Galileo Galilei (1564–1642)

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GALILEO GALILEI
(1564–1642)

Galileo Galilei, the Italian astronomer and physicist, was born at Pisa. Although he created no systematic philosophy, his influence on the trend of modern philosophical thought is very marked. To it may be traced the definitive separation of physical science from philosophy, the abandonment of authority as a criterion of scientific truth, the distinction between objective and subjective qualities in observable phenomena, and the introduction (or reintroduction) of empirical and skeptical elements into philosophical investigations. The seventeenth-century revival of atomism and the removal of occult qualities from the concept of causation owed much to Galileo. His writings marked the beginning of an antimetaphysical movement in philosophy, exemplified in later times by Positivism and operationalism, and they remained relatively free from such concealed ontological assumptions as are to be found in some ostensibly nonmetaphysical systems; for example, in philosophical empiricism, mechanism, and phenomenalism. The events of Galileo's personal life involved him in an active struggle for freedom of thought, and this in turn underlay those scientific and philosophical convictions for which he became a symbol to his contemporaries and followers.

After a meager, conventional preparatory education, Galileo was enrolled in the school of medicine at the University of Pisa in 1581. His interests turned to mathematics about 1583, and he left the university in 1585 without having received a degree. For a time he continued his studies alone, giving lessons at Siena and Florence, and in 1589 secured the chair of mathematics at Pisa. His early investigations in physics, particularly mechanics, set him in sharp opposition to the views prevailing among professors of philosophy, who, as followers of Aristotle, looked with disfavor upon the introduction of mathematics into physics. In 1591 Galileo left Pisa to become professor of mathematics at the University of Padua. Here he continued his mechanical researches, undertook the manufacture of mathematical instruments for sale, and composed several useful treatises that were circulated in manuscript among his pupils and friends.

In 1604 he disclosed, in a letter to Fra Paolo Sarpi, the mathematical law of freely falling bodies. He had made substantial progress toward a rational mechanics when, in 1609, his attention was diverted from those studies by the invention of the telescope in Holland. Galileo promptly duplicated and improved the device, producing an instrument suitable for astronomical observation, and early in 1610 he published Sidereus Nuncius (The starry messenger), in which he described the mountainous character of the moon, the existence of countless previously unobserved stars, and the discovery of four satellites of Jupiter. These discoveries provoked widespread controversy among philosophers and astronomers. Shortly after Sidereus Nuncius was published, Galileo resigned his post and returned to Florence as chief mathematician and philosopher to Cosimo II de' Medici, grand duke of Tuscany. Late in 1610, he detected the curious appearance of Saturn (although he could not account for it) and the phases of Venus, and shortly thereafter he noted the phenomenon of sunspots.

Strong philosophical opposition was aroused by his publication in 1612 of Discourse on Bodies in Water, in which he ridiculed the Aristotelian theory of the elements, advocated observation and experiment as the chief criteria of physical truth, and gave some support to the views of Democritus. Publication in 1613 of his Istoria e dimostrazioni intorno alle macchie solari (Letters on sunspots) aroused theological opposition by its open support of the Copernican theory, which appeared to contradict the Bible by asserting the motion of Earth and the stability of the sun. Toward the end of that year certain philosophers and priests, in the presence of members of the ruling family, attacked Galileo and Copernicanism on religious grounds. Galileo replied in a long letter addressed to his former pupil Benedetto Castelli, who had defended him in his absence. In this letter Galileo contended that biblical passages had no authority in scientific controversies, and that the language of the Bible should instead be interpreted in the light of man's knowledge of natural phenomena, gained through reason and observation.

Early in 1615, Galileo was violently attacked from the pulpit in Florence, and a copy of his earlier letter, together with a denunciation of the Galileists, was sent to the Inquisition at Rome. Rumors spread that the Catholic Church would soon officially condemn the Copernican theory and silence Galileo. He countered by greatly expanding the arguments of his previous letter to Castelli, and widely circulated the new version in manuscript copies ostensibly addressed to Grand Duchess Christina, the mother of Cosimo de' Medici. At the end of the same year Galileo went personally to Rome in an attempt to stem the anti-Copernican tide. In this he was unsuccessful, for the church officially moved to condemn the views of the motion of Earth and stability of the sun as contrary to the Scriptures. Galileo was instructed not to hold or defend these views, but no action was taken against his person or his previously published books.

After a period of silence, Galileo entered into a polemic with a Jesuit professor at Rome over the nature of comets, and in 1623 he published The Assayer (Il saggiatore ), ridiculing the Aristotelian philosophy and expounding his methodological ideas. This book was dedicated to the new pope, Urban VIII, who as cardinal had been very friendly toward Galileo. In 1624, Galileo visited Rome and obtained permission to write on the Copernican and Ptolemaic systems, provided that the treatment was impartial. The composition of his next book, Dialogue concerning the Two Chief World Systems, occupied Galileo intermittently for several years. After great difficulty in getting a license to publish it, he brought it out at Florence in 1632. Five months later the printer was ordered to issue no more copies, and Galileo was summoned to Rome to face the Inquisition. Despite remonstrances of the Tuscan ambassador and of Grand Duke Ferdinand II, he was eventually compelled to appear and stand trial. In June 1633 the book was condemned, and Galileo was sentenced to life imprisonment. He was, however, permitted to reside first at Siena with the archbishop, his friend and former pupil, and then under house arrest at his own villa near Florence. There he managed to compose and smuggle out his most mature work, a treatise on physics known as Two New Sciences, which was printed in Holland in 1638. Galileo died four years later.

Philosophical Roots

The conditions of Galileo's education and career led to his intimate familiarity with the works of Aristotle. There is little evidence, however, that he ever preferred to read the works of any particular philosopher. His personal library was scanty, and his correspondence is devoid of philosophical references or discussions. In polemic works he refers often to Aristotle, usually with disfavor. His occasional references to Democritus, Socrates, Plato, and Seneca are more favorable but superficial, and appear to express general anti-Aristotelianism rather than allegiance to any other philosopher. Near contemporaries such as Girolamo Cardano and Bernardino Telesio were dismissed by Galileo with the remark that he had read but little of their work. Although there is extant a Latin treatise in Galileo's handwriting that contains references to scores of ancient and medieval philosophers, this dates from his student days (1584) and is not demonstrably original or representative of his own views.

It is evident that during most of his life Galileo's favorite reading was literary rather than philosophical. He is said to have known Ludovico Ariosto by heart; he lectured on Dante Alighieri, annotated Torquato Tasso, delighted in the rustic dialect writings of Ruzzante (Angelo Beolco), and frequently cited Homer, Vergil, Pulci, and other poets in his works. Nowhere in his writings is there an overt expression of allegiance to any of his philosophical predecessors. However, the question of Galileo's true metaphysical position has been much debated in recent decades. His emphasis on the mathematical element in physics has induced many excellent scholars, led by Alexandre Koyré, to classify him unequivocally as a Platonist. On the other hand, Galileo's insistence on the power of observation to refute any reasoned conclusion has caused others, notably Ludovico Geymonat, to resist this conclusion and even to stress a strong Aristotelian element in Galileo's own work. In opposition to both these views, Edward Strong has questioned the propriety of reading any metaphysical position into Galileo's work and emphasizes his evident preoccupation with methodological considerations, to the exclusion of dogmatic philosophy. Finally, Alistair Crombie has aptly remarked that it is precisely the absence of systematic philosophy in Galileo that has made it possible for adherents of nearly every philosophical school to find some support for their views in his works.

Galileo's anti-Aristotelianism makes its first appearance in his early studies of motion. In order to defend a theory of motion (later abandoned) founded on Archimedean conceptions, he was obliged to demolish several prevailing Aristotelian assertions: that the speed of free fall is proportional to the weight of the falling body and inversely proportional to the density of the medium; that the motion of projectiles depends on some action of the medium; and that motion is impossible in the void. Galileo's attack on Aristotle widened with his adoption of the Copernican astronomy and his abandonment of the distinction between elemental and celestial matter, so essential to Aristotle's world view. In the end he questioned the reliability of Aristotelian logic and asserted that rigorous demonstration was to be found only in mathematics. Thus, it may be argued that Aristotle's physical errors led Galileo to distrust logical deduction as a basis for physics, and his famous dictum that "the book of nature is written in mathematical characters, without a knowledge of which men cannot understand it" probably represents a methodological canon rather than a metaphysical position.

Galileo's opposition to Aristotle was also to some degree a literary pose rather than a true philosophical position. In later years he often declared himself a better Aristotelian than his contemporary Peripatetic adversaries, and in the opening sections of the Dialogue he made extensive use of Aristotelian arguments to secure assent to essential points in the Copernican theory. Perhaps the chief significance of Galileo's anti-Aristotelianism is its intimate relation to his consistent rejection of authority of any kind in matters of science. His unwillingness to accept any intermediary between himself and nature was the motivation of his bold warning to the church against the utilization of scriptural authority in scientific disputes. In short, Aristotle was not so much the philosophical opponent as the historical symbol in Galileo's unremitting battle against authority as a criterion of truth.

separation of physics from philosophy

Until Galileo's time, physical science (including theoretical astronomy) was regarded as a proper part of philosophy and was so taught in the universities. Aristotle's principles of motion supplied the axioms, and the science was purely deductive. Several of Galileo's predecessors had questioned those principles as being in apparent contradiction with experience; Galileo continued these attacks and undertook experimental investigations of the actual phenomena of motion. In this way he came upon some new results and sought principles from which both the old and new phenomena might be deductively established. If he was not entirely successful in this quest, that was not a matter of deep concern to him. René Descartes later criticized Galileo sharply for his investigation of physical effects without a prior knowledge of their causes, and Cartesian physics was made an integral part of Descartes's systematic philosophy. In the end, however, the example of Galileo, and not that of Descartes, was followed by scientists. Physics became first a distinct branch (the "natural philosophy" of Isaac Newton) and ultimately a separate discipline from philosophy. The philosophical effects of this separation have been enormous. The emphasis on physics that prevailed in philosophy at Galileo's time has vanished; in its place, the theory of knowledge has risen to preeminence in modern philosophy, where from time to time it has threatened to subordinate or even to expel metaphysics. It is very doubtful that this would have come about without the separation of physical researches from philosophical investigations, in which separation Galileo was the pioneer.

Nor was Galileo content merely to remove terrestrial physical phenomena from the realm of speculative philosophy. Telescopic observation suggested to him a direct analogy between terrestrial and celestial matter, a conception that was antagonistic to Aristotle's entire scheme. The mountainous character of the moon's surface refuted the axiom of perfect sphericity of celestial bodies; the appearance and disappearance of sunspots destroyed the axiom of celestial immutability and perfection. Galileo did not hesitate to attribute terrestrial qualities to all celestial bodies, thus laying the basis for physical astronomy, even though this had grave religious implications and challenged the traditional cosmological and cosmogonical assumptions of the Peripatetics, who dominated the philosophical thought of the time.

Abandonment of Authority

The age into which Galileo was born was one in which the power of authority was uppermost in every sphere of activity—political, religious, and philosophical. It was therefore virtually impossible to attack that power in one sphere without disturbing it in others. To Galileo it was clear that in matters of scientific investigation, authority as such could not be allowed any weight; observation, experiment, and reason alone could establish physical truth. Accordingly, he disputed the right of philosophers and theologians to exercise control over scientific investigations or even scientific theories. Confronted with almost overwhelming opposition in this dispute, he was compelled to adopt, if not to formulate systematically, an alternative criterion of truth that might have some hope of acceptance. In so doing he avoided the error of such philosophers as Bernardino Telesio and Francis Bacon, who in effect had called for reliance solely upon sensory evidence, for Galileo was well aware of the possibility of illusion or of misinterpretation. It appeared to Galileo that mathematics alone had the kind of certainty that could be completely trusted, and he took the position that only to the extent to which men can detect mathematical regularities in phenomena can they be certain they have reached the truth in physical matters: "Without mathematics, one wanders about in a dark labyrinth."

This dictum of Galileo's is often taken today as the expression of a fundamentally Platonic viewpoint, but there is no evidence that Galileo believed in a world of Platonic forms as distinguished from that of events. He appears to have been concerned with relationships rather than essences, and it is in this sense that his mathematical conception of the world is to be taken. He expressly stated that the failure of a physicist to describe the real world was not the fault either of that world or of mathematics, but was merely a result of the limited competence of the physicist, analogous to the shortcomings of a merchant or an accountant who had failed to take into account the weights of the containers in computing the value of his merchandise. Galileo held that although we must be satisfied with limited objectives, we may achieve complete certainty with regard to them.

Galileo's battle for free inquiry, independent of the interference of authority, was, in his own time, doomed to defeat. Nevertheless, the practicability of his alternative criterion of truth gained him a substantial number of followers whose intelligent application of his suggestions was eventually beneficial not only to physical science, but to philosophy as well. The systematic search for solutions to physical problems within preestablished limits of inquiry, under the rule that the demands of sense data, reason, and mathematical interpretation must be simultaneously satisfied, led to the development of a body of dependable knowledge of the physical world that philosophers could no longer ignore in their speculations about underlying reality. At the same time, this complex criterion of physical truth gave rise to a more serious examination of epistemological problems that had been relatively neglected in previous ages of untrammeled speculative philosophy. "We must deal with the real world, and not one on paper," Galileo proclaimed in his Dialogue.

It should be noted that Galileo did not extend his demands beyond the domain of physical science. In order to reconcile his scientific position with his acceptance of religious authority, he distinguished sharply between the two uses of language, or even the two languages, of faith and science. This position was expounded at length in his Letter to Christina (1615, published 1636). Wherever natural phenomena are involved, the language of the Bible is to be interpreted by the findings of science, while the exposition of supernatural texts is to be left to theologians. This concept of duality of language was given an interesting extension in his Two New Sciences ; the purely mathematical sections are written in Latin, while the physical and more general sections are in Italian. It is reasonable to assume that Galileo's attitude toward the difficult question of why mathematical relations prevail in physics, and indeed toward metaphysical questions in general, was similar to his expressed opinion with regard to supernatural assertions of any kind: that they were outside the domain of science.

Objective and Subjective Qualities

It is well known that Galileo clearly set forth the distinction, later made into a fundamental principle by the philosophical empiricists, between primary and secondary qualities. In accordance with his conception that only mathematics afforded complete certainty, he believed that to the fundamental particles of matter one must attribute size, shape, number, and rate of motion; but that one is under no compulsion to invest them with color, sound, odor, and the like. This separation of subjective qualities from those capable of mathematical treatment was a decisive step in the removal of man from his traditionally central place in the entire scheme of things. It is also noteworthy that Galileo showed no interest whatever in pursuing an analysis of subjective (or secondary) qualities; he did not (as did Aristotle and Descartes) compose books on the mind, the spirit, or man in general. Thus, Galileo's treatment of this fundamental principle of empiricism is by no means indicative that he subscribed to an empiricist philosophy in the technical sense. His distinction of objective-subjective was simply an integral part of his separation of physics from philosophy; and if it had any metaphysical implications, they were lost to Galileo. The source of inspiration for this fundamental notion was certainly Greek atomism; but if any classifiable system of philosophy is to be found in Galileo's own writings, it is that of naive realism—a recurrent theme in physical science and the philosophy of science from Galileo's time through the nineteenth century.

Highly important philosophically in Galileo's physical investigations was his insistence on the doctrine of relativity of motion. On the one hand, this removed Earth from its privileged position in astronomy, and in this regard was little more than an extension of the Copernican revolution. On the other hand, it began a new revolutionary movement in which the investigation of natural laws as mathematical relationships was to replace traditional inquiries into the natures or essences of physical entities. Thus, the way was opened to a coherent science of dynamics, while the accepted world view based on the doctrine of "natural places" was rendered untenable.

Skepticism and the Scientific Method

It is worth noting briefly that Galileo introduced (or rather reintroduced) into Western philosophy certain traditions of skepticism which had lapsed during the reign of authority. He often said that it was easier for him to determine that something was false than to discover the truth of any matter, and he openly contended that it was less shameful to be ignorant than to argue on the wrong side. He was accustomed to advise his pupils freely and candidly to confess "I do not know" rather than to offer merely verbal explanations. Thus, among his followers it became once more respectable for a philosopher to acknowledge ignorance. The effects of this were very noticeable in the activities of his disciples and in the Galilean school they created. Establishment of the first great experimental scientific society, the Academy of the Cimento, with its motto of "Test and test again," would scarcely have been possible before Galileo. It was his habit to stress the infinite amount that must remain unknown, no matter how deeply one might penetrate into any subject of inquiry.

methodological views

Galileo's philosophical importance is nowhere clearer than in his contributions to the method of scientific investigation. His suggestions and his example in this field laid the basis for procedures in physical science that have enriched epistemology as much as science itself. Yet Galileo's conceptions of scientific method, like his other philosophic views, make their appearance in his books only implicitly and incidentally to other considerations. They are first apparent in his polemic work on floating bodies (1612), in which experiments are designed for and applied to the refutation of verbal explanations and arguments. This book marked a definite epoch in the philosophy of science, inasmuch as it is perhaps the first systematic exposition of physical experiments specifically designed to refute a philosophical position. In classical form, it presents the Peripatetic theory and Galileo's countertheory, with a set of experimental tests to show the falsity of the former and the truth of the latter. Although methodological considerations are not discussed in the abstract, the work is a model of the "experimental philosophy" carried on by the school of Galileo.

The work on sunspots published in the following year is also rich in methodological material; here Galileo destroyed the arguments of his anonymous Jesuit adversary by establishing the analogy of terrestrial phenomena to solar phenomena and by applying mathematical reasoning to the problem of the location of the spots. In this instance the use of experiment was precluded by inaccessibility of the phenomena, but observational data were correctly applied in its stead. It thus constituted a methodological continuation of the book on floating bodies, although the points at issue were in this case strictly scientific and not philosophical, at least in modern terms. Particularly noteworthy are certain semantic critiques directed against arguments based upon purely verbal deductions made by Galileo's adversary in this controversy.

Because of the prohibition against discussion of Copernicanism, ten years elapsed before the publication of Galileo's next acknowledged work, The Assayer, which differed markedly from his earlier works. Ostensibly a polemic over the nature of comets, it was in reality a detailed critique of the then prevailing treatment of astronomical phenomena. Instead of adopting a specific theory of comets, Galileo undertook to refute his opponent by showing that all his arguments depended upon assumptions that could not be demonstrated or upon confusions of a linguistic character. It is probable that he was motivated at least in part by a desire to place in the hands of his readers a method by which they might themselves arrive at conclusions that he had been forbidden to advocate. Among the principal themes of this work are the proper and improper use of observation and experiment, the distinction between primary and secondary qualities, the necessity for clarification of language in dealing with physical concepts, and the infinite scope of natural phenomena.

In his ill-fated Dialogue of 1632, Galileo developed the last-named theme at length. It was his view that physical truths are boundless in number but perfectly consistent; that human knowledge at any time can comprise but a finite part of this infinite whole; that sense experience, indispensable though it is to a knowledge of the world, can be deceptive or misleading in any given instance. Thus, the concept of physical science as essentially a process of successive approximations is already implicit in the teachings of Galileo. At each stage of inquiry, sense experience must be combined with reasoning and with mathematics to afford a sound basis of deduction. Galileo noted that the method used in proof is rarely the same as that used in making a discovery, and held that unless the proof is mathematical, it lacks absolute certainty.

The Two New Sciences of 1638, Galileo's chief contribution to physics, was of less direct philosophical importance. Its indirect importance lies in the fact that it definitively established physics as a distinct discipline on the basis of its own methods of investigation, methods that have persisted virtually unchanged. Of particular importance to later philosophical developments were the introduction in this work of the concept of one-to-one correspondence in the analysis of the arithmetical infinite and Galileo's suggestions relating to the roles of physical and mathematical indivisibles in the explanation of observable phenomena.

In conclusion, it should be observed that a profound difference existed between the methods by which Galileo and his contemporary, Johannes Kepler, applied mathematical reasoning to physical science, particularly to astronomy. Kepler's thought was pervaded by the conviction that numerical relationships determined the structure of the universe in the sense of Pythagorean mysticism. Accordingly he attempted, repeatedly, to deduce that structure from a priori numerical hypotheses. As a result, he was early led into fantastic speculations from which he was sometimes able to extricate himself only after years of labor. Galileo, on the other hand, regarded mathematics as an indispensable practical tool and as the definitive test in the quest for physical certainty; but he was not inclined to follow wherever mathematical deduction might lead. His errors, unlike Kepler's, are usually to be found in attempts to create mathematical proofs for physical laws of which he had previously made certain, in the desire to achieve the unique degree of certainty that he ascribed to mathematical demonstration.

Bibliography

works by galileo

Edizione nazionale delle opere di Galileo Galilei. Edited by A. Favoro. Florence, 1890–1909; reprinted 1929–1939.

Discorso intorno alle cose che stanno in su l'acqua. Florence, 1612. Translated by T. Salusbury as Discourse on Bodies in Water. Urbana: University of Illinois Press, 1960.

Istoria e dimostrazioni intorno alle macchie solari (Letters on sunspots). Rome, 1613. Translated by S. Drake in Discoveries and Opinions of Galileo. Garden City: NY: Doubleday, 1957. Introduction and notes by S. Drake.

Il saggiatore. Rome, 1623. Translated by S. Drake in Controversy on the Comets of 1618, edited by C. D. O'Malley and S. Drake. Philadelphia: University of Pennsylvania Press, 1960.

Dialogo sopra i due massimi sistemi del mondo. Florence, 1632. Translated by S. Drake as Dialogue concerning the Two Chief World Systems. Berkeley: University of California Press, 1953.

Nov-antiqua … Doctrina … de Sacrae Scripturae Testimoniis. Strasburg, 1636. Translated by S. Drake as "Letter to the Grand Duchess Christina" in Discoveries and Opinions of Galileo. Garden City: NY: Doubleday, 1957.

Discorsi e dimostrazioni matematiche intorno a due nuove scienze. Leiden: Appresso gli Elsevirii, 1638. Translated by H. Crew and A. De Salvio as Dialogues Concerning Two New Sciences. New York: Macmillan, 1914.