# Galileo Galilei

Galileo di Vincenzo Bonaiuti de’ Galilei (/ˌɡælɪˈleɪoʊ ˌɡælɪˈleɪiˌ/ , Italian: [ɡaliˈlɛːo ɡaliˈlɛi]; Pisa, February 15, 1564 – Arcetri, January 8, 1642) was an Italian physicist, astronomer, philosopher, mathematician, writer and academic, considered the father of modern science. Key figure in the scientific revolution, for having explicitly introduced the scientific method (also called “Galilean method” or “experimental method”), his name is associated with important contributions in physics and astronomy. Also of primary importance was his role in the astronomical revolution, with his support for the heliocentric system and the Copernican theory.

First son of Vincenzo Galilei, musicologist, and Giulia Ammannati, of an illustrious but decayed family, in Florence (where his father had moved to devote himself to trade) he had his first cultural education mainly humanistic-literary. In 1581, on his father’s advice, he enrolled at the faculty of medicine of the University of Pisa, where he had the opportunity to learn Aristotelian physics following the courses of F. Bonamico; Galilei, in fact, never showed particular interest in the study of medicine, which he abandoned definitively in 1585. Previously he had begun the study of mathematics under the guidance of O. Ricci, who initiated him to the reading of the great works of the Greeks, in particular of Archimedes from whom he derived a practical and instrumental conception of mathematics, typical of all his later thought.

Left the university without having achieved any degree, he returned to Florence, where he wrote his first works in which he alternated the literary interest with the scientific one. Are of 1588 the two lessons “On the Shape, Location, and Size of Dante’s Inferno,” while, in close connection with his studies Archimedean, since 1586 had continued the research of mechanics and had created the hydrostatic balance for the determination of the specific weight of bodies, in 1586-87 had discovered some theorems on the center of gravity that were published only in 1638.

In 1589, thanks to the help of Guidobaldo Dal Monte, he obtained the chair of mathematics at the University of Pisa with a three-year contract with little remuneration because it was a secondary teaching. While in the lessons he kept to the traditional topics, privately Galilei continued his research on the isochronism of the pendulum (whose first intuition he had in 1583, while, according to tradition, he was in the cathedral of Pisa), the experiences on the fall of bodies and especially the studies on the problem of motion, even in light of the theory of impetus that, thanks to Giambattista Benedetti and Niccolò Tartaglia, had reached a wide diffusion in Italy; representative document of his positions, still scholastic, is the De motu, remained unpublished. However, he did not neglect literary studies, as it appears from the writings “Considerazioni sul Tasso and Postille sull’Ariosto”.

## Scientific and philosophical thought

The central nucleus of Galilean research is represented by the dynamics that, as Lagrange said, Galilei “baptized”. Even if Galilei did not give explicitly the formulations of the three laws, as they are found in Newton, we owe to him the overcoming of the ancient conceptions and the clarification of the basic concepts of dynamics. We must also remember: his studies on magnetism; his investigations on hydrostatics; his researches on the oscillations of the pendulum, which led him to observations on acoustic phenomena, in particular on resonance and musical intervals; his researches on the resistance and on the force of machines (among these Galilei included animal bodies) similar, but of different scale, which are at the basis of the study of biological mechanics.

The breadth and depth of the turning point that Galilei’s work contributed so significantly to Western culture was possible because of the methodology he elaborated and the general philosophical attitude that is the most controversial part of his thought. Considered a Platonic, because of the function that mathematics plays in his physics, the Aristotelian elements of his thought have also been emphasized, while others have mainly stressed the methodological aspects to the detriment of a dogmatic philosophical vision.

The fact that Galilei looked for points of contact and support for his work in the various philosophical systems known at that time, rather than trying to adapt his work to one or another of these systems, perhaps highlights the priority that Galilei gave to scientific problems over philosophical ones, and therefore the priority of the methodological aspect over that of a systematic coherence. In this fact is perhaps the neuralgic point of Galilean investigation. From this point of view, the destruction of Aristotelian physics, the liberation of science from the principle of authority, its release from philosophical problems, which was reproached by Descartes, are configured more as consequences, as points of arrival than as the driving forces of his thought.

Galilei freed physical research from Aristotelianism, but his anti-Aristotelian position, which dates back to the Pisan period, was determined by the denial of logical deduction as a fundamental criterion for scientific research. An example is the distinction between primary and secondary qualities of substances, according to which the former are “magnitudes, figures, multitudes, and late or fast movements” to which Galilei attributes a reality that he denies instead to the latter, namely “colors, tastes, smells, sounds”. This distinction, to which Galilei is certainly indebted to Greek atomism, had considerable importance in the subsequent development of philosophical thought, but it is not indicative in him of a skeptical attitude; rather it is due to the measurable character of the primaries and therefore to their use in physical research.

Galilei does not give an abstract exposition of his scientific method, but through his works one can follow the process of its formation. His method is not something improvised or uprooted from the tradition, but a synthesis of those reworkings and those elements that characterize part of the thought of the XV-XVI centuries. First of all, it should be noted that the influence of the craft and engineering tradition of the Middle Ages and the Renaissance influenced Galilei’s method only to the extent that it allowed him to prepare instruments suitable for the preparation and conduct of experiments. To the traditional theses, mostly supported by verbal arguments and based on common experience, Galilei contrasted the results obtained by experiments, in which a particular phenomenon is isolated and studied in its physical-mathematical configuration. On the other hand, the belief that equal causes correspond equal effects led him to eliminate the existence of “celestial physics” and “terrestrial physics” having different natures, to affirm the existence of a universal physics.

The proceeding of Galilean method is so represented by the verification of a hypothesis through an experiment in which are considered only those elements that are measurable: it was so possible to apply to the procedure the instrument that for Galilei gave more guarantee of correctness and precision, that is mathematics. The fact that he applied the laws of mechanics to all fields had as a consequence a mechanistic vision of the world.

The figure of Galilei, his work, and his trial have represented an emblem for subsequent philosophical and scientific thought, a symbol often extended beyond its real historical meaning, so as to be used from time to time as a banner in the fight against the principle of authority in the issues related to the relationship between science and faith, between science and society, and between the fragmentation of scientific knowledge and philosophy. However, there is no doubt that Galilei’s liberation of science from philosophy and theology marked a profound transformation of both the way of thinking and the way of considering the problem of knowledge, just as it marked the beginning of the development of modern science and its increasing specialization as the investigation of reality deepened.

## Literary style and linguistic innovation

His familiarity with letters and poets, his love for Ludovico Ariosto and Ruzante (Angelo Beolco) can be seen in the prose of his works, even in the sparse scientific exposition. In it, next to the fervor that the subject arouses in the author, to the polemical force that permeates certain pages, to the pressing rhythm or to the sharp tone that sometimes it assumes, there is the attempt to make the used language more and more adherent to the treated matter. He has bent the language instrument to the needs of the scientific argument and at the same time has made a revision of the related terminology, which he felt was a necessity in order to achieve greater clarity.

Beyond the unquestionable literary value of Galilei’s prose, it is worth noting the reasons that led Galilei to use both Latin and Italian in his scientific works: while the former was used only for communications to the official scientific world, the latter was considered a valid instrument for the diffusion of new scientific conquests and a new conception of the world. In Galilei’s eyes, the Italian language also had the merit of being freer from the conditioning of the old way of doing science. This attention to the linguistic problem is an essential component of Galilei’s cultural battle aimed not only at the acquisition of new knowledge, but also at the diffusion of this knowledge in wider and wider strata of people.

The death of his father, the economic hardship and the hostility of the academic environment led him to seek and obtain the chair of mathematics at the University of Padua (1592), where he remained 18 years in a “lively and stimulating environment” in which the Serenissima guaranteed a wide freedom of thought. In this period he was Marina Gamba’s companion with whom he had three children: Virginia (1600), Livia (1601) and Vincenzo (1606). Galilei’s research in those years took place in different directions. First, he dealt with practical matters of immediate civil and military utility for the Venetian Republic. He published, among other things, the Treaty of fortification (Trattato di fortificazione, 1593-94) and The operations of the geometric-military compass (Le operazioni del compasso geometrico-militare, 1606), which gave rise to a bitter dispute with a certain Baldassarre Capra on the priority of the discovery of the instrument. He also dealt with problems related to electrical and magnetic phenomena with particular regard to magnets. In the meantime he carried out regular lectures, of decidedly Ptolemaic orientation, then published in the Treatise of the Sphere or Cosmography (1597).

At the center of his interests were, however, the dynamics and theoretical issues of astronomy. In the treatise of clear Archimedean setting Le mecaniche, published by M. Mersenne only in 1634, he extended the principle of virtual speeds, already used by Guidobaldo Dal Monte, to the study of levers and pulleys, to the investigation of inclined planes and all other related machines. In 1604, in a letter to P. Sarpi gave the first, imprecise, formulation of the law of falling bodies.

As for astronomy, in 1597, in two letters addressed to Iacopo Mazzoni and Kepler, had the opportunity to declare his adherence to the Copernican thesis, he also claimed to be in possession of valid arguments in favor of them, but not made known. The first public statement was, however, only in 1604, when Galilei in three lectures interpreted the phenomenon of the appearance of a new star as a confirmation of the Copernican theory, meeting with violent criticism of scientific circles more loyal to tradition.

A real turning point occurred in 1609 when his attention was attracted by the news of the invention of the telescope by Dutch eyeglass makers. Once the instrument was perfected and built, Galilei fully evaluated its possibilities and used it for astronomical observations (January 1610) that led him to discover the mountainous nature of the Moon, Jupiter’s four satellites, the Milky Way as a cluster of “tiny stars” and the phases of Venus. In March of that year he published the Sidereus nuncius with the news of his discoveries that collapsed the Aristotelian theory of the perfection of celestial bodies and demonstrated the correctness of the heliocentric system.

## The Florentine period

The importance of these discoveries, while causing lively controversy, increased enormously the fame of Galilei and Cosimo II, to whom were dedicated the satellites of Jupiter with the name of “Medicean planets”, called him in Florence appointing him “primary mathematician and philosopher” of the Grand Duchy of Tuscany. At first Galilei obtained the recognition of Kepler and partly of the Jesuit astronomers. The trip undertaken for this purpose in Rome in the early months of 1611, despite the triumphant welcome, allowed Galilei to realize some considerable resistance, in particular of Cardinal R. Bellarmino.

Upon his return to Florence published the Discourse about the things that are on the water or that move in it (1612), in which demolished, from an Archimedean point of view, the Aristotelian theory of the elements and that found a strong opposition in philosophical circles. By now Galilei made open profession of Copernicanism and the publication (1612) of three letters to Marco Welser, duumvir of Augsburg, on sunspots caused, in addition to a long dispute with the Jesuit C. Scheiner on the priority of the discovery, the reaction of theologians against the Copernican theory, considered heretical because in contradiction with what is said in the Bible on the movement of the Earth, which resulted in a real complaint presented to the Holy Office by the Dominican N. Lorini. To these attacks Galilei replied in the letter, circulated in many copies among friends and acquaintances, addressed to his student Benedetto Castelli (1613), in which, starting from the assumption that “proceeding equally from the divine Word, the Holy Scripture and nature”, states that the discordance between faith and science is not an indication of a double truth, but it is the effect of a difference in language and that, as far as scientific aspects are concerned, it is in the light of the progress of science that “the true senses of the sacred places must be found”.

Galilei still defended his scientific position and attempted a propaganda and diffusion action in three other letters, two of them addressed to Monsignor P. Dini, mathematician in Pisa, and one to the Grand Duchess of Tuscany, Christine of Lorraine (1615). But by now the Church was going to take a stand against the Copernican theories and against Galilei, which was not worth a second trip to Rome at the end of 1615 to support the defense of his thesis.

At the beginning of 1616 the two propositions on the motion of the Earth and the stability of the Sun were condemned, the reading of Copernicus’ work was forbidden, awaiting revision, and Galilei was warned, in a non-formal way, not to “profess, defend, teach, both orally and in writing” the condemned theses. To this bitter defeat followed years of silence interrupted only by the indirect participation of Galilei to the controversy with the Jesuit Orazio Grassi on the nature of comets (three of them had appeared during 1618), and following which he wrote Il Saggiatore, which he published in 1623, encouraged by the recent appointment to the papal throne of Maffeo Barberini, Urban VIII.

Beyond the misinterpretation of the phenomenon of comets presented in this work, Il Saggiatore is of great interest both for the general issues addressed (mathematics as a language of nature, criticism of the incorruptibility of the heavens, distinction between primary and secondary qualities), and for the extremely clear exposition of his methodological criteria. Favorably impressed by the benevolent reception of the work by the pontiff, whom he had the opportunity to meet in Rome in 1624, Galilei decided to complete the great work he had been thinking about for a long time, destined, in his intentions, to take stock of the controversial issue of astronomical systems.

After some vicissitudes of censorship, the Dialogue on the two greatest systems of the world was completed in 1630. In this work are compared the two great astronomical systems Ptolemaic and Copernican. The lively criticism of the scholastic culture and of the Aristotelian distinction between terrestrial and celestial physics, the still imprecise enunciation of the principle of inertia and the very important one of the principle of relativity, developed with the famous simile of the ship, according to which the mechanical phenomena occur in the same way on land or on a ship that moves with respect to it of uniform rectilinear motion, as well as the argument of the ebb and flow of the sea presented (wrongly) as evidence of the motion of the Earth, make the work a true Copernican manifesto. Aristotle’s mechanics was definitely compromised by the Dialogue and a new mechanics was outlined, able to assign physical and real consistency to the Copernican “hypothesis”.

## The process and the last years

Despite the imprimatur obtained by Father N. Riccardi, which allowed the publication of the work on February 21, 1632, the reactions were immediate and violent. On October 1 of that year Galilei was summoned to Rome by the Inquisition. The scientist, now advanced in years and in poor health, arrived in Rome in February 1633. Once again, his defenses, his attempts and his influential protectors and friends were useless; first for the investigation, then for the trial, Galilei was “vehemently suspected of heresy, that is, of having held and believed false doctrine contrary to the Holy and Divine Scriptures, that the Sun is the center of the Earth and that it does not move from east to west, and that the Earth moves and is not the center of the world.

Galilei, forced to abjure, was sentenced to prison for life, a sentence commuted first in absolute isolation at the Bishop Piccolomini, his former student and friend, then in his villa in Arcetri. Here he spent the last years of his life, saddened by the death of his daughter Virginia, who had been a great comfort to him, by the loss of his eyesight and by increasingly precarious health conditions. He continued, however, his studies in physics and in 1638 in Holland were published in the Discourses and mathematical demonstrations around two new sciences, the second major work of Galilei, which are gathered, extended and revised, the studies on mechanics that he had pursued for over forty years. The work is in the form of a dialogue and takes place in four days: the first two are devoted to the exposition of the first new science around the resistance of materials and the constitution of corpuscular matter, the other two days deal with the second new science, dynamics, and more specifically of local motions, the motion of the projectiles, the isochronism of the oscillations of the pendulum. In appendix there are some demonstrations related to the center of gravity of solids taken from the youthful treatise De motu.

With the setting up of a new dynamics, essential basis to support the Copernican system, the Discorsi, although not dealing with astronomical issues, brought a fundamental contribution to the affirmation of the heliocentric theory. Galilei still dealt with mechanical problems related to the construction of pendulum clocks and published a famous paper on moonlight (Sopra il candore della Luna, 1640). The closeness of friends and students, including V. Viviani and E. Torricelli, helped to make less sad and lonely the last days of the great scientist, who died on January 8, 1642. In 1992 the Church, at the end of the work of a commission specially established by Pope John Paul II, solemnly rehabilitated Galilei, admitting the errors of the Holy Office.