspectival cavern gives way to the clarity and brightness of the eight (acht), which lacks the negating “n”. The heptagonal cosmos of the ancients and its mystery religions are left behind, and man steps forth to integrate and concretize space.”

— Jean Gebser
The Ever Present Origin
Ohio University Press, 1984, pp. 17