René Descartes: ‘I think therefore I am’

By Brother John Raymond

René Descartes has been called by many the father of modern philosophy. He is most famous for his statement ‘I think therefore I am’. This discovery will become the basis for his philosophy. He was led to this statement by a reaction against the skepticism of his day. Descartes wanted to safeguard philosophy by finding clear and distinct ideas which nobody could refute. He found the Scholastic terminology too vague in many of the terms which they used. Thus, Descartes built his own philosophy on what he considered a solid foundation. He also desired his philosophy to be a comprehensive scientific philosophy. At one point in his life he was so sure of himself that he thought his philosophy would replace Scholasticism. He said with sharp criticism that his new textbook would be called the ’Summa Philosophiae’. Descartes was involved in many disputes over his works, but according to Father Copleston Descartes was known to be a man of moderation and kind disposition. He also always made sure that no matter where he moved to he would be within walking distance of a Catholic Church. He tried to avoid discussions of a purely theological nature; although he was forced into many such discussions over his philosophy. He always submitted to the Church in matters of faith or morals. We will look at Descartes’ life as reconstructed from his many letters of correspondence. We will see how he developed his philosophy. We will look at his conclusions on the relationship between the soul and body. Finally, we will see how he explains and defends the Most Blessed Sacrament.

René Descartes was born on March 51st, 1596 in Touraine, France. His mother died thirteen months after his birth. In 1604 he attended La Fleche College which was directed by the Fathers of the Society of Jesus. Descartes studied logic, philosophy, and math in his last few years of college and finished his studies in 1612. He commented that he liked traditional learning, and yet on later reflection he felt that some parts had no solid foundation. He said, ‘philosophy teaches us to speak with an appearance of truth about all things and causes us to be admired by the 1ess learned’.1 We see that Descartes is unimpressed with traditional philosophy because any point of it can be a matter of dispute. Descartes left the college with a determination to experience life outside a textbook, He joined the army of Prince Maurice of Nassau in Holland, but did not accept any soldier’s pay. Descartes combined his duties with mathematical studies, A1so he composed a short treatise on music with a Dutch doctor. In 1619 he left the service and went to Germany where he joined the army of Maximilian of Bavaria. It was at the secluded station of Neuberg on the Danube where Descartes had three consecutive dreams that inspired him to seek the truth by reason. He felt this mission was a Divine call so he vowed to make a pilgrimage to the shrine of Our Lady at Loreto in Italy. After much military service travelling, he finally made his way to Our Lady’s shrine in 1623. Next, he stayed In Paris where he renewed his acquaintance with his fellow classmate Marin Mersenne. Prizing his seclusion great1y, Descartes found Paris too distracting, so he moved to Holland in 1628, where he spent most of the rest of his life. From Holland we find Descartes beginning his letter-writing career.

The next ten years are marked by the development of a philosophy of science and nature. Always concerned about Church teaching, we find Descartes writing to Mersenne, a Friar Minim already mentioned and a principle correspondent, about whether the Church had decided the created universe to be finite or infinite. In 1630 Descartes was entrenched in a hot dispute with Isaac Beeckman, a Dutch doctor already mentioned, over who should receive the credit for the treatise on music written twelve years previously. In 1633 Descartes wrote Mersenne to inform him that he would suppress the Treatise on the World because it contained the doctrine that the earth was in motion around the sun. This doctrine was condemned by the Church in Galileo’s book. In 1637, Descartes published the Discourse on the Method, with Optics, Meteorology, and Geometry. Descartes claims his ‘method’ has universal application. He sent copies of his work to the King of France, Cardinal Richelieu, and the Jesuit teachers at La Fleche. Descartes always wished to submit his works to the scrutiny of the Church as well as those in civil authority. In 1638 Descartes became involved in a number of controversies concerning his scientific work. A heated debate ensued between him and Pierre de Fermat, a mathematician, over his Dioptrics and Geometry. The two were finally reconciled through Mersenne as an intermediary. Jean Baptiste Morin, a professor of mathematics at the College de France, attacked Descartes’ theories of Nature and the transmission of light. In 1639 we find Descartes trying to answer innumerable questions concerning mechanics, chemistry, anatomy, and optics all of which he treated in Discourse on the Method.

In 1641 Descartes published his Meditations on First Philosophy along with six sets of objections and replies. Descartes now enters the arena of the Scholastics. He is accused of all types of transgressions of Church doctrine implied in his philosophy. In 1642, a second set of seven objections and replies were published. Also at this time Gisbert Voetius, a professor of Theology, was trying to have Henricus Regius, a professor of medicine, condemned for heresy at the University of Utrecht for indiscreetly teaching Cartesian views. Three points were brought under attack; first that the human being was not an essential unity, second that the earth moved around the sun, and third that substantial forms should be rejected. Finally, Regius was restricted to teaching traditional physics and medicine. Descartes wrote Regius congratulating him for ‘suffering persecution for the truth’s sake’, but he encouraged him to listen to the magistrate’s decree.2 In October Princess Elizabeth of Bohemia, in exile at Hague, read Descartes’ Meditations. She was greatly perplexed about the relationship of the mind to the body in his work. They debated this issue in many letters until finally Descartes gave up the debate and preferred silence on the issue.

1n1644, Descartes published his Principles of Philosophy while visiting France. One year later Descartes had to defend his writings against the Jesuit Father Denis Mesland, a missionary to America, concerning transubstantiation. Also at this time a quarrel ensued with his disciple Regius who was claiming the mind was only a mode of the body. This view was the extreme opposite position from Descartes. In one of the letters addressed to Princess Elizabeth at this time we see Descartes’ view on the Protestant position. She tells Descartes that her brother Prince Edward became a Catholic to marry the Princess of Mantua which greatly distressed her. Descartes responded by saying he could not be sympathetic to her grief over her brother’s conversion. He says, ‘Those who were not Catholic should remember that if they or their ancestors had not left the Roman faith they would not hold their present religion’.3 In 1646 Regius published his Foundations of Physics which Descartes disowned since it contained mostly Cartesian ideas. In 1647, Descartes took a tongue lashing when the head of the Theological department at a college in Leyden publically defended a thesis that said Cartesianism was atheistical.

As Descartes’. life was coming to a close he went back to Paris where he was awarded a pension by the King of France which he never drew. A year later Descartes returned to Paris but found the city in a tumult and returned home. On his return to Holland he received a letter from the Cambridge Platonist Henry More who welcomed him as an ally against the materialism of Hobbes. In 1649, Descartes went to Sweden after receiving an invitation from Queen Christina. He suffered greatly from the Swedish winter coupled with his having to rise at 5:00 a.m., every day to teach the Queen philosophy. While helping a friend recover his health, Descartes contracted pneumonia. He died of pneumonia only four months after his arrival in Sweden on February 11th, 1650. Descartes died a pious Catholic.

So far we have seen Descartes many times mentioning a ‘method’ which he believes to have universal application. A good way of understanding this method is to use an analogy of a basket of apples. If we know some apples to be rotten in the basket, it would be best for us to dump the whole basket out on the ground and choose the apples that are not rotten for placement back in the basket. The basket is like the human mind and the apples represent our ideas. Descartes wants to dump out all the ideas of the mind which are not clear and distinct. This type of ideas would be like the rotten apples. The method of checking for rotten apples or ideas is to scrutinize them as being either doubtful or not. If they are doubtful, he will discard them from his mind. Thus, he found reasons to doubt everything in his mind except for his own existence which he could not doubt. To doubt is to exist. With respect for the Church, Descartes put moral axioms and Church doctrines outside of any question of doubt. This approach to thought by Descartes is very much influenced by his interest in Geometry. Geometry begins with basic axioms or assumptions which are too basic to be demonstrated as true or false. From these assumptions one then deduces other propositions which lead to other statements until one has constructed an entire system. Descartes applies something basic to any Geometrical system to human thought. The first rule of thought is to break down multiple data into its simplest elements. Second to take these simple elements and deduce propositions step by step. Descartes believed he had arrived at an axiom with the statement of his existence. Unlike Geometry, Descartes looked at this axiom to determine why he held it to be true. He concluded the only reason it was true was because he perceived it to be clear and distinct. Unfortunately, he concluded anything he perceived clearly and distinctly in his mind must be true. Thus, truth was reduced to intuition or pure intellectual activity.

Descartes considered the proof of God’s existence as necessary for his philosophy of innate ideas. He proves the existence of God in the following way. In his mind there is an idea of an infinite and perfect Being, How can a finite and imperfect creature have such an idea? This idea can only come from such a Being. Included in this idea is a necessary existence. Not only does the idea of God come from God in our minds but part of this idea of God necessarily includes His existence. Since he perceives this clearly and distinctly, it must be true. Why is God so important to Descartes? Not only does God explain how these innate ideas got there, but God reassures Descartes trustworthiness in these ideas as true; God would not deceive him. Now he can rebuild the world from his mind without any reservations. The mind left to itself is infallible. Error arises from prejudice, passion, education, and impatience. Thus, his comprehensive scientific philosophy can now be developed. It can be likened to a tree where metaphysics forms the roots, physics the trunk, and the branches of science the tree branches. We can see how Descartes ‘method’ is very similar to a Geometrical system and why he needed God as part of this system.

Now that Descartes can trust these ideas, he believes that the nature of the body and soul can be known by clear and distinct ideas in his mind. These natures in his mind are different. The essence of the soul is to think and that of the body is just extension. They are two separate substances. Descartes defines man as ‘a spirit which makes use of a body’.4 We see that the function of the body is purely mechanical. Also, he says, ‘I know that thinking substance is a complete thing no less than that which is extended’.5 Again, we see the soul and body as self-subsisting. He goes further in the claim that the human being is not an essential unity but an ‘ens per accidens’. If we consider the relationship of the soul and body, it is inessential for one to be joined to the other since they can subsist apart. Thus, the relationship of the soul and body is accidental. The Scholastics insisted on the unity of the human being. For them the soul was seen as the principle of biological, sensitive, and intellectual life. Saint Thomas said that the soul gives existence to the body in the sense that we call it a human body. For him the soul and body are one composite substance. Descartes was forced by the Scholastics to clarify his views. He claimed that when we consider the relationship of the parts of a human being the above is valid. It we look at human beings as a whole, they are a unique ‘ens per se’ because for a man to be a man it requires an essential union between the soul and body. Descartes feels it is more important to emphasize the relationship of the parts to refute those that claim the soul to be mortal, More people mistakenly deny the distinction between the soul and body than affirm the distinction end deny their substantial union. Descartes feels he is doing the Church a favor by promoting the distinction of the parts. He says, ‘I thought I would please the theologians more by saying that a human being was an ‘ens per accidens’, in order to make the distinction, then if I said that he was an ‘ens per se’, in reference to the union of the parts’.6 Thus, we see Descartes’ reasons for his strange statement that the soul and body have an accidental relationship.

Descartes next has to contend with Princess E1izabeth to give an explanation of how the soul interacts with the body. She says, ‘it would be easier to attribute matter and extension to the soul, than to attribute to an immaterial being the capacity to move and to be moved by a body’ .7 Descartes feels that the pineal gland in the brain is the connecting organ between the soul and body. It is the only organ In the brain which is not double. Thus, it must be the spot where all the senses are united into the common sense. The blood contains the sensitive soul or animal spirits as proven by Leviticus (Chapter 17; Verse 14), ‘The soul of all flesh is in the blood’. The act of walking takes place indirectly by the pineal gland altering the direction of the animal spirits in the blood to move the limbs. These animal spirits also move the pineal gland which causes the soul to feel the ideas of pain, pleasure, hot, cold, and so forth. The body is almost entirely automatic and was set in motion by God. Memory is produced by traces left by brain particles which are set in motion by external stimu1us to the sense organs, or by the rising of the animal spirits from the heart to the brain, or by the mind’s idea of a corporeal thing. Purely intellectual things are not remembered, but only the corporeal names associated with them. According to Descartes, when a similar condition is repeated in the body it impels the soul to repeat the same thought. Converse1y, when the same thought is repeated it disposes the body to return to the same condition. Thus, the interaction between the soul and the body can be likened to a pilot and ship. The soul does not create new movement or energy in the body but like a pilot only alters its direction. The body like the ship is purely automatic but can effect the pilot through the gauges which tell the condition of the ship.

Another topic which Descartes was forced to contend with by his opponents was transubstantiation. Antoine Arnauld, a Jansenist theologian, held that Descartes’ philosophy conflicted with Catholic belief about the sacrament of the Eucharist. He said that Descartes

denied real accidents, but recognized only modes which are unintelligible unless they inhere in some substance. Descartes argued that the Church has never taught that the forms of bread and wine are real accidents. To be a real accident they would have to subsist by a miracle on their own after the substance has been removed. The Council of Trent holds that ‘the whole substance of the bread is changed into the Body of Christ while the form of the bread remains unaltered.8 Descartes sees the ‘form’ of the bread as the surface common to the individual particles of bread and the bodies surrounding them. He says ‘form’ means exactly what is required to act on the senses, He elaborates further that this surface is intermediate between the Body of Jesus Christ and the air surrounding Him. This ‘surface’ is only a mode or manner of being and not a substance or real nature. After consecration, the numerical identity of the surface remains the same because it does not depend on the identity of the bodies between which it exists but only on the similarity of the dimension. He likens this similarity to a river which can still be recognized ten years later even though the water and the particles of earth surrounding it on the bank are totally different. Thus, Descartes holds that real accidents do not exist as Arnauld accuses him of saying. Yet, denying real accidents in no way takes away from the Blessed Sacrament. Descartes defines the ‘form’ of bread and wine to refer to en intermediate surface which acts on our senses. This surface depends on the similarity of dimensions before and after consecration for its mode of being.

Descartes becomes involved in explaining the manner of existence of Jesus in the Blessed Sacrament reluctantly in a letter to a Jesuit Father Mesland. He was reluctant because the Council of Trent said that Jesus Christ is in the Blessed Sacrament ‘in a manner of existence which can scarcely be expressed in words’.9 Descartes will make an attempt to express this mystery in words as long as it Is in accord with Church teaching, and Father Mesland does not tell anyone Descartes is the author. According to Descartes, we must first define the term ‘body’ because it is very ambiguous. If we are talking about a non-living body, then if any matter is removed the body is no longer numerically the same. It is said to be smaller. If we are talking about the body of a man, the body is numerically the same body even though the matter changes. This is true only as long as the body remains substantially united to the same soul. An example of this would be the body of an infant in contrast to that of an adult. We consider a person to have the same body all through his life even though the matter from infancy may no longer be the same in the body. Thus, if we apply this reasoning to the Blessed Sacrament, as long as the Soul of Jesus Christ Is supernaturally conjoined to the bread and wine the Host can be divided in half without ceasing to be the Body of Jesus Christ. This is because any matter, small or large, informed by the human soul is taken to be an entire human body. Thus, Jesus Christ can be present only once in the entire undivided Host, and yet is entire in every particle that is divided. This whole and entire presence of Jesus in each particle is the reason why the priest holds the finger and the thumb together after the consecration. This is also the reason why the ablutions are performed. Jesus is no longer present when the ‘form’ of the Host ceases to exist by being dissolved in water. Thus, we see Descartes’ theory that the union of the Soul of Jesus Christ to the Host is what makes iu the ‘Body’ in the sense of a human body.

In Descartes’ theory there is the problem of consubstantiation. He talks of the Soul of Jesus Christ being conjoined to the bread and wine. This is in direct contradiction of his earlier quote from the Council of Trent that the whole substance of the bread is changed. We can no longer talk about the substance of bread after consecration. Our Lord’s Soul cannot exist with the substance of the bread. This would be the Lutheran idea of consubstantiation. This theory held that Our Lord is present ‘with’ the bread. Thus, we cannot hold Descartes’ theory as Catholics. In trying to define transubstantiation, he has turned it into consubstantiation.

We have seen Descartes’ life as reconstructed by his many letters of correspondence. He was greatly influenced by Geometry in his approach to philosophy. Many of his theories on Science and Nature were attacked as well as his Geometry. As he moved into his comprehensive philosophy, he came under criticism by the Scholastics. Yet, he maintained that nothing in his philosophy affected Church teachings, and he fought against all who claimed it did. Descartes defended the Catholic Church as can be seen in his letter to Princess Elizabeth concerning her brother’s conversion. Descartes’ method of doubt which he likened to sorting through a basket of apples led him to believe that truth was to be found only in pure intellectual activity. In order to trust in these ideas, Descartes had to prove the existence of God. Once he had done this he could trust these innate ideas since God would not deceive him. From these ideas he deduces the rest of his philosophy. One of his ideas is that the body and soul are two distinct substances. The body Is like an automatic machine set in motion by God, and the soul’s essence is to think. Put them together and we have the relationship of a pilot to his ship. The relationship is an accidental one when we consider the soul and body as parts. To be a human being, though, it is essential for the soul to be united to the body. This union takes place at the pineal gland in the brain. The blood contains the sensitive soul which Is redirected by this gland to indirectly bring about a certain willed action. The blood also moves the gland which causes corporeal thoughts in the soul.

The final area which we treated was that of transubstantiation. Descartes denies that there is such a thing as real accidents but claims this in no way contradicts Our Lord’s presence in the Blessed Sacrament. He does define the ‘form’ of the Host to be maintained by an intermediate surface which acts on our senses. This surface depends on the similarity of the Host’s dimension for its mode of being. In discussing how Our Lord exists in the Blessed Sacrament, Descartes claims that the Soul of Jesus Christ conjoined with. the bread and wine is what makes the Sacrament His Body. The soul united to any matter is what makes it a human body. The problem with this theory is that it replaces transubstantiation with consubstantiation. Thus, we have seen and explored a small part of the works of Descartes. We have looked at those parts of his life and philosophy which were considered to be of the most interest to the Christian tradition.


  1. Frederick Copleston, S.J., Descartes to Leibniz (Vol. IV of A History of Philosophy, ed. Edmund F. Sutcliffe, S.J.; Maryland; The Newman Press, 1959 ), p. 64.
  2. René Descartes, Philosophical Letters. Translated by Anthony Kenny (Minneapolis: University of Minnesota Press, 1970), p. 133.
  3. Ibid., p. 188.
  4. Copleston, op. cit., p. 121.
  5. Ibid., p. 122.
  6. Descartes, op. cit., p. 130
  7. Ibid., p. 140
  8. René Descartes, The Philosophical Writings of Descartes (Vol. II, Translated by John Cottingham, Robert Stoothoff, Dugald Murdoch.; New York; Cambridge University Press, 1984), p. 175.
  9. Descartes, op. cit., p. 156.


Copleston, Frederick, S.J. Vol. IV of A History of Phi1osophy. Edited by Edmund F. Sutcliffe, S.J., Maryland, The Newman Press, 1959.

Descartes, René, Philosophical Letters. Translated by Anthony Kenny, Minneapolis, University of Minnesota Press, 1970.

Descartes, René, Vol. II of The Philosophical Writings of Descartes. Translated by John Cottingham, Robert Stoothoff, Dugald Murdoch, New York, Cambridge University Press, 1984.

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