The present volume issues from a 2007 conference at the Centre for History of Science, Ghent University, part of the project "Re-evaluation of the Sciences in the Low Countries in the Renaissance." The Ghent Centre is outstanding in high-standard programs and initiatives inspired by Fernand Hallyn and by the editor of this volume, Hiro Hirai, who worked (2005-2008) at Hallyn's Centre. Hirai is a former disciple of Robert Halleux and Bernard Joly, witness his outstanding study on "semen" in Renaissance theories of matter,  and is currently a Research Fellow at the Philadelphia Chemical Heritage Foundation.
The recent demise of Fernand Hallyn inflicts a loss on history of science scholarship, for his works have far-reaching merit. In Renaissance et Réforme we reviewed Hallyn's study (his last) of Cornelius's father, a Frisian doctor and mathematician. 
When Hallyn's Gemma Frisius, arpenteur de la terre et du ciel (2008) is placed beside the essay collection here reviewed a careful inventory of both Gemma's works, and their cross-fertilisations throughout sixteenth century European science, unfolds.
In Gemma Frisius Hallyn showed how the father chose calculation and measurement,--the path of scientific innovation--over submission to ancient theories. He repeatedly replaced models and measuring instruments derived from Ptolemaic principles by ones that corrected them, by dint of calculations. For this, as Hallyn demonstrated, Copernicus's work was of distinct importance, albeit Frisius did not eschew corrections to Copernican miscalculations either. Thus, in his De Astrolabo catholico (posthumously published by Cornelius in 1556) Frisius proposed a universal astrolabe. He moved its projective centre to the vernal point of the equinox; this allowed an infinite retreat from that point, consequently including the observer in the projection. He thus replaced the Ptolemaic astrolabe with its projective centre located at one of the celestial poles, which placed the observer outside the projected space. The universal astrolabe exemplifies a conceptual awareness of space and an urge to "recalculate" previous achievements in science on the part of early sixteenth-century humanists and Gemma Frisius in particular. That the modern distinction between "scientific truth" and "ontological truth" signalled by Hallyn in treating Gemma senior, does not account for the son's work, Hallyn immediately shows in his lead article to this Cornelius Gemma collection: "A poem on the Copernican system: Cornelius Gemma and his cosmocritical art." Hallyn explores Gemma's use of similitude as a chief tool to explain astronomical phenomena. Rooted in medieval Christian tradition is the analogy between "natura viciosa" and "materia viciosa"; this directly invokes mankind's fall (Genesis ) as the cause of earth's material corruption (De natura divinis characterismis , 1569).
Professor of medicine at the University of Louvain--successively regius professor (1569) and professor ordinarius (1574)--Cornelius Gemma (1535-1578) was part of the nexus of significant contemporaries like Guillaume Postel, Jacques Charpentier and Antoine Mizauld, and referred to by not the least among early modern scientists: Tycho Brahe and Johannes Kepler.
Cornelius Gemma. Cosmology, Medicine and Natural Philosophy in Renaissance Louvain is structured by the title's themes. Part one treats Gemma's cosmology and astrology, part two, medicine and prodigy, and part three focuses on method and sapientia . If the collection is here and there repetitive, inevitable in such symposia, different views and focuses of the eight authors assure a fruitful variety. The collection offers particular interest and stimulation in that it demonstrates--dispelling some current misconceptions--how, on the one hand, Neo-Platonist thinkers, notably those who foregrounded the heterodox origins of sapientia (Ficino's prisca theologia theory) were integrating principles of calculation and measurement and how, on the other hand, evolution of scientific reasoning was propelled by expanding the hypothetical dimensions of space per experimental (conceptual) models, no longer a priori bound by canons of Aristotelian and Ptolemaic cosmologies. In fact, this "emancipation" had already started in the late thirteenth century, but only now received a new, decisive impulse.
Christian Platonism in general and Marsilio Ficino's Platonic Theology in particular reverberate the most throughout all the essays. Hallyn, Hirai, Clucas and Leinkauf show that the universal method proposed by Gemma sources in that text and that his De arte cyclognomica (1569) strives to harmonize Plato, Aristotle, Hippocrates and Galen. A universal method for what?
Cornelius was preoccupied with the explanation of "critical signs" (critica signa ) in a framework at once both astrological and cosmological, a science called cosmocritical art. By "critical signs" we must understand phenomena deemed irregularities in nature, just like the monsters and prodigies described and interpreted as presages by Polydore Vergil, Conrad Lycosthenes, Ambroise Paré and Jerome Cardano, all well-known humanists and encyclopaedists. Where heavenly extravagances were at issue, Cornelius Gemma contributed to the discussions of his time and, as the present volume shows, in no unimportant way. He was puzzled, as many a contemporary, not least Tycho Brahe, by the conjunction of Saturn and Jupiter in 1563 and in 1572 by a huge luminous glow visible to the naked eye, a "new star" in the constellation of Cassiopeia. Kepler pursued this phenomenon in his 1606 De stella nova , two years after the appearance of yet another "new star." In fact it was death not birth, a star exploding at the end of its life, a supernova, as found by astronomical research a mere forty years ago and, not yet fully understood, studied down to the present day. (See Nature , December 4, 2008). Soon after, in 1577, a comet appeared, fascinating Gemma as well, witness his De prodigiosa specie naturaque cometae , published in Antwerp in 1578. To explain the 1572 "new star" Cornelius conceived space to transcend the heavens visible to the eye; this is what he proposed in his De naturae divinis characterismis (1575), a treatise of "cosmocritics" that followed on his De arte cyclognomica . With these works, the Louvain scholar challenged the old system of celestial bodies and space and harvested, alternately, Brahe's agreement and disagreement.
In his contribution "La notion de prodige selon Cornelius Gemma" Jean Céard, whose La nature et les prodiges (1977) stands as a landmark in "prodigies" historiography, shows how Gemma merged with debates of his time on the question whether nature itself caused prodigies or whether these were matters of divine marvel. An answer was prerequisite to reading signs, to knowledge of them. Cornelius proposes a solution in his De naturae divinis characterismis ; its point of departure is an interpretation of Hippocrates's divinum quid and the platonic concept of the universe as an animate being that is at the same time multiple. He distinguishes two sorts of prodigies, materialised and non-materialised, the first one "supernatural," the second one coming from God and taking the form of dreams, ecstasies, vaticinations or oracles.
The way the Louvain scholar interpreted and inferred from Hippocrates led him to an extended system, as Hiro Hirai shows in his comparative essay on Neoplatonic readings of Hippocrates by Fernel, Cardano and Gemma. Gemma connected medicine with prophecy. This inheres in the chain going from animate celestial heat to the spiritus , by him equated with innate heat. In other words, coherence obtains between macrocosm, microcosm and human society. By these associations he arrived at a subdivision of medical art that accommodates the divine and allows for divination. He read this into Hippocrates who, for him, had been at the birth of the prisca theologia transmitted down to Gemma's time, and was thus at the origin of continuity from Greek to Christian culture, a continuity sought by the latter's gatekeepers. Concetta Pennuto in her essay on "Cornelius Gemma et l'épidémie de 1574" further describes Gemma's achievements in the domain of medicine, more precisely aetiology.
Comparative also is Gemma Ernst's contribution: "Il linguaggio universale dei cieli. Cornelio Gemma, Tycho Brahe, Tommaso Campanella." It treats the now-similar, now-divergent explanations these three thinkers gave for the surprising celestial events earlier discussed above. Ernst traces the transmission of the practice of prophesy from celestial phenomena, now joined with new astronomical insights and scientific measurement. Prophesy, of course, conflicts with attempts to emancipate human reasoning from credulity, and attempts to disjoin humanity, history, human consciousness and will from celestial events and advents of natural phenomena tout court . Ernst explains why Cornelius's particular attention to visions and imagery, as signs of the flaws of his time or of hope for better times, provoked Tycho Brahe, otherwise appreciative of Gemma's erudition, to accuse him of superstition and hubris. Brahe's matter-of-factness is relative, however; he too read predictions of political events into the new star (interpretations that would have displeased Pico della Mirandola), even adopting some of Gemma's ideas. Tommaso Campanella would echo these themes in 1637.
In his article concentrating on discussion about the 1572 supernova during the 1570s, Dario Tessicini shows, in virtuoso ways, how Gemma's contributions and the reactions they provoked, attempted both to conserve the conventional geocentric cosmology of finite space of Artistotle and promote, at the same time, hypotheses that opened the door to an indefinite extension of the cosmic space. Kepler later advanced these, with reference to Gemma, as did Giordano Bruno (De immenso et innumerabilibus ), also a reader of Cornelius. Tessicini points out that contemporary readers were especially attracted to Gemma's premise (De naturae divinis characterismis ) of rectilinear movement to explain the nova's apparition in 1572, as an alternative to the luminosity-thesis. Following on this premise, non-orthodox ideas about an indefinite extension of the cosmic space developed. The different readings of Gemma's work in the late sixteenth and early seventeenth centuries evince the philosophical and scientific relevance of his theses, despite Gemma's recourse to divine intervention when natural phenomena mis-matched with Aristotelian physical and philosophical principles. It was Bruno who resolved the dilemma on a philosophical level; his infinite physical universe implied realisation of all possible forms of being, and the fusion of efficient and second causes, supernatural and natural; the miraculous can be excluded.
The methodological implications of Gemma's work are clarified in the last part of the volume, "Method and Sapientia." Gemma's universal method--for what?--, developed in his De arte cyclognomica and De naturae divinis characterismis is scrutinised with elevating clarity by Stephen Clucas. He begins with a brief review of the modern history of science. Gemma, he points out, is usually absent. Petrus Ramus, whose mid-sixteenth-century work drew on Rudolf Agricola's dialectical method, is usually represented as the "debut" of early modern methodology. Clucas brings to light the Platonic branch of methodology in natural philosophy that flowered in Cusanus, Ficino and Bovelles, and subsequently, via the pen of Gemma, in Bruno and Kircher (cf. Leinnkauf 1993 and Muslow 2000, 2005).
Clucas describes how Gemma, starting from a threefold structure of the cosmos and from analogical inference, develops a circular method, imitative of the circular structures in the mind of God, in the human soul, and in nature. Thus, the method imitates creation's infinitude, thus the method is a universal one: the knowledge it potentially generates, infinite and universal. Therefore, argues Clucas, Gemma's art is simultaneously a work of logic, physics and metaphysics. If he explicitly strove to unify Aristotelian considerations--the univocal and physical--with Platonic metaphysics and analogy, he ultimately comes down on the Platonic side (thus citations from Ficino, Hermes Trismegistus, the Orphic hymns, Pythagoras, and Plotinus), embracing the possibility of unlimited expansion of knowledge and an ultimate reduction to unity.
Thomas Leinkauf pursues the quest. His analysis of the first book of the Ars Cyclognomica ("Cornelius Gemma, Philosophie und Methode: eine Analyse des ersten Buches der Ars Cyclognomica ") shows how Neoplatonism inspired building methodological thinking. He gives special attention to theories, taken from both Aristotle and Plato, on the human soul and mentations: imaginatio , ratio , intellectus , each of these three directed to a different domain and category of knowledge. His treatment of the synthesising potency of Gemma's ars traces it back to Plato (in particular), Plotinus and Ficino. Multiplicity in one-ness and the all-permeating spiritus make their appearance again. This ties in with Hallyn's opening essay, which notices in Gemma's thought Stoic-influenced conceptions of pneuma , the all-infusing heat assimilated to God's action. However, as Ficino did, Gemma (unlike Pico in his De Ente et Uno )--aware of the lurking pantheism built into the Stoic principle--sought to maintain a division between this world and the self-referential Ens .
In sum, this volume convincingly illuminates how Cornelius Gemma's works interacted with those of his peers. We see how Neo-Platonic metaphysics and conceptions of sapientia form a junction with scientific measurement, in a field replete with both innovation and conservation. Gemma took up implications and debates extant in fourteenth-century Paris, in a scientific-methodologically and socially completely different context. At the pivot-point we find Ficino, who brought the junction of sapientia and actual observation to his readers in his De Vita where he harks back to Jewish and Oriental sources--with implications for the evolution of natural sciences.
1. Le Concept de semence dans les théories de la matière à la Renaissance. De Marsile Ficin à Pierre Gassendi (Brepols, 2005).
2. We mention here an erratum to that review, the book does include an index nominum ; it lacks an index rerum .
3. See Stéphane Toussaint, "Ficino's Orphic Magic or Jewish Astrology and Oriental Philosophy? A Note on spiritus , the Three Books on Life , Ibn Tufayl and Ibn Zarza," Accademia II-2000, pp. 19-31 and ibid. , "Ficino, Archimedes and the Celestial Arts," in Michael J. B. Allen and Valery Rees, eds., Marsilio Ficino: His Theology, His Philosophy, His Legacy , Leiden-Boston-Cologne: Brill, 2002, pp. 307-326.