Early Modern Philosophy: René Descartes

By Dr. Garth Kemerling / 11.12.2011
Professor of Philosophy
Capella University

Philosophy Pages

Life and Works

After receiving a sound education in mathematics, classics, and law at La Flèche and Poitiers, René Descartes embarked on a brief career in military service with Prince Maurice in Holland and Bavaria. Unsatisfied with scholastic philosophy and troubled by skepticismof the sort expounded by Montaigne, Descartes soon conceived a comprehensive plan for applying mathematical methods in order to achieve perfect certainty in human knowledge. During a twenty-year period of secluded life in Holland, he produced the body of work that secured his philosophical reputation. Descartes moved to Sweden in 1649, but did not survive his first winter there.

Although he wrote extensively, Descartes chose not to publish his earliest efforts at expressing the universal method and deriving its consequences. The Regulae ad directionem ingenii (Rules for the Direction of the Mind) (1628) contain his first full statement of the principles underlying the method and his confidence in the success of their application.  In Le Monde (The World) (1634), Descartes clearly espoused a Copernican astronomy, but he withheld the book from the public upon learning of Galileo’s condemnation.

Descartes finally presented (in French) his rationalist vision of the progress of human knowledge in the Discours de la méthode pour bien conduire sa Raison et chercher la Vérité dans les Sciences   (Discourse on Method) (1637). In this expository essay, Descartes assessed the deficient outcomes of a traditional education, proposed a set of rules with which to make a new start, and described the original experience upon which his hope for unifying human knowledge was based. The final sections of the Discourseand the essays (on dipotric, meteors, and geometry) appended to it illustrate the consequences of employing this method.

A few years later, Descartes offered (in Latin) a more formal exposition of his central tenets in Meditationes de Prima Philosophia (Meditations on First Philosophy)(1641). After an expanded statement of the method of doubt, he argued that even the most dire skepticism is overcome by the certainty of one’s own existence as a thinking thing. From this beginning, he believed it possible to use our clear and distinct ideas to demonstrate the existence of god, to establish the reliability of our reason generally despite the possibility of error, to deduce the essence of body, and to prove that material things do exist. On these grounds, Descartes defended a strict dualism, according to which the mind and body are wholly distinct, even though it seems evident that they interact. The Meditations were published together with an extensive set of objections (by Hobbes, Gassendi, Arnauld, and others) and Descartes’s replies. Descartes later attempted a more systematic exposition of his views in the Principia Philosophiae (Principles of Philosophy) (1644) and an explanation of human emotion in Les Passions de L’Ame (The Passions of the Soul)

Method: A New Approach

The first great philosopher of the early modern era was René Descartes, whose new approach won him recognition as the progenitor of modern philosophy. Descartes’s pursuit of mathematical and scientific truth soon led to a profound rejection of the scholastic tradition in which he had been educated. Much of his work was concerned with the provision of a secure foundation for the advancement of human knowledge through the natural sciences. Fearing the condemnation of the church, however, Descartes was rightly cautious about publicly expressing the full measure of his radical views. The philosophical writings for which he is remembered are therefore extremely circumspect in their treatment of controversial issues.

After years of work in private, Descartes finally published a preliminary statement of his views in the Discourse on the Method of Rightly Conducting the Reason (1637). Since mathematics has genuinely achieved the certainty for which human thinkers yearn, he argued, we rightly turn to mathematical reasoning as a model for progress in human knowledge more generally. Expressing perfect confidence in the capacity of human reason to achieve knowledge, Descartes proposed an intellectual process no less unsettling than the architectural destruction and rebuilding of an entire town. In order to be absolutely sure that we accept only what is genuinely certain, we must first deliberately renounce all of the firmly held but questionable beliefs we have previously acquired by experience and education.

The progress and certainty of mathematical knowledge, Descartes supposed, provide an emulable model for a similarly productive philosophical method, characterized by four simple rules:

  1. Accept as true only what is indubitable.
  2. Divide every question into manageable parts.
  3. Begin with the simplest issues and ascend to the more complex.
  4. Review frequently enough to retain the whole argument at once.

This quasi-mathematical procedure for the achievement of knowledge is typical of a rationalistic approach to epistemology.While engaged in such a comprehensive revision of our beliefs, Descartes supposed it prudent to adhere to a modest, conventional way of life that provides a secure and comfortable environment in which to pursue serious study. The stoic underpinnings of this “provisional morality” are evident in the emphasis on changing oneself to fit the world. Its general importance as an avenue to the contemplative life, however, is more general. Great intellectual upheavals can best be undertaken during relatively calm and stable periods of life.

Anticipated Results

In this context, Descartes offered a brief description of his own experience with the proper approach to knowledge. Begin by renouncing any belief that can be doubted, including especially the testimony of the senses; then use the perfect certainty of one’s own existence, which survives this doubt, as the foundation for a demonstration of the providential reliability of one’s faculties generally. Significant knowledge of the world, Descartes supposed, can be achieved only by following this epistemological method, the rationalism of relying on a mathematical model and eliminating the distraction of sensory information in order to pursue the demonstrations of pure reason.

Later sections of the Discourse (along with the supplementary scientific essays with which it was published) trace some of the more significant consequences of following the Cartesian method in philosophy. His mechanistic inclinations emerge clearly in these sections, with frequent reminders of the success of physical explanations of complex phenomena. Non-human animals, on Descartes’s view, are complex organic machines, all of whose actions can be fully explained without any reference to the operation of mind in thinking.

In fact, Descartes declared, most of human behavior, like that of animals, is susceptible to simple mechanistic explanation. Cleverly designed automata could successfully mimic nearly all of what we do. Thus, Descartes argued, it is only the general ability to adapt to widely varying circumstances—and, in particular, the capacity to respond creatively in the use of language—that provides a sure test for the presence of an immaterial soul associated with the normal human body.

But Descartes supposed that no matter how human-like an animal or machine could be made to appear in its form or operations, it would always be possible to distinguish it from a real human being by two functional criteria. Although an animal or machine may be capable of performing any one activity as well as (or even better than) we can, he argued, each human being is capable of a greater variety of different activities than could be performed by anything lacking a soul. In a special instance of this general point, Descartes held that although an animal or machine might be made to utter sounds resembling human speech in response to specific stimuli, only an immaterial thinking substance could engage in the creative use of language required for responding appropriately to any unexpected circumstances. My puppy is a loyal companion, and my computer is a powerful instrument, but neither of them can engage in a decent conversation. (This criterion anticipated the more formal requirements of the Turing test.)

Doubt and Existence

For a more complete formal presentation of this foundational experience, we must turn to the Meditationes de prima Philosophia (Meditations on First Philosophy) (1641), in which Descartes offered to contemporary theologians his proofs of the existence of god and the immortality of the human soul. This explicit concern for religious matters does not reflect any loss of interest in pursuing the goals of science. By sharply distinguishing mind from body, Descartes hoped to preserve a distinct arena for the church while securing the freedom of scientists to develop mechanistic accounts of physical phenomena. In this way, he supposed it possible to satisfy the requirements of Christian doctrine, but discourage the interference of the church in scientific matters and promote further observational exploration of the material world.

The arrangement of the Meditations, Descartes emphasized, is not the order of reasons; that is, it makes no effort to proceed from the metaphysical foundations of reality to the dependent existence of lesser beings, as Spinoza would later try to do. Instead, this book follows the order of thoughts; that is, it traces the epistemological progress an individual thinker might follow in establishing knowledge at a level of perfect certainty. Thus, these are truly Meditations: we are meant to put ourselves in the place of the first-person narrator, experiencing for ourselves the benefits of the philosophical method.

The Method of Doubt

The basic strategy of Descartes’s method of doubt is to defeat skepticism on its own ground. Begin by doubting the truth of everything—not only the evidence of the senses and the more extravagant cultural presuppositions, but even the fundamental process of reasoning itself. If any particular truth about the world can survive this extreme skeptical challenge, then it must be truly indubitable and therefore a perfectly certain foundation for knowledge. The First Meditation, then, is an extended exercise in learning to doubt everything that I believe, considered at three distinct levels:

Perceptual IllusionFirst, Descartes noted that the testimony of the senses with respect to any particular judgment about the external world may turn out to be mistaken. (Med. I) Things are not always just as they seem at first glance (or at first hearing, etc.) to be. But then, Descartes argues, it is prudent never wholly to trust in the truth of what we perceive. In ordinary life, of course, we adjust for mistaken perceptions by reference to correct perceptions. But since we cannot be sure at first which cases are veridical and which are not, it is possible (if not always feasible) to doubt any particular bit of apparent sensory knowledge.

The Dream Problem Second

Descartes raised a more systematic method for doubting the legitimacy of all sensory perception. Since my most vivid dreams are internally indistinguishable from waking experience, he argued, it is possible that everything I now “perceive” to be part of the physical world outside me is in fact nothing more than a fanciful fabrication of my own imagination. On this supposition, it is possible to doubt that any physical thing really exists, that there is an external world at all. (Med. I)

Severe as it is, this level of doubt is not utterly comprehensive, since the truths of mathematics and the content of simple natures remain unaffected. Even if there is no material world (and thus, even in my dreams) two plus three makes five and red looks red to me. In order to doubt the veracity of such fundamental beliefs, I must extend the method of doubting even more hyperbolically.

A Deceiving God

Finally, then, Descartes raises even more comprehensive doubts by inviting us to consider a radical hypothesis derived from one of our most treasured traditional beliefs. What if (as religion teaches) there is an omnipotent god, but that deity devotes its full attention to deceiving me? (Med. I) The problem here is not merely that I might be forced by god to believe what something which is in fact false. Descartes means to raise the far more devastating possibility that whenever I believe anything, even if it has always been true up until now, a truly omnipotent deceiver could at that very moment choose to change the world so as to render my belief false. On this supposition, it seems possible to doubt the truth of absolutely anything I might come to believe.

Although the hypothesis of a deceiving god best serves the logical structure of the Meditations as a whole, Descartes offered two alternative versions of the hypothetical doubt for the benefit of those who might take offense at even a counter-factual suggestion of impiety. It may seem more palatable to the devout to consider the possibility that I systematically deceive myself or that there is some evil demon who perpetually tortures me with my own error. The point in each case is that it is possible for every belief I entertain to be false.

Remember that the point of the entire exercise is to out-do the skeptics at their own game, to raise the broadest possible grounds for doubt, so that whatever we come to believe in the face of such challenges will indeed be that which cannot be doubted. It is worthwhile to pause here, wallowing in the depths of Cartesian doubt at the end of the First Meditation, the better to appreciate the escape he offers at the outset of Meditation Two.

I Am, I Exist

The Second Meditation begins with a review of the First. Remember that I am committed to suspending judgment with respect to anything about which I can conceive any doubt, and my doubts are extensive. I mistrust every report of my senses, I regard the material world as nothing more than a dream, and I suppose that an omnipotent god renders false each proposition that I am even inclined to believe. Since everything therefore seems to be dubitable, does it follow that I can be certain of nothing at all?

It does not. Descartes claimed that one thing emerges as true even under the strict conditions imposed by the otherwise universal doubt: “I am, I exist” is necessarily true whenever the thought occurs to me. (Med. II) This truth neither derives from sensory information nor depends upon the reality of an external world, and I would have to exist even if I were systematically deceived. For even an omnipotent god could not cause it to be true, at one and the same time, both that I am deceived and that I do not exist. If I am deceived, then at least I am.

Although Descartes’s reasoning here is best known in the Latin translation of its expression in the Discourse, “cogito, ergo sum” (“I think, therefore I am”), it is not merely an inference from the activity of thinking to the existence of an agent which performs that activity. It is intended rather as an intuition of one’s own reality, an expression of the indubitability of first-person experience, the logical self-certification of self-conscious awareness in any form.

Skepticism is thereby defeated, according to Descartes. No matter how many skeptical challenges are raised—indeed, even if things are much worse than the most extravagant skeptic ever claimed—there is at least one fragment of genuine human knowledge: my perfect certainty of my own existence. From this starting-point, Descartes supposed, it is possible to achieve indubitable knowledge of many other propositions as well.

I Am a Thinking Thing

An initial consequence may be drawn directly from the intuitive certainty of the cogito itself. If I know that I am, Descartes argued, I must also know what I am; an understanding of my true nature must be contained implicitly in the content of my awareness.

What then, is this “I” that doubts, that may be deceived, that thinks? Since I became certain of my existence while entertaining serious doubts about sensory information and the existence of a material world, none of the apparent features of my human body can have been crucial for my understanding of myself. But all that is left is my thought itself, so Descartes concluded that “sum res cogitans” (“I am a thing that thinks”). (Med. II) In Descartes’s terms, I am a substance whose inseparable attribute (or entire essence) is thought, with all its modes: doubting, willing, conceiving, believing, etc. What I really am is a mind[Lat. mens] or soul [Lat. anima]. So completely am I identified with my conscious awareness, Descartes claimed, that if I were to stop thinking altogether, it would follow that I no longer existed at all. At this point, nothing else about human nature can be determined with such perfect certainty.

In ordinary life, my experience of bodies may appear to be more vivid than self-consciousness, but Descartes argued that sensory appearances actually provide no reliable knowledge of the external world. If I hold a piece of beeswax while approaching the fire, all of the qualities it presents to my senses change dramatically while the wax itself remains. (Med. II) It follows that the impressions of sense are unreliable guides even to the nature of bodies. (Notice here that the identity of the piece of wax depends solely upon its spatial location; that’s a significant hint about Descartes’s view of the true nature of material things, which we’ll see in more detail in Meditation Five.)

Mind and Body: God and Human Nature

Clear and Distinct Ideas

At the outset of the Third Meditation, Descartes tried to use this first truth as the paradigm for his general account of the possibilities for achieving human knowledge. In the cogito, awareness of myself, of thinking, and of existence are somehow combined in such a way as to result in an intuitive grasp of a truth that cannot be doubted. Perhaps we can find in other cases the same grounds for indubitable truth. But what is it?

The answer lies in Descartes’s theory of ideas. Considered formally, as the content of my thinking activity, the ideas involved in the cogito are unusuallyclear and distinct. (Med. III) But ideas may also be considered objectively, as the mental representatives of things that really exist. According to a representative realist like Descartes, then, the connections among our ideas yield truth only when they correspond to the way the world really is. But it is not obvious that our clear and distinct ideas do correspond to the reality of things, since we suppose that there may be an omnipotent deceiver.

In some measure, the reliability of our ideas may depend on the source from which they are derived. Descartes held that there are only three possibilities: all of our ideas are either adventitious (entering the mind from the outside world) or factitious (manufactured by the mind itself) or innate (inscribed on the mind by god). (Med. III) But I don’t yet know that there is an outside world, and I can imagine almost anything, so everything depends on whether god exists and deceives me.

God Exists

The next step in the pursuit of knowledge, then, is to prove that god does indeed exist. Descartes’s starting point for such a proof is the principle that the cause of any idea must have at least as much reality as the content of the idea itself. But since my idea of god has an absolutely unlimited content, the cause of this idea must itself be infinite, and only the truly existing god is that. In other words, my idea of god cannot be either adventitious or factitious (since I could neither experience god directly nor discover the concept of perfection in myself), so it must be innately provided by god. Therefore, god exists. (Med. III)

As a backup to this argument, Descartes offered a traditional version of thecosmological argument for god’s existence. From the cogito I know that I exist, and since I am not perfect in every way, I cannot have caused myself. So something else must have caused my existence, and no matter what that something is (my parents?), we could ask what caused it to exist. The chain of causes must end eventually, and that will be with the ultimate, perfect, self-caused being, or god.

As Antoine Arnauld pointed out in an Objection published along with the Meditations themselves, there is a problem with this reasoning. Since Descartes will use the existence (and veracity) of god to prove the reliability of clear and distinct ideas in Meditation Four, his use of clear and distinct ideas to prove the existence of god in Meditation Three is an example of circular reasoning. Descartes replied that his argument is not circular because intuitive reasoning—in the proof of god as in the cogito—requires no further support in the moment of its conception. We must rely on a non-deceiving god only as the guarantor of veridical memory, when a demonstrative argument involves too many steps to be held in the mind at once. But this response is not entirely convincing.

The problem is a significant one, since the proof of god’s existence is not only the first attempt to establish the reality of something outside the self but also the foundation for every further attempt to do so. If this proof fails, then Descartes’s hopes for human knowledge are severely curtailed, and I am stuck in solipsism, unable to be perfectly certain of anything more than my own existence as a thinking thing. With this reservation in mind, we’ll continue through the Meditations, seeing how Descartes tried to dismantle his own reasons for doubt.

Deception and Error

The proof of god’s existence actually makes the hypothetical doubt of the First Meditation a little worse: I now know that there really is a being powerful enough to deceive me at every turn. But Descartes argued that since all perfections naturally go together, and since deception is invariably the product of imperfection, it follows that the truly omnipotent being has no reason or motive for deception. God does not deceive, and doubt of the deepest sort may be abandoned forever. (Med. IV) It follows that the simple natures and the truths of mathematics are now secure. In fact, Descartes maintained, I can now live in perfect confidence that my intellectual faculties, bestowed on me by a veracious god, are properly designed for the apprehension of truth.

But this seems to imply too much: if I have a divinely-endowed capacity for discovering the truth, then why don’t I always achieve it? The problem is not that I lack knowledge of some things; that only means that I am limited. Rather, the question is why I so often make mistakes, believing what is false despite my possession of god-given mental abilities. Descartes’s answer derives from an analysis of the nature of human cognition generally.

Every mental act of judgment, Descartes held, is the product of two distinct faculties: the understanding, which merely observes or perceives, and the will, which assents to the belief in question. Considered separately, the understanding (although limited in scope) is adequate for human needs, since it comprehends completely everything for which it has clear and distinct ideas. Similarly, the will as an independent faculty is perfect, since it (like the will of god) is perfectly free in every respect. Thus, god has benevolently provided me with two faculties, neither of which is designed to produce error instead of true belief. Yet I do make mistakes, by misusing my free will to assent on occasions for which my understanding does not have clear and distinct ideas. (Med. IV) For Descartes, error is virtually a moral failing, the willful exercise of my powers of believing in excess of my ability to perceive the truth.

The Essence of Matter

Since the truths of reason have been restored by the demonstration of god’s veracity, Descartes employed mathematical reasoning to discover the essence of bodies in the Fifth Meditation. We do not yet know whether there are any material objects, because the dream problem remains in force, but Descartes supposed that we can determine what they would be like if there were any by relying upon reason alone, since mathematics achieves certainty without supposing the reality of its objects.

According to Descartes, the essence of material substance is simply extension, the property of filling up space. (Med. V) So solid geometry, which describes the possibility of dividing an otherwise uniform space into distinct parts, is a complete guide to the essence of body. It follows that there can be in reality only one extended substance, comprising all matter in a single spatial whole. From this, Descartes concluded that individual bodies are merely modes of the one extended being, that there can be no space void of extension, and that all motion must proceed by circular vortex. Thus, again, the true nature of bodies is understood by pure thought, without any information from the senses.

By the way, this explanation of essences suggested to Descartes another proof of god’s existence, a modern variation on the Ontological Argument. Just as the essence of a triangle includes its having interior angles that add up to a straight line, Descartes argued, so the essence of god, understood as a being in whom all perfections are united, includes necessary existence in reality. (Med. V) As Descartes himself noted, this argument is no more certain than the truths of mathematics, so it also rests on the reliability of clear and distinct ideas, secured in turn by the proofs of god’s existence and veracity in the Third and Fourth Meditations.

The Existence of Bodies

In the Sixth Meditation, Descartes finally tried to eliminate the dream problem by proving that there is a material world and that bodies do really exist. His argument derives from the supposition that divinely-bestowed human faculties of cognition must always be regarded as adequately designed for some specific purpose. Since three of our faculties involve representation of physical things, the argument proceeds in three distinct stages. (Med. VI)

First, since the understanding conceives of extended things through its comprehension of geometrical form, it must at least be possible for things of this sort to exist. Second, since the imagination is directed exclusively toward the ideas of bodies and of the ways in which they might be purposefully altered, it is probable that there really are such things. Finally, since the faculty of sense perception is an entirely passive ability to receive ideas of physical objects produced in me by some external source outside my control, it is certain that such objects must truly exist.

The only alternative explanation for perception, Descartes noted, is that god directly puts the ideas of bodies into my mind without there acutally being anything real that corresponds to them. (This is precisely the possibility that Malebranche would later accept as the correct account of the material world.) But Descartes supposed that a non-deceiving god would never maliciously give me so complete a set of ideas without also causing their natural objects to exist in fact. Hence, the bodies I perceive do really exist.

Mind-Body Dualism

Among the physical objects I perceive are the organic bodies of animals, other human beings, and myself. So it is finally appropriate to consider human nature as a whole: how am I, considered as a thinking thing, concerned with the organism I see in the mirror? What is the true relation between the mind and the body of any human being? According to Descartes, the two are utterly distinct.

The Sixth Meditation contains two arguments in defence of Cartesian dualism: First, since the mind and the body can each be conceived clearly and distinctly apart from each other, it follows that god could cause either to exist independently of the other, and this satisfies the traditional criteria for a metaphysical real distinction. (Med. VI) Second, the essence of body as a geometrically defined region of space includes the possibility of its infinite divisibility, but the mind, despite the variety of its many faculties and operations, must be conceived as a single, unitary, indivisible being; since incompatible properties cannot inhere in any one substance, the mind and body are perfectly distinct. (Med. VI)

This radical separation of mind and body makes it difficult to account for the apparent interaction of the two in my own case. In ordinary experience, it surely seems that the volitions of my mind can cause physical movements in my body and that the physical states of my body can produce effects on my mental operations. But on Descartes’s view, there can be no substantial connection between the two, nor did he believe it appropriate to think of the mind as residing in the body as a pilot resides within a ship. Although he offered several tenatative suggestions in his correspondence with Princess Elizabeth, Descartes largely left for future generations the task of developing some reasonable account of volition and sensation, either by securing the possibility of mind-body interaction or by proposing some alternative explanation of the appearances.

On the other hand, Cartesian dualism offers some clear advantages: For one thing, it provides an easy proof of the natural immortality of the human mind or soul, which cannot be substantially affected by death, understood as an alteration of the states of the physical organism. In addition, the distinction of mind from body establishes the absolute independence of the material realm from the spiritual, securing the freedom of scientists to rely exclusively on observation for their development of mechanistic explanations of physical events.

Cartesian Philosophy

Consequences of Dualism

Descartes worked out his own detailed theories about the physical operation of the material world in Le Monde (The World), but uncertainty about ecclesiastical reactions prevented him from publishing it. The final sections of the Discourse, however, include several significant hints about the positions he was prepared to defend. Their explanations of the activities of living organisms make the mechanistic implications of the Cartesian view more evident.

Since, as everyone acknowledges, non-human animals do not have souls, Descartes concluded that animals must be merely complex machines. Since they lack any immaterial thinking substance, animals cannot think, and all of the movements of their bodies can, in principle, be explained in purely mechanical terms. (Descartes himself incorrectly supposed that the nervous system functions as a complex hydraulic machine.) But since the structure of the human body and the behavior of human beings are similar to the structure and behavior of some animals, it is obvious that many human actions can also be given a mechanistic explanation. La Mettrie later followed this line of reasoning to its ultimate conclusion, supposing human beings to be nothing more than Cartesian machines.


The philosophy of Descartes won ready acceptance in the second half of the seventeenth century, expecially in France and Holland. Although few of his followers, known collectively as Cartesians, employed his methods, they showed great diligence and ingenuity in their efforts to explain, defend, and advance his central doctrines.

In the physical sciences, for example, Cavendish, Rohault, and Régis were happy to abandon all efforts to employ final causes in their pursuit of mechanistic accounts of physical phenomena and animal behavior. On this basis, however, such philosophers were able to progress beyond a simple affirmation of the mysterious reality of mind-body interaction.

Metaphysicians like Cordemoy and Geulincx fared little better in their efforts to deal with this crucial problem with dualism. If there is no genuine causal interaction between independent substances, we seem driven to suppose that the actions of mind and body are merely parallel or divinely synchronized.

Not everyone was entirely satisfied by the epistemological foundations of the Cartesian scheme, either. Critics like Arnauld, Nicole, and Foucher drew attention to the inherent difficulty of explaining in representationalist terms how our ideas of things can be known to resemble the things themselves and the implausibility of reliance upon innate ideas. Conway went even further, rejecting the dualistic foundations of Descartes’s substance-ontology along with his approach to human knowledge.

Pascal: The Religious Mathematician

One seventeenth-century thinker of greater independent significance wasBlaise Pascal, with his unusual blend of religious piety, scientific curiosity, and mathematical genius. Led by his deep religious feelings to participate fully in the pietistic Jansenism of the Port-Royal community, Pascal maintained that formal reasoning about god can never provide an adequate substitute for genuine personal concern for the faith: “The heart has its reasons that reason cannot know.”

Pascal’s mathematical acumen was no less remarkable than that of Descartes; his work anticipated the development of game theory and the modern methods of calculating probability. In fact, his famous “Wager” applies these mathematical techniques to the prudence of religious conviction in the absence of adequate evidence: since the consequences of believing are infinitely beneficial if there is a god and only slightly inconvenient if there is not, while the outcome of atheism is only somewhat more pleasant if there is no god and eternally costly if there is, the expected value of theism is much greater than that of atheism, and it is reasonable to stake one’s life on the possibility that god does exist.

Malebranche: Seeing All Things in God

The most original and influential philosopher of the Cartesian tradition wasNicolas Malebranche. Noting the steady progress of efforts to provide mechanistic accounts of the behavior of the human body, Malebranche concluded that the mind and body are not only substantially distinct but causally independent of each other. The appearance of genuine interaction arises from what is in fact merely the perfect parallelism of events in the mental and physical realms.

According to Malebranche, then, our ideas of bodies do not result from any causal influence that physical objects have on our senses; rather, they are produced in our minds directly by god. Thus, he supposed, in sense perception what literally happens is that we “see all things in god.” Similarly, our wills have no causal influence on the material world, but god provides for the coordination of our volitions with the movement of bodies. In general, since there is no causal interaction, it is the power of god alone that secures a perpetual, happy coincidence of the states and operations of minds and bodies.

Since only god’s activity is efficacious in either mental or physical things, apparent causes in either realm are merely the occasions for the appearance of their supposed effects in the other. Thus, the views of Malebranche are often referred to collectively as occasionalism. Although the entire theory found few enthusiastic adherents, Malebranche’s analysis of the regularities exhibited in nature by causally independent beings and events was greatly influential on later philosophers, including Berkeley and Hume.