In no field has the modern period so completely broken with tradition as in physics. The correctness of the Copernican theory is proved by Kepler's laws of planetary movement, and Galileo's telescopical observations; the scientific theory of motion is created by Galileo's laws of projectiles, falling bodies, and the pendulum; astronomy and mechanics form the entrance to exact physics—Descartes ventures an attempt at a comprehensive mechanical explanation of nature. And thus an entirely new movement is at hand. Forerunners, it is true, had not been lacking. Roger Bacon (1214-94) had already sought to obtain an empirical knowledge of nature based upon mathematics; and the great painter Leonardo da Vinci (1452-1519) had discovered the principles of mechanics, though without gaining much influence over the work of his contemporaries. It was reserved for the triple star which has been mentioned to overthrow Scholasticism. The conceptions with which the Scholastic-Aristotelian philosophy of nature sought to get at phenomena—substantial forms, properties, qualitative change—are thrown aside; their place is taken by matter, forces working under law, rearrangement of parts. The inquiry into final causes is rejected as an anthropomorphosis of natural events, and deduction from efficient causes is alone accepted as scientific explanation. Size, shape, number, motion, and law are the only and the sufficient principles of explanation. For magnitudes alone are knowable; wherever it is impossible to measure and count, to determine force mathematically, there rigorous, exact science ceases. Nature a system of regularly moved particles of mass; all that takes place mechanical movement, viz., the combination, separation, dislocation, oscillation of bodies and corpuscles; mathematics the organon of natural science! Into this circle of modern scientific categories are articulated, further, Galileo's new conception of motion and the conception of atoms, which, previously employed by physicists, as Daniel Sennert (1619) and others, is now brought into general acceptance by Gassendi, while the four elements are definitively discarded (Lasswitz, Geschichte der Atomistik, 1890). Still another doctrine of Democritus is now revived; an evident symptom of the quantification and mechanical interpretation of natural phenomena being furnished by the doctrine of the subjectivity of sense qualities, in which, although on varying grounds, Kepler, Galileo, Descartes, Gassendi, and Hobbes agree.[1] Descartes and Hobbes will be discussed later. Here we may give a few notes on their fellow laborers in the service of the mechanical science of nature.
[Footnote 1: Cf. chapter vi. in Natorp's work on Descartes'
Erkenntnisstheorie, Marburg, 1882, and the same author's Analekten zur
Geschichte der Philosophie, in the Philosophische Monatshefte, vol.
xviii. 1882, p. 572 seq.]
We begin with John Kepler[1] (1571-1630; chief work, The New Astronomy or Celestial Physics, in Commentaries on the Motions of Mars, 1609). Kepler's merit as an astronomer has long obscured his philosophical importance, although his discovery of the laws of planetary motion was the outcome of endeavors to secure an exact foundation for his theory of the world. The latter is aesthetic in character, centers about the idea of a universal world-harmony, and employs mathematics as an instrument of confirmation. For the fact that this theory satisfies the mind, and, on the whole, corresponds to our empirical impression of the order of nature, is not enough in Kepler's view to guarantee its truth; by exact methods, by means of induction and experiment, a detailed proof from empirical facts must be found for the existence not only of a general harmony, but of definitely fixed proportions. Herewith the philosophical application of mathematics loses that obscure mystical character which had clung to it since the time of Pythagoras, and had strongly manifested itself as late as in Nicolas of Cusa. Mathematical relations constitute the deepest essence of the real and the object of science. Where matter is, there is geometry; the latter is older than the world and as eternal as the divine Spirit; magnitudes are the source of things. True knowledge exists only where quanta are known; the presupposition of the capacity for knowledge is the capacity to count; the spirit cognizes sensuous relations by means of the pure, archetypal, intellectual relations born in it, which, before the advent of sense-impressions, have lain concealed behind the veil of possibility; inclination and aversion between men, their delight in beauty, the pleasant impression of a view, depend upon an unconscious and instinctive perception of proportions. This quantitative view of the world, which, with a consciousness of its novelty as well as of its scope, is opposed to the qualitative view of Aristotle;[2] the opinion that the essence of the human spirit, as well as of the divine, nay, the essence of all things, consists in activity; that, consequently, the soul is always active, being conscious of its own harmony at least in a confused way, even when not conscious of external proportions; further, the doctrine that nature loves simplicity, avoids the superfluous, and is accustomed to accomplish large results with a few principles—these remind one of Leibnitz. At the same time, the law of parsimony and the methodological conclusions concerning true hypotheses and real causes (an hypothesis must not be an artificially constructed set of fictions, forcibly adjusted to reality, but is to trace back phenomena to their real grounds), obedience to which enabled him to deduce a priori from causes the conclusions which Copernicus by fortunate conjecture had gathered inductively from effects—these made our thinker a forerunner of Newton. The physical method of explanation must not be corrupted either by theological conceptions (comets are entirely natural phenomena!) or by anthropomorphic views, which endow nature with spiritual powers.
[Footnote 1: See Sigwart, Kleine Schriften, vol. i. p. 182 seq.; R.
Eucken, Beiträge zur Geschichte der neueren Philosophie, p. 54 seq.]
[Footnote 2: Aristotle erred when he considered qualitative distinctions (idem and aliud) ultimate. These are to be traced back to quantitative differences, and the aliud or diversum is to be replaced by plus et minus. There is nothing absolutely light, but only relatively. Since all things are distinguished only by "more or less," the possibility of mediating members or proportions between them is given.]
Intermediate between Bacon and Descartes, both in the order of time and in the order of fact, and a co-founder of modern philosophy, stands Galileo Galilei (1564-1641).[1] Galileo exhibits all the traits characteristic of modern thinking: the reference from words to things, from memory to perception and thought, from authority to self-ascertained principles, from chance opinion, arbitrary opinion, and the traditional doctrines of the schools, to "knowledge," that is, to one's own, well grounded, indisputable insight, from the study of human affairs to the study of nature. Study Aristotle, but do not become his slave; instead of yielding yourselves captive to his views, use your own eyes; do not believe that the mind remains unproductive unless it allies itself with the understanding of another; copy nature, not copies merely! He equals Bacon in his high estimation of sensuous experience in contrast to the often illusory conclusions of the reason, and of the value of induction; but he does not conceal from himself the fact that observation is merely the first step in the process of cognition, leaving the chief rôle for the understanding. This, supplementing the defect of experience—the impossibility of observing all cases—by its a priori concept of law and with its inferences overstepping the bounds of experience, first makes induction possible, brings the facts established into connection (their combination under laws is thought, not experience), reduces them to their primary, simple, unchangeable, and necessary causes by abstraction from contingent circumstances, regulates perception, corrects sense-illusions, i. e., the false judgments originating in experience, and decides concerning the reality or fallaciousness of phenomena. Demonstration based on experience, a close union of observation and thought, of fact and Idea (law)—these are the requirements made by Galileo and brilliantly fulfilled in his discoveries; this, the "inductive speculation," as Dühring terms it, which derives laws of far-reaching importance from inconspicuous facts; this, as Galileo himself recognizes, the distinctive gift of the investigator. Galileo anticipates Descartes in regard to the subjective character of sense qualities and their reduction to quantitative distinctions,[2] while he shares with him the belief in the typical character of mathematics and the mechanical theory of the world. The truth of geometrical propositions and demonstrations is as unconditionally certain for man as for God, only that man learns them by a discursive process, whereas God's intuitive understanding comprehends them with a glance and knows more of them than man. The book of the universe is written in mathematical characters; motion is the fundamental phenomenon in the world of matter; our knowledge reaches as far as phenomena are measurable; the qualitative nature of force, back of its quantitative determinations, remains unknown to us. When Galileo maintains that the Copernican theory is philosophically true and not merely astronomically useful, thus interpreting it as more than a hypothesis, he is guided by the conviction that the simplest explanation is the most probable one, that truth and beauty are one, as in general he concedes a guiding though not a controlling influence in scientific work to the aesthetic demand of the mind for order, harmony, and unity in nature, to correspond to the wisdom of the Creator.
[Footnote 1: Cf. Natorp's essay on Galileo, in vol. xviii. of the Philosophische Monatshefte, 1882.]
[Footnote 1: This doctrine is developed by Galileo in the controversial treatise against Padre Grassi, The Scales (Il Saggiatore, 1623, in the Florence edition of his collected works, 1842 seq., vol. iv. pp. 149-369; cf. Natorp, Descartes' Erkenntnisstheorie, 1882, chap. vi.). In substance, moreover, this doctrine is found, as Heussler remarks, Baco, p. 94, in Bacon himself, in Valerius Terminus (Works, Spedding, vol. iii. pp. 217-252.)]
One of the most noted and influential among the contemporaries, countrymen, and opponents of Descartes, was the priest and natural scientist, Petrus Gassendi,[1] from 1633 Provost of Digne, later for a short period professor of mathematics at Paris. His renewal of Epicureanism, to which he was impelled by temperament, by his reverence for Lucretius, and by the anti-Aristotelian tendency of his thinking, was of far more importance for modern thought than the attempts to revive the ancient systems which have been mentioned above (p. 29). Its superior influence depends on the fact that, in the conception of atoms, it offered exact inquiry a most useful point of attachment. The conflict between the Gassendists and the Cartesians, which at first was a bitter one, centered, as far as physics was concerned, around the value of the atomic hypothesis as contrasted with the corpuscular and vortex theory which Descartes had opposed to it. It soon became apparent, however, that these two thinkers followed along essentially the same lines in the philosophy of nature, sharply as they were opposed in their noëtical principles. Descartes' doctrine of body is conceived from an entirely materialistic standpoint, his anthropology, indeed, going further than the principles of his system would allow. Gassendi, on the other hand, recognizes an immaterial, immortal reason, traces the origin of the world, its marvelous arrangement, and the beginning of motion back to God, and, since the Bible so teaches, believes the earth to be at rest,—holding that, for this reason, the decision must be given in favor of Tycho Brahé and against Copernicus, although the hypothesis of the latter affords the simpler and, scientifically, the more probable explanation. Both thinkers rejoice in their agreement with the dogmas of the Church, only that with Descartes it came unsought in the natural progress of his thought, while Gassendi held to it in contradiction to his system. It is the more surprising that Gassendi's works escaped being put upon the Index, a fate which overtook those of Descartes in 1663.
[Footnote 2: Pierre Gassendi, 1592-1655: On the Life and Character of Epicurus, 1647; Notes on the Tenth Book of Diogenes Laërtius, with a Survey of the Doctrine of Epicurus, 1649. Works, Lyons, 1658, Florence, 1727. Cf. Lange, History of Materialism, book i. § 3, chap, 1; Natorp, Analekten, Philosophische Monatshefte, vol. xviii. 1882, p. 572 seq.]