One of the major intellectual debates of the Middle Ages concerned the problem of universals. The problem has its source in Plato and Aristotle, but medieval philosophers such as Boethius, Abelard, Aquinas, Scotus, and Ockham discussed it with a degree of precision and sophistication that is perhaps unrivaled. It is sometimes noted that there is not one problem here but several, touching on metaphysical, epistemological, and logical questions. The main problem, however, is metaphysical and can be stated as follows: some things in the world resemble each other. Many things are, for example, red, six feet tall, virtuous, and/or human. If two or more things are similar in one of these respects, or in some other respect, in virtue of what are they similar? Is there some entity that they literally share in common – namely, the universal (such as Redness)? If so, what status does this universal have? Does it exist in particulars, or is it distinct and separable from them? Broadly speaking, “realists” are philosophers who affirm the existence of universals and confer some ontological status upon them apart from the mind, whereas “conceptualists” and “nominalists” – for reasons of ontological parsimony – reduce them to ideas, names, or even the spoken word. In the Middle Ages, the debate about universals centered on Aristotle's five “predicables” – genus, species, differentia, property, and accident – that had been treated in an influential work by Porphyry, a third-century Neoplatonist. What, for example, is the ontological status of the species man, or of rationality, which constitutes the differentia that distinguishes man from other species within the genus animal? But for philosophers before and after this period, the problem of universals has always had a much wider application. Plato, for example, was a realist who conceived of Beauty, Truth, and mathematical objects as separately existing universals.

Most early modern philosophers write as if the problem of universals has been solved and typically give the issue only passing attention.

One basic enterprise in philosophy going back to the ancient Greeks is to answer the question, What is it? It is reasonable to construe the answers to such questions as definitions, or what are nowadays called “real definitions,” since they typically go beyond the mere meanings of words and aim to specify the essences of things. Who, having read Plato's dialogues, can forget Socrates’ quest for definitions of things like “knowledge,” “justice,” or “virtue”? Aristotle and his Scholastic heirs continued and further developed this tradition. Aristotle distinguished different types of definitions, but definitions in the primary sense represent the essences of species. One defines an essence in terms of a genus and a differentia, as in the case of the species man, defined as rational animal. Since, on Aristotle's view, essences constitute the first principles of the sciences, definitions play a key role in his epistemology and in syllogistic reasoning, which is intended in part to systematize our knowledge of the natural world (see syllogism).

Descartes is very suspicious of this entire tradition, expressing three related criticisms of philosophical definitions: they obscure concepts that are already clear and distinct, tend to be vacuous, and fail to advance our knowledge. Concerning the first, he writes in the Principles I.10: “I have often noticed that philosophers make the mistake of employing logical definitions in an attempt to explain what was already very simple and self-evident; the result is that they only make matters more obscure” (AT VIIIA 8, CSM I 296; cf. AT III 597, CSMK 139). It helps to remember that for Descartes our simple ideas are innate. These include the ideas of thought, existence, and certainty, which one must grasp first in order to appreciate the force of the cogito. Elsewhere he notes that in order to understand these and other such concepts, we do not “have to rack our brains trying to find the ‘proximate genus’ and ‘essential differentia’ which go to make their true definition. We can leave that to someone who wants … to debate in the Schools.” All one must do is look within one's own mind. It is a mistake to “try to define what should only be conceived” (AT X 523–24, CSM II 417).

Descartes states his second criticism by mocking what he takes to be Aristotle's definition of motion: “A man who walks across a room shows much better what motion is than a man who says ‘It is the actuality of a potential being in so far as it is potential’” (AT III 597, CSMK 139). Definitions tend to be devoid of content, a jumble of words used without meaning.

Descartes’ cosmological (or causal) arguments for the existence of God are often thought to be among the least plausible aspects of his philosophy and, perhaps as a result, have been relatively neglected by commentators. But these arguments are as important to his epistemic project in the Meditations as they are controversial. Having proven in the Second Meditation that he exists as a thinking thing, the meditator aims in the Third Meditation to discover whether anything outside of him exists. Before philosophizing, he had judged that there are physical objects that cause his sensations, and that these sensations resemble their causes, but he now recognizes that such judgments are based on “blind impulse” and thus untrustworthy (AT VII 38–40, CSM II 26–27). Descartes will eventually develop a proof of the existence of bodies (in the Sixth Meditation), but he postpones that here, turning his sights on the more ambitious project of demonstrating God's existence (see body, proof of the existence of). His goal is not just to overcome solipsism but to chart a course that will enable him to dispel the hyperbolic doubts of the First Meditation – most notably, the “omnipotent deceiver hypothesis” – by proving that he was created by an all-perfect being who would not deceive him.

Descartes presents two theistic proofs in the Third Meditation, though most commentators treat the first as the main argument, and in a letter Descartes asserts that the two arguments are “reducible to one” (AT IV 112, CSMK 232). Given these considerations and the constraints of space, the main focus here will be on the first argument. The main problem with this argument is that it relies on seemingly bizarre principles of causality and antiquated doctrines about degrees of reality and different kinds of being, which have their source in Neoplatonism and Scholasticism and do not seem to be a good fit within Descartes’ own metaphysical system. How can such principles enable us to achieve knowledge if they themselves are so dubious? In the Objections and Replies, Descartes responds to several criticisms of his arguments. He also reformulates his cosmological arguments in the Geometrical Exposition and in the Principles (1644), though he presents them there after the ontological argument, reversing the order he had followed in the Meditations, leaving commentators to debate the significance of this reordering.

Descartes’ thought is characterized by a tension between faith and reason with which the philosopher never fully comes to grips. In spite of his many criticisms of the tradition and his endeavor to proceed from entirely new, entirely rational foundations, Descartes appeals in a number of his writings to the importance of faith in human life and the inner compatibility between faith and reason. Nonetheless, as Jacques Maritain (1944, 44) has observed, Descartes’ “fundamental principles will develop into a sheer enmity of reason against faith,” and Descartes’ legacy will eventually become that of a thinker interested exclusively in the earthly project of rendering human beings “the masters and possessors of nature” (AT VI 62, CSM I 142f).

Like Saint Augustine and Saint Thomas Aquinas before him, Descartes proclaims an inner compatibility between religious faith and the principles of his philosophy. He asserts his “extreme devotion to the Catholic religion,” his “firm faith in the infallibility of the Church,” and his conviction that the relation between the truths of faith and the insights of his philosophy are such that he “cannot have any fear that one truth may be in conflict with another” (AT III 259, CSMK 161). Nonetheless, from his earliest writings he seeks to bifurcate the domains of faith and reason along the lines sketched by Galileo in his 1615 “Letter to the Grand Duchess Christina” (Galileo 1957): faith concerns matters pertaining to our salvation, whereas reason pertains to earthly questions that can be settled by argumentation and empirical investigation. Truths such as the mystery of the Trinity and the immortality of the soul are disclosed not by reason but by revelation (AT III 167, CSMK 166; AT V 53, CSMK 320), whereas metaphysical and scientific questions are to be settled by reason and empirical investigation. One of the principal concerns of Cartesian metaphysics is the nature and existence of God. God plays a key role in Descartes’ metaphysics, guaranteeing the truth of clear and distinct ideas; thus natural theology plays a key role in Descartes’ thought. But Descartes scrupulously avoids revealed theology, arguing that it lies outside the domain of his immediate philosophical concerns.

René Descartes (1596–1650) is the most important French philosopher who ever lived and, arguably, among the top five philosophers of all time. He is known as “the father of modern philosophy” in part because of the influence he continues to exert over the way the discipline is practiced. But he was much more than a philosopher; he was a universal genius whose writings range over anatomy and physiology, astronomy, biology, epistemology, ethics, mathematics, medicine, metaphysics, meteorology, music, optics, psychology, and physics. There is a very long tradition of Cartesian studies but over the past thirty years there has been an explosion of academic works devoted to Descartes’ ideas, and the rate of production is accelerating. An insatiable appetite for all things Cartesian is being fed by a steady diet of new monographs, guidebooks, biographies, anthologies of essays, translations, journal articles, and other scholarly works. One of the primary aims of this volume is to help the reader digest this vast literature, while also introducing him or her to the breadth of Descartes’ thought.

The Cambridge Descartes Lexicon is more like an encyclopedia or a compendium than a traditional dictionary, both in its scope and in its content. Many of the entries are fairly lengthy, especially those devoted to important subject terms such as “Cogito Ergo Sum,” “Free Will,” “God,” “Human Being,” “Idea,” “Law of Nature,” and “Representation.” Most of the subject entries are also “scholarly” in the sense that they introduce readers to debates in the secondary literature. The authors of these entries sometimes present these debates without defending their own views, but in many cases they take an interpretive stand. Authors of overlapping entries were encouraged to stake out opposing positions (see, e.g., “Body,” “Individuation,” and “Substance”). The result is that by reading just a few pages readers can familiarize themselves with almost any given scholarly dispute and get a sense of the arguments and textual evidence for various interpretations.

In perusing the Lexicon, readers will find articles on all of the familiar topics, such as “Certainty,” “Clarity and Distinctness,” “Doubt,” “Dualism,” “Geometry,” “Knowledge,” “Method,” and “Mind,” as well as entries on several unexpected and perhaps unfamiliar topics such as “Language,” “Physico-Mathematics,” “Rosicrucian,” and “The Stampioen Affair.”

In the Nicomachean Ethics, Aristotle famously says that virtue is a hexis. This term, which comes down to us from the Latin habitus, is often transliterated in English as “habit,” though some commentators note that Aristotle is claiming that virtue is something active requiring attention, practice, and reflection, whereas the word “habit” suggests something passive and involuntary. Descartes’ use of the term habitus, or habitude in French, is more complex. He sometimes uses it to refer to our natural dispositions or to ones that we formed in childhood prior to being in full possession of our reason. Typically, these are vicious habits that Descartes encourages us to overcome by replacing them with virtuous ones, but unlike the former the latter must be actively cultivated. Habits play a prominent role in two aspects of Descartes’ philosophy: his account of happiness and virtue on the one hand and his epistemology on the other. What follows is a separate discussion of each.

1.Virtue and Habits

Although Descartes often treats habits as obstacles to the search after truth, habits are instrumental and essential to living a virtuous and happy life. Descartes’ treatment of habits within his moral psychology and philosophy is developed most fully in his final work, Passions of the Soul (1649). There, Descartes explains that humans are a union or composite of mind and body, and passions are modes of this union (see human being). Passions and actions that promote the preservation of the union are virtuous and good, and those that harm the union are bad. Because humans cannot directly arouse or suppress passions or directly will the body to move, they must do so indirectly by means of representing things that are joined to certain passions (Passions I.45). The joining of particular ideas to particular passions results from habit – a disposition to act in a certain way that is acquired by nature or by repetition. Thus, habit formation is a key ingredient in the training and guiding of our actions. In the pursuit of happiness, habits serve as the mechanism for regulating our passions – instituting the right idea-passion couplings (by nature or repeated experience) that produce beneficial actions.

Descartes presents his reader with many faces, among them that of an epistemologist, a mathematician, a physicist, and a physiologist, but he is best known for his distinctive metaphysical claims – for example, that he exists as a purely thinking thing, that mind and body are really distinct, that mind and body causally interact, and that God or a supremely perfect creator exists. But perceptions of Descartes’ legacy have changed repeatedly over the centuries, and the fact that he is known now primarily as a metaphysician is more reflective of us, his twenty-first-century readers, than of his own preoccupations. Recent scholarship has revealed that Descartes sees himself mainly as a scientist (or natural philosopher) who spends much of his time experimenting, studying the experiments of others, and theorizing about the results (see, e.g., Gaukroger 1995, Clarke 2006). This is in keeping with his professed view that metaphysics is something one does “once in a lifetime” (semel in vita) and that the bulk of one's intellectual life should be devoted to pursuits such as mechanics, medicine, and morality, which yield practical benefits for humankind (AT IXB 14, CSM I 186; cf. AT III 695, CSMK 228 and AT V 165). Still, Descartes believes that metaphysics is important and necessary insofar as it provides a foundation for science, and this belief is reflected in the number of pages he devotes to it in his two most important works, the Meditations and the Principles.

Descartes uses the term “metaphysics” (Latin metaphysica, French métaphysique) and its cognates infrequently in his writings, raising questions in at least one commentator's mind about whether “Cartesian thought belong[s] to metaphysics” as that term was traditionally understood by his predecessors (Marion 1999, 1–2). In his correspondence, Descartes refers to drafts of the Meditations as his “Metaphysics,” but he ultimately settles on the title Meditations on First Philosophy “because I do not confine my discussion to God and the soul, but deal in general with all the first things to be discovered by philosophizing” (AT III 235, CSMK 157). Here Descartes is assuming the Thomistic account of special metaphysics (as opposed to general metaphysics) as the science of immaterial beings, but in what follows we will use the term “metaphysics” in its broader contemporary sense, which seems very close to what he means by “first philosophy.”