Metaphysics

Early modern metaphysics is in large part a response to the challenges that dramatic advances in physics and astronomy presented for theology and Aristotelian philosophy. Francis Bacon (1561–1626) is the figure who perhaps best marks the transition between Renaissance and early modern philosophy. Bacon's major work, The New Organon, as its title suggests, was an attempt to supplant Aristotle's authority. Two doctrines of that work form the basis of many of the period's debates. First, Bacon asserted that a traditional part of metaphysics, the inquiry into the final causes, or purposes, of nature, is barren, thereby divesting natural science of an important theological element. Second, Bacon denied that forms, understood as abstract natural kinds, are principally what exists. Metaphysics, on Bacon's view, is the study of first and formal causes in nature. It is, in short, the study of the most general laws or reasons by which natural events may be understood.

René Descartes (1596–1650) followed Bacon in denying that explanations in terms of final causes are appropriate in the sciences. He defended the existence and perfection of God but focused his metaphysics, as Bacon did, on providing an account of the natural world. In his most important philosophical works, Meditations on First Philosophy (1641) and Principles of Philosophy (1644), he argued that there are two basic kinds of substances or things with independent existence: minds and bodies. Any substance is characterized by a property necessary for its existence, which Descartes called in different places its nature, essence, or attribute. The nature of mind is thought; of body, extension. Substances have, in addition, properties that are not essential to them and that they can gain or lose without themselves going out of existence. Such properties are called modes or affections.

Descartes's metaphysics is best known for one argument and a problem that the argument generates. He argued that the existence of the self can be known for certain just because, at the very time one doubts the proposition "I exist," one is at the same time thinking and therefore existing (Descartes, vol. 1, pp. 194–195). From this argument, often referred to as "the cogito," Descartes eventually concluded that the essence of mind—and so perhaps of the person—is thought. However, it is clear that people have and are intimately aware of their own bodies. The mind-body problem arises from an attempt to give an account of human nature. Are people essentially, minds, bodies, or both? Moreover, if people are both, the relationship between mind and body needs to be explained. For Descartes, this latter problem was especially difficult. He endorsed a view of perfection in the world much like that of Ficino, in which minds are much more perfect than bodies, so he had to explain in his theory of perception how something less perfect apparently causes changes in something more perfect.

Benedictus, or Baruch, de Spinoza (1632–1677) published during his lifetime a well-received commentary on Cartesian philosophy. Many of his best-known metaphysical positions, which are found in their most mature form in his posthumously published Ethics, can be understood as reactions to Cartesianism. Spinoza identified God with Nature and defended against Cartesian dualism a substance monism in which God is the only substance (EIp14) and possesses all attributes (Spinoza, EId6), including both extension and thought. Spinoza rejected any purpose in nature. (Spinoza is cited here in the customary fashion. For example, EIIp7 means Ethics, Part II, proposition 7.) He defended, largely against Descartes, necessitarianism (EIp33), the view that things could not have been any other way than the way they are, and universal determinism (EIp28–Ip29), in which every existing mode is the effect of some efficient cause. Spinoza addressed the mind-body problem by means of the doctrine known as parallelism, in which mind and body do not interact (EIIp7). Instead, the chain of causes and effects may be understood equally well as a chain of exclusively mental or exclusively physical events.

John Locke (1632–1704), whose philosophy is, much like Bacon's, an attempt to vindicate mechanistic science, adopted a slightly different notion of substance. For Locke, substances are much the same things as Cartesian substances—persons, God, angels, and ordinary material objects. Substances are so designated, however, not because of their independent existence but because they form a substratum for clusters of properties in which those properties inhere and which can survive change in those properties. Although Locke, like his predecessors, described modes at length, he considered them properly ideas rather than properties of substance. The important, knowable properties of substance, in his view, are qualities. In his Essay concerning Human Understanding, Locke famously (or notoriously) defended Robert Boyle's (1627–1691) distinction between primary qualities, such as extension, which really are in substances, and secondary qualities, such as white, which are mere powers to produce certain sensations in people (Locke, 2.8.12–13). This doctrine may be a result of Locke's attempt to ground corpuscularian physical theories, in which extension and other primary qualities were supposed to explain color perceptions and other secondary qualities.

Gottfried Leibniz (1646–1716) was perhaps the most sophisticated metaphysician of the early modern period. He read his predecessors and contemporaries widely and engaged with their views in a variety of contexts. Leibniz was by temperament conciliatory, and his sympathy toward Scholasticism emerges most clearly in his attempt to reintegrate final causes into metaphysics, both at the level of scientific explanation and at the very general level (pp. 52–55). Leibniz's fundamental metaphysical principle, the Principle of Sufficient Reason, is the doctrine that there is reason for every state of affairs. At the most general level, this reason must be in terms of God's purposes: God chooses that the world be this world rather than other possible worlds because this one is the best of all possible worlds (pp. 96–97). The Principle of Sufficient Reason formed the basis for some powerful criticisms of Newtonian physics. It is also the principal target of Voltaire's satire, Candide.

From at least the 1680s, Leibniz understood substances in grammatical terms, as subjects of predication that cannot themselves be predicated of something else. For example, "a king" can be either the subject or predicate of a proposition, so it is not substance; "Alexander," however, can only be a subject, so Alexander is a genuine substance (pp. 41–42). Because Leibniz conceived of each substance as a complete concept or a subject with a comprehensive list of true predications of it, he denied that substances change. So he denied, at the strictly metaphysical level, causal interaction among substances (p. 47). His grammatical conception of substance caused him to endorse a version of idealism, the view that substances cannot be material (p. 79) and in 1714 the theory of monads, his mature metaphysics in which all things either are or are composed of simple, indivisible, inalterable, indestructible, perceiving things.

George Berkeley (1685–1753) formulated an influential version of idealism. He was influenced by Leibniz's views on material objects and also by the Cartesian Nicolas Malebranche's (1638–1715) criticisms of accounts of causal influence among bodies. His most explicit target, however, was Locke's version of the distinction between primary and secondary qualities. Berkeley argued that Locke was right to think that sensations such as pain and white that are caused in people do not resemble their objects but that Locke had no reason for thinking that the perception of extension and other purported primary qualities is different. In either sort of case, one has access only to one's sensations, and these are certainly ideas. Berkeley identified ordinary objects and the physics of ordinary objects, then, as concerning ideas rather than their cause, which can only be God.

David Hume (1711–1776) followed Berkeley in construing ordinary objects as ideas (1.2.6). He was, however, skeptical about the causes of ideas and indeed is best known for his skepticism about causality generally. Hume denied that any sort of necessary connection could be known through experience and so denied all causal connections of the sort asserted by many of his predecessors. He redescribed causal relations as laws concerning the causal conjunction of events of one sort with events of the other. Hume has traditionally been understood as moving from causal language in the description of nature, in which events of one sort are said to cause events of another sort, to the language of laws, in which, more modestly, events of one sort are said to follow events of another sort with lawlike regularity.