Rationalism and the Historical Mind/Body Controversy


Philosophy seeks to uncover wisdom in the otherwise tumultuous field of epistemology.  Those familiar with epistemology understand that something must serve as a conduit between the world containing knowledge (or metaphysics) and the individual seeking it.  In the following essay, I shall seek to explain one method of epistemology (or “theory of knowledge”) dignified by the term rationalism so that a broader context can be developed for our excursus into the mind/body controversy.  This is not to say that other theories are irrational or that rationalism corners the market on what makes one a rational being.  It is one among many methodologies that seeks to bridge the gap between the “knower” and the external world (or that which is to be known).  My investigation here shall focus on how rationalism has historically led to the well-known view of dualism through its methodological process.

The reader may be concerned as to what relevance mind-body dualism poses.  Its relevance is important to mainline Christian theology since it adopts a supernatural element for the afterlife prior to the general resurrection of all believers.  Critics and proponents have been content to argue the matter purely out of doctrinal exchanges and by appealing to the Bible.  Moreover, Christianity’s Jewish predecessors had divided over the veridical status of the supernatural elements of Israel’s religion as witnessed in the Pharisee-Sadducee schism.  But Christianity had (and continues to possess) convictions about consciousness subsequent to death.  This doctrine of the afterlife presupposes one of its other doctrines: the existence of an inner-human, immaterial spirit that animates the body.  There are decisive passages throughout the Bible that posit this sort of duality in human beings.[1]  But there appear to be clear examples from the New Testament of an immaterial spirit or soul that coexists with its human body.[2]  Critics of dualism appeal to those passages that equate soul with body and spirit with wind such that human beings appear to be simply natural agents.  But epistemological rationalists have offered additional material for naturalists to consider.  They suppose that there are philosophical considerations to warrant the belief in mind-body dualism.  This philosophical enterprise shall be the primary focus of this essay.[3]

Out of necessity, we shall consider three primary thinkers who are credited with instigating rationalism, and we shall see how each thinker historically interacts with the metaphysical mind/body problem.  I believe that such an historical investigation shall bring a bit of background light to the current philosophical mind/body controversy.



There are two ways rationalism can be understood.  First, rationalism can be perceived as a methodology about how people can access the external world.  Secondly, rationalism can be perceived as a theory of interpretation.  That is, rationalism is used to justify our collective knowledge by seeking to provide a sure foundation.  Professor of philosophy at the University of Reading, John Cottingham, has rightly enunciated that rationalism “is by no means of purely historical interest . . . and many of the problems with which [the rationalists] grappled are of enduring concern today.”[4]  Indeed, the ideas generated by most schools of thought have present ramifications for philosophical thinking.  So we must now answer the question, “What is rationalism?” if we are to understand its interaction with mind/body dualism.

Rationalism can be contrasted to empiricism as seeking to move away from sensory knowledge.  The rationalists of the 17th century sought to build a philosophical epistemology by seeking non-experiential (and perhaps a priori) means to discover a more fundamental and superior type of knowledge.  Professor Cottingham writes of the rationalists:

[T]hey shared a belief that it was possible, by the use of reason, to gain

a superior kind of knowledge to that derived from the senses.[5]


For rationalism, knowledge begins with the reasoning intellect apart from the sensory data derived from experiential input.  Thus, when the empiricist seeks to know something she relies on what her senses tell her.  But a rationalist seeks to find a more fundamental concept about how the external world of experience is to be understood.  What is even more appealing to the rationalists is that internal reasoning, they believed, would lead one to knowledgeable certainty.[6]  This is why Cottingham cites G. W. F. Leibniz as saying that internal reasoning will allow us to ascertain “the universal and necessary truths of science.”[7]  For the rationalists, the acquisition of knowledge via intellectual reason was to acquire a purely reasonable source of information.  This affinity for certainty also led the rationalists, particularly Benedictus Spinoza, to marry mathematics with rationalist inquiry because:

[I]f men clearly understood the whole of nature they would find that

everything is just as necessary as the things treated of in mathematics.[8]

Therefore, many aspects of rationalism came to be understood as logical necessities and logically correct constructions.  As a rationalist, to affirm a claim that “X is true” means “X is true such that ~X is impossible.”  For example, a triangle necessarily has three sides.  Areas of knowledge that are probabilistic are illusions and are less manifest than pure reason.[9]  So the faithful rationalist will always seek to know something qua mathematically and/or logically correct.  Let us now review three primary representatives of 17th century rationalism.

René Descartes (b. 1596)

Descartes was born in Touraine, France, in 1596.  While being born, his mother passed away.  Theologian and philosopher Dr. Ed L. Miller says that “from her he appears to have inherited a frail constitution.”[10]  When he was eight years old, he was enrolled in a Jesuit school in La Fléche.  Due to his unhealthy condition, he earned certain privileges such as extra morning sleep.  Descartes’ passions included mathematics, geometry, medicine, and philosophy.  With the help of his father’s financial contribution, he eventually earned a doctorate in Law at Poitiers in December of 1616.  With a desire to learn from the “book of the world,”[11] he ended up serving in Holland as part of the army of Prince Maurice of Nassau.  After the Thirty Years War broke out, he joined the Catholic forces and ended up in Prague.  On November 10, 1619, Descartes, following personal meditation, claims to have experienced consecutive dreams which led him to pursue unifying all knowledge that was revealed to him.  Professor Cottingham explains that this embedded in him a “mission to found a new philosophical system.”[12]

Descartes has been called the “Father of Modern Philosophy” because of his remarkable approach to attempting to ground all knowledge in a foundation of certitude.  The geometrical method that Descartes would employ would be the operations of intuition and deduction.  For him, finding the unshakable foundation for all knowledge was similar to defining a triangle as a three-sided figure.  In his writings, he tells how there is only one thing we can be absolutely certain of even if we doubt it: Cogito ergo sum (I am thinking, therefore I exist).[13]  Even if someone is doubting that they exist, there still must be an ego around to perform the doubting.[14]  From this single foundation, says Descartes, one can now assert the realities of God and the external world.  For certain there exists a mind that is ontologically distinct from the physical body (more on this in a moment).

Benedictus Spinoza (b. 1632).

The Dutch philosopher, Spinoza, was born in Amsterdam in November of 1632 with the given name “Baruch Spinoza,” but later renounced it and took on the Latin “Benedictus” at age 21.  He was brought up in a professing Jewish tradition in a Portuguese family.[15]  This led to his affinity for Old Testament and Talmudic studies.  And he also studied the philosophical works of medieval philosopher Moses Maimonides.  Spinoza eventually studied under the Christian Cartesian philosopher, Francis Van den Ende, where he picked up an acquaintance with several languages.  At the age of 24, Spinoza was excommunicated from the Jewish community for his departure from Jewish theology and its interpretation of the Scriptures.  As a means of self-support, he took up lens grinding for optical instruments and remained a quiet and solitary scholar.  His denial of the chair of philosophy at Heidelberg in 1673 and the fact that he had not held any position of professorship during his life point to the fact that he chose to stay away from the limelight.[16]  In his years of writing, Spinoza never ceased producing religiously controversial writings (some of which were later banned).[17]

The metaphysics of Spinoza concerns the nature of God.  His falling out with Jewish theology eventually led him to embrace a view of God that envisioned such a being as impersonal and natural.  That is, God is Nature itself which may be a pantheism of sorts or a naturalistic spin on the nature of God.  This world view was established so that Spinoza could create a foundation for ethics that did not depend on theological volunteerism or a divine command theory of ethics.[18]  Thus, given that everything is of the same substance (God or Nature), proportions of motion and rest and the exhibition of particular patterns are key descriptive factors of extension that make different objects in the world distinct.

Gottfried Wilhelm Leibniz (b. 1646)

Leibniz was born at Leipzig in Saxony.  He lived a scholarly life from the beginning, even as a boy (where he learned Greek and Scholastic philosophy).  He was the son of a professor of moral philosophy and, at the age of 15, entered into the same university that his father taught at.  He studied under James Thomasius and, when in Jena, studied mathematics under Earhard Weigel.  Later he would take a doctorate in Law at Altdorf in 1667.  When he was later appointed as Elector of Mainz, he met Nicolas Malebranche and Antoine Arnauld.  When he returned to Paris he made his famous discovery of the theory of Infinitesimal Calculus.  The controversy over whether or not Newton was the first to discover calculus is tarnished by the fact that Leibniz made his publication of the subject prior to Newton’s.[19]  En route back to Germany, Leibniz encountered Spinoza whom he spent a great deal of time studying posthumously.  Although their relationship is historically ambiguous, there is no reason to think that Leibniz’s criticism of Spinoza was in anyway insincere.  Leibniz, after having erected academies, spent his remaining years engaging in the philosophy of religion with respect to synthesizing the Protestant-Catholic dichotomy (to no avail) and reuniting the Christian Confessions.

Leibniz enunciates the distinction between necessary truths and contingent truths.  The former concerns truths such that their denial is self-contradictory and, thus, impossible.  The latter concerns truths that depend on other propositions such that their denial is possible.  A necessary truth would be “A is A” while a contingent truth might be “A is B.”  That all bachelors are unmarried males is a necessary truth based purely on identification.[20]  The historical event that Caesar crossed the Rubicon might possibly be false and is contingent on the actual events transpiring.  All truth statements, says Leibniz, contain within them the predicate.  Perhaps the most ardent metaphysical view espoused by Leibniz is his doctrine of the monads.  Monads are, quite simply, indivisible parts that compose the substance of the universe.  Cottingham calls the monads “‘true unities’ in nature.”[21]  Everything about the world and its eventual unfolding are all contained in the monads (much like DNA).  The interaction of the monads and the unfolding of their cause-effect relationships in the universe all depend on God creating a pre-established harmony.  That is, God creates events that occur in conjunction with what we perceive as their causes.  Unlike Hume, causality is not an illusion but a method of perceiving the concurrence of God’s activity in the world.[22]



One of the most fascinating fields of philosophy is the philosophy of the cognitive sciences.  Our present age of neuroscience has led many to add to the most vexing debate about whether our universe is solely material or whether there exists a concurrent reality that is imperceptible.  Although much has been written and discussed on the issues of mind and its relationship to the body,[23] I shall discuss here how the Rationalists have historically engaged the mind/body controversy and what their interaction means for us today.

René Descartes laid the groundwork for positive studies in dualism.  Dualism is the view that perceives reality as being composed of two different substances.[24]  John Cottingham explains that Descartes’ philosophical metaphysics ties in to mind/body dualism when he writes:

Descartes . . . seems to present his reasons for maintaining the

distinctness of mind and body as flowing from the technique of

wholesale doubt with which his metaphysical reflections begin.[25]

So the famous “Cogito; Ergo sum” proclamation leads us to know with certainty that an ego in fact exists, but the reality of the external world (and the ego’s body in particular) is not so obvious.  Thus the mind and body are distinct substances.  Descartes refers to the thinking mind as the res cogitans and the physical body as the res extensa.[26]  Moreover, Descartes suggests that damage to the body does not affect the ontological status of the mind.  He writes:

[I]f a foot or arm or any other part of the body is cut off, nothing has

thereby been taken away from the mind.”[27]

The final and most perplexing category of the mind/body dichotomy is the relationship between the two substances.  Descartes, when grafting his definition, is careful to script his terminology such that mind and body appear to be in two different planes of existence.  The difference between the two is as easy as geometry.  Material objects (including physical bodies) exist as extensions in space.  Thus the physical is one sort of substance because it has width, height, and depth; Hence Descartes’ use of res extensa for the material body (which in Latin literally refers to “the thing extended”).  So it is conceivable for Descartes to envisage a conjoined substantial being that possesses distinct natures.  When his critics pressed for the method of interaction, he proposed a physical interaction point in the brain at the point of the pineal gland.[28]  It is at this conduit that the res cogitans and res extensa enjoy a “union” and an “intermingling,”[29] to use Descartes’ terminology.



Descartes’ views are not without their scrutiny.  As one can imagine a scenario envisioned by Descartes leads to all sorts of metaphysical speculation.  But this is not to say that the theory is itself false.  Rather, it may only require some sort of clarification.

Benedictus Spinoza approaches the mind/body problem contra Descartes by perceiving the apparent distinction as a mere distinction in aspect and not one in ontology.  So, the observance of the “evening star” in the sky and the observance of the “morning star” do not evince two distinct objects but mere perceptual aspects of the same object.  For Spinoza, the universe itself is composed of an all-embracing singular and indivisible substance.  Any thought of distinctiveness between mind and body would, therefore, be illusory.  So no matter how appealing or persuasive a dualistic tendency might be, Spinoza’s commitment to monism is reason enough to marry the “two natures” into one.  This view has been identified as the Double-Aspect Theory.[30]

G. W. F. Leibniz’s objection is quite interesting.  Instead of abandoning the idea of dualism (as Spinoza does) his proposal takes us on a theological tour of antecedent hypotheses and fanciful monadic activity.  In Leibniz’s solution to the mind/body problem, God creates individual events and objects in close relation to each other so that perfect correspondence exists.  This relationship is endowed with perfect harmony and consecution such that there appears to be causal interaction between the objects.  Instead, God creates either two or more objects of the same substance or of different substances together such that a certain outcome of the given antecedents always ensues.  So the antecedent does not cause the event but merely exists concurrently with it via divine sovereignty.  This view is recognized by philosophers as the Theory of Preestablished Harmony.[31]



While these and other Rationalist thinkers do not adhere to the definitions that Descartes has afforded, their objections (when simplified) result in the outworking of each’s preestablished world view.  To borrow a cliché, if one wears a pair of rose-colored glasses then the world will appear rose-colored.  For Spinoza, a monistic (and pantheistic) world view precludes the division of any of the universe’s substance.  For Leibniz, only God serves as the true source of causation.  When these factors are assumed then the results are the objections above.  But are these formidable objections?  Some reasons suggest that they are just as speculative as dualism.

Contemporary critics argue against dualism by enunciating the recent advancements of neuroscience and artificial intelligence.[32]  The Rationalists looked at in this essay seem to take a presuppositional approach.  Spinoza disavows a Cartesian dualism because of his presuppositions about all matter being a singular indivisible substance.  Thus any view suggesting a dualistic universe would be precluded a priori.  Leibniz’s objections are similar.  If God serves as the only true source of causal activity then no amount of influence would convince him that individual minds could actually cause physical brain states (or vice versa).  Descartes is not without his shortcomings either since he tries to explain mind/body interaction by presupposing dualistic natures.  However, he argues that mental states appear distinct from brain states.  This seems like a formidable and common-sense approach.  To see how the distinction seems immediately intuitive, think about the color orange.  It does not turn some feature of my brain into an orange color.  The two are immediately distinct to the ego whether they turn out to be identical through some other means or not.  But Descartes really feels that his “Cogito; Ergo sum” statement establishes the priority of the res cogitans over the res extensa.  Then again his Catholic upbringing no doubt includes similar influences on his philosophy of mind.  As a Christian believer he would use this to advance a considerable apologetic toward maintaining belief in the afterlife and in the incarnation of Jesus of Nazareth.  Whatever the case, it seems that a person’s world view will certainly define what one thinks about metaphysical issues such as the one discussed here.



We have looked at the branch of epistemology known as Rationalism and reviewed some of its primary adherents.  Descartes, Spinoza, and Leibniz all believe that reality can be known through deductive reasoning processes that avoid the traps of Empiricism.  Descartes believes that our deduction of the ego as a known certainty makes the experiential world an independent substance.  As a consequence, Spinoza and Leibniz interact with the arguments presented by Descartes in assessing the mind/body controversy.  What the controversy comes down to is contingent upon the world view held. 

Our 21st century perceptions about the mind/body controversy have taken a different route.  We now have the advantages of contemporary advancements in neuroscience and artificial intelligence.  Although the debate will never be resolved this side of the grave, the interactions between proponents and skeptics allow us to find the meaning to our own existence.  If we are composed of dualistic natures, which seems to be intuitively feasible, then we are once again faced with the question of existential meaning and purpose because we must find the source of this imperceptible substance.  If God exists then this renders the notion of dualism more probable.  If God does not exist then the notion of dualism is either anomalous or it must be wrong.  Therefore, our personal answers to the mind/body controversy have existential repercussions that cannot be ignored by sojourners of truth.


[1]. Genesis 2:7; 37:35 (this occurs as a reference to the hopeful reuniting of a father and son in the She’ol of the afterlife – its inhabitants are implied by passages such as Isaiah 14:9); 1 Samuel 28:7-25; Psalm 11:5; Isaiah 1:14; Zechariah 12:1.  The Rabbinic literature also captures the notion of dualism (see Mid. Gen. 409, 516, 549; Num. 733; Ecc. 83, 229; Bab. Tal. Ber. 59, AZ 21; Yeb amoth XVI.3, 157, to name a few).

[2]. Matt. 10:28; 17:1-8 (Mark 9:2-8 confirms this experience and exists as an older record to that of Matthew.  Most New Testament scholars now acknowledge that (i) the existence of events contained in multiple layers of tradition bespeak validity, and (ii) the Gospel of Mark antedates the Gospel of Matthew); 2 Corinthians 5:1-6; Philippians 1:23-24; Revelation 6:9.

[3]. Interested researchers who desire to see an Evangelical defense of mind-body dualism from both a philosophical and theological standpoint are encouraged to see Gary Habermas and J.P. Moreland, Immortality: The Other Side of Death (Tennessee: Thomas Nelson Publishers, 1992).  For a strictly theological defense of mind-body dualism and afterlife consciousness, researchers are encouraged to investigate the comprehensive work by Robert A. Morey, Death and the Afterlife (Minnesota: Bethany House Publishers, 1984).

[4]. John Cottingham, The Rationalists: A History of Western Philosophy 4 (Oxford: Oxford University Press, 1988), p. 11.

[5]. Ibid., p. 4.

[6]. Ibid., p. 5.

[7]. G.W.F. Leibniz, Die Philosophischen Schriften von G. W. Leibniz, ed. C. I. Gerhardt, vol. VI (Berlin: Weidmann, 1875-90), pp. 504-5, cited in Cottingham, The Rationalists, p. 5.

[8]. B. Spinoza, Spinoza, Opera, ed. C. Gebhardt, vol. I (Heidelberg: Carl Winters, 1925; reprinted 1972), p. 266, cited in Cottingham, The Rationalists, p. 8.

[9]. Cottingham, The Rationalists, pp. 7-8.

[10]. Ed L. Miller, Questions that Matter: An Invitation to Philosophy (New York: McGraw-Hill, Inc., 1992), p. 104.

[11]. Descartes’ phrase here implies that he preferred personal experience rather than university education.  Cf. Cottingham, The Rationalists, p. 12 and Miller, Questions, p. 105.

[12]. Cottingham, The Rationalists, p. 13.

[13]. Robert Audi, ed., The Cambridge Dictionary of Philosophy (New York: Cambridge University Press, 1995), p. 195.

[14]. Cottingham, The Rationalists, pp. 39-42.


[15]. See Cottingham, The Rationalists, pp. 18-19 and Frederick Sopleston, S.J., A History of Philosophy, Vol. IV, Image ed. (New York: Doubleday, 1993), p. 205.

[16]. Copleston, History of Philosophy, p. 205-6.

[17]. Cf. Cottingham, The Rationalists, p. 21.

[18]. Audi, Dictionary of Philosophy, p. 759-60; Cottingham, The Rationalists, pp. 91-4.

[19]. Cottingham, The Rationalists, p. 26; Also see Copleston, History of Philosophy, pp. 264-5.

[20]. Ibid., pp. 66-70.

[21]. Ibid., p. 107.

[22]. Ibid., p. 109-14.

[23]. For example, see a previous essay I had written, “Reductionism as Explanation and the Mind/Body Problem” (http://sguthrie.net/REDCTN.htm).  Contemporary champions of mind/body dualism include Karl Popper, John Eccles, Stewart Goetz, Charles Taliaferro, John Foster, and William Hasker.  Some of its most ardent critics include Paul and Patricia Churchland.

[24]. Miller, Question that Matter, p. 112.

[25]. Cottingham, The Rationalists, p. 116.

[26]. See ibid., p. 83.

[27]. René Descartes, Oevres de Descartes, ed. C. Adam and P. Tannery, Vol. 11 (Paris: Vrin/CNRS, 1964-76, revised ed.), p. 86 cited in Cottingham, The Rationalists, p. 116.

[28]. The pineal gland, or the epiphysis, is the cone-shaped gland in the brain that secretes melatonin.

[29]. See Descartes, Meditations on First Philosophy, I, p. 192.  Also see Miller, Questions that Matter, p. 114.

[30]. Cottingham, The Rationalists, pp. 127-36.

[31]. Ibid., pp. 136-49.

[32]. See Paul Churchland, A Neurocomputational Perspective: The Nature of Mind and the Structure of Science (Massachusetts: MIT Press, 1990); idem, Matter and Consciousness (Massachusetts: Massachusetts Institute of Technology, 1988, revised edition).

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