Here are enunciated respectively (1) the principle of empiricism, (2) the primacy of mechanistic science, and (3) faith in mathematical explanation. It is upon these three doctrines, as upon a rock, that Renaissance and early modern science and philosophy were built. From each of Leonardo’s theses descended one of the great streams of Renaissance and early modern philosophy: from the empirical principle the work of Bacon, from mechanism the work of Hobbes, and from mathematical explanation the work of Descartes.
Any adequate philosophical treatment of scientific method recognizes that the explanations offered by science are both empirical and mathematical. In Leonardo’s thinking, as in scientific procedure generally, there need be no conflict between these two ideals; yet they do represent two opposite poles, each capable of excluding the other. The peculiar accidents of Renaissance scientific achievement did mistakenly suggest their incompatibility, for the revival of medical studies on the one hand and the blooming of mathematical physics on the other emphasized opposite virtues in scientific methodology. This polarity was represented by the figures of Andreas Vesalius (1514–64) and Galileo.
Vesalius, a Flemish physician, astounded all of Europe with the unbelievable precision of his anatomical dissections and drawings. Having invented new tools for this precise purpose, he successively laid bare the vascular, neural, and muscular systems of the human body. This procedure seemed to demonstrate the virtues of empirical method, of experimentation, and of inductive generalization on the basis of precise and disciplined observation.
Only slightly later, Galileo, following in the tradition already established by Copernicus and Kepler, attempted to do for terrestrial and sidereal movement what Vesalius had managed for the structure of the human body—creating his physical dynamics, however, on the basis of hypotheses derived from mathematics. In Galileo’s work, all of the most original scientific impulses of the Renaissance were united: the interest in Hellenistic mathematics, the experimental use of new instruments such as the telescope, and the underlying faith that the search for certainty in science is reasonable because the motions of all physical bodies are comprehensible in mathematical terms. Galileo’s work also deals with some of the recurrent themes of 16th- and 17th-century philosophy: atomism (which describes the changes of gross physical bodies in terms of the motions of their parts), the reduction of qualitative differences to quantitative differences, and the resultant important distinction between “primary” and “secondary” qualities. The former qualities—including shape, extension, and specific gravity—were considered to be part of nature and therefore real. The latter—such as colour, odour, taste, and relative position—were taken to be simply the effect of the motions of physical bodies on perceiving minds and therefore ephemeral, subjective, and essentially irrelevant to the nature of physical reality.
Galileo.The Metropolitan Museum of Art, New York; The Jefferson R. Burdick Collection, gift of Jefferson R. Burdick (accession no. 63.350.202.25.19); www.metmuseum.org
Modern philosophy
The rise of empiricism and rationalism
The scientific contrast between Vesalius’s rigorous observational techniques and Galileo’s reliance on mathematics was similar to the philosophical contrast between Bacon’s experimental method and Descartes’s emphasis on a priori reasoning. Indeed, these differences can be conceived in more abstract terms as the contrast between empiricism and rationalism. This theme dominated the philosophical controversies of the 17th and 18th centuries and was hardly resolved before the advent of Immanuel Kant. The empiricism of Francis Bacon
Sir Francis Bacon was the outstanding apostle of Renaissance empiricism. Less an original metaphysician or cosmologist than the advocate of a vast new program for the advancement of learning and the reformation of scientific method, Bacon conceived of philosophy as a new technique of reasoning that would reestablish natural science on a firm foundation. In the Advancement of Learning (1605), he charted the map of knowledge: history, which depends on the human faculty of memory, poetry, which depends on imagination, and philosophy, which depends on reason. To reason, however, Bacon assigned a completely experiential function. Fifteen years later, in his Novum Organum, he made this clear: Because, he said, “we have as yet no natural philosophy which is pure,…the true business of philosophy must be…to apply the understanding…to a fresh examination of particulars.” A technique for “the fresh examination of particulars” thus constituted his chief contribution to philosophy.
Francis Bacon, oil painting by an unknown artist; in the National Portrait Gallery, London.Courtesy of the National Portrait Gallery, London
Bacon’s hope for a new birth of science depended not only on vastly more numerous and varied experiments but primarily on “an entirely different method, order, and process for advancing experience.” This method consisted of the construction of what he called “tables of discovery.” He distinguished three kinds: tables of presence, of absence, and of degree (i.e., in the case of any two properties, such as heat and friction, instances in which they appear together, instances in which one appears without the other, and instances in which their amounts vary proportionately). The ultimate purpose of these tables was to order facts in such a way that the true causes of phenomena (the subject of physics) and the true “forms” of things (the subject of metaphysics—the study of the nature of being) could be inductively established.
Bacon’s empiricism was not raw or unsophisticated. His concept of fact and his belief in the primacy of observation led him to formulate laws and generalizations. Also, his conception of forms was quite un-Platonic: a form for him was not an essence but a permanent geometric or mechanical structure. His enduring place in the history of philosophy lies, however, in his single-minded advocacy of experience as the only source of valid knowledge and in his profound enthusiasm for the perfection of natural science. It is in this sense that “the Baconian spirit” was a source of inspiration for generations of later philosophers and scientists. The materialism of Thomas Hobbes
Thomas Hobbes was acquainted with both Bacon and Galileo. With the first he shared a strong concern for philosophical method, with the second an overwhelming interest in matter in motion. His philosophical efforts, however, were more inclusive and more complete than those of his contemporaries. He was a comprehensive thinker within the scope of an exceedingly narrow set of presuppositions, and he produced one of the most systematic philosophies of the early modern period—an almost completely consistent description of humankind, civil society, and nature according to the tenets of mechanistic materialism.
Thomas Hobbes, detail of an oil painting by John Michael Wright; in the National Portrait Gallery, London.Courtesy of the National Portrait Gallery, London
Hobbes’s account of what philosophy is and ought to be clearly distinguished between content and method. As method, philosophy is simply reasoning or calculating by the use of words as to the causes or effects of phenomena. When a person reasons from causes to effects, he reasons synthetically; when he reasons from effects to causes, he reasons analytically. (Hobbes’s strong inclination toward deduction and geometric proofs favoured arguments of the former type.) His dogmatic metaphysical assumption was that physical reality consists entirely of matter in motion. The real world is a corporeal universe in constant movement, and phenomena, or events, the causes and effects of which it is the business of philosophy to lay bare, consist of either the action of physical bodies on each other or the quaint effects of physical bodies upon minds. From this assumption follows Hobbes’s classification of the fields that form the content of philosophy: (1) physics, (2) moral philosophy, and (3) civil philosophy. Physics is the science of the motions and actions of physical bodies conceived in terms of cause and effect. Moral philosophy (or, more accurately, psychology) is the detailed study of “the passions and perturbations of the mind”—that is, how minds are “moved” by desire, aversion, appetite, fear, anger, and envy. And civil philosophy deals with the concerted actions of people in a commonwealth—how, in detail, the wayward wills of human beings can be constrained by power (i.e., force) to prevent civil disorder and maintain peace.