Descartes res cogitans and res extensa

The rise of modern science in the 17th century gave rise to the identification of two elements in the scientific method the empirical element, or observation and experimentation, and the rational element, which refers to mathematics and deductive reasoning. These changes boldly challenged the medieval philosophy of scholasticism and called for a single, unified science in which philosophy and all the sciences would be interconnected in one systematic reality. This was the vision of a man named Rene Descartes. It is said that being a mathematician himself, the longing for certainty was always paramount with Descartes. Descartes regarded only mathematics as certain but he wondered what kind of relations it had on the other sciences. With this in his mind, he then started a journey of looking for ways to unify the sciences. After around 30 years in 1647 he published the Meditations on First Philosophy but 22 years later in 1669 the Inquisition forbade the reading of the book. The Meditations contains the principles of Cartesian Psychophysical Dualism  or Descartes idea that everything that exists is either thinking substance, or res cogitans, or physical substance, or rex extensa.

The metaphysics of Descartes centered on the principle of dualism. Dualism is defined as the name for any theory which claims that there are two ultimate and irreducible components in the subject to be explained. Dualism extends to the known branches of philosophy  good and evil in ethics, true or false in logic, and metaphysical dualism.

Metaphysical dualism is in fact the term applied to the metaphysical theory which claims that there are two ultimate and irreducible kinds of reality. If one adheres to the concept of metaphysical dualism, he believes that everything that exists in the world is either A or B, and the most usual classification is either physical or nonphysical. One of the first known proponents of metaphysical dualism was Plato, who theorized that everything we can see has an equivalent in the world of Forms and that this material chair I am sitting on for example is not as real as the chair in the world of Forms. Aside from Plato, metaphysical dualism extended to theological concepts in the idea of body and soul, and almost always it is the nonphysical substance that is more real or that survives.

Descartes idea of metaphysical dualism is a reality consisting of only two kinds of substances  mental or res cogitans, and physical or res extensa. According to Descartes, res cogitans can never be shown to be, derived from, or a form of, or a function of, or reducible to, res extensa, and vice versa. The proofs of these two classifications are ontological, or that its proof is already in its statement.

Ontological Proof of res cogitans. Ontological means that the mere fact that ones being conscious of the fact that he is thinking proved that he exists as a substance in which thinking is going on (Lavine 122). This is the proof of the cogito and it is the proof of the existence of res cogitans. It simply means that even if I doubt everything, I know that I am the one doing the doubting. Therefore, I must necessarily exist, for if I do not exist then no one or nothing will do the doubting.

Ontological Proof of res extensa. On the other hand, ones clear and distinct idea of the attribute of extension proved that substances with the attribute of extension exist as the cause of his ideas. This is the proof of res extensa. According to Descartes, his faculty of imagination which he uses in considering material things is sufficient to persuade him of their existence, for imagination is simply the application of facultas cognoscitiva, or cognitive faculty to a body which is immediately present to it, and which therefore necessarily exists.

The only existing proof for res cogitans is the ontological proof of the cogito (see Ontological Proof of res cogitans). Res cogitans, or the mind, is immaterial and non-physical and therefore contains properties distinct from res extensa, which is material and physical.

Mental Nature. Res cogitans is not only immaterial but also mental in nature, which means that it is a thinking substance which does not occupy space, is not in motion, is capable of reasoning, remembering and denying, has free will and is morally responsible for its action. In the words of Descartes himself, the existence of res cogitans implies that I am not even a bodily man, I am only reason. This is because the thinking subject is the only thing that is certain and can be proven at this point, and is itself self-evident.

Other Qualities. According to Descartes, res cogitans, aside from its ability to think, also possesses the capacity to doubt, understand or conceive, affirm, deny, will, refuse, imagine and perceive. Res cogitans also desires to knowdoes not wish to be deceived and imagines many thingsdespite his will. (Descartes 129)

Empirical Proof of res extensa. According to Descartes, apart from the aforementioned ontological proof, the empirical proof of res extensa is that all throughout his life, his senses pointed out to him the existence of the parts of his body and the sensations of pleasure and pain. He also learned through the senses the attributes of things and size, shape, motion and temperature, as well as the physical sensations of hunger and thirst, and joy and sadness.

Problems of the Empirical Proof. However, Descartes said that sensation does not necessarily mean that material bodies exist exactly as our senses show them to be and that these various experiences gradually ruined all his faith in the senses when he found that judgments based upon the external senses were erroneous and that the internal senses are also defective. Descartes also underlined the possibility that memory may interfere with the imagination which on the whole may interfere with the senses. In that case, Descartes concluded that it would be better to examine sense-perception itself and inquire whether or not from the ideas that are apprehended bythinking, one can obtain a certain proof of the existence of res extensa. This therefore means that although the existence of res cogitans is certain, that of res extensa is not. About a century after Descartes cast his doubt upon the proof of res extensa, a man in the name of Immanuel Kant resolved his problem of whether the existence of res extensa can be proven by the senses. Kant assigned the term thing-in-itself to the original object that no one can ever know and may be an object of thought or inference, and one that cannot be experienced as itself for in being experienced, it would be changed by its passage through sense and thought. This simply means that the object as it appears to us is actually a mere phenomenon or appearance which is perhaps very different from the external object, or the thing-in-itself, before it was perceived by our senses. In this respect, Kant somehow agreed with Descartes in saying that the existence or truth of the object being perceived is not necessarily guaranteed or ascertained by the senses. Kant himself said, It remains completely unknown to us what objects may be by themselves and apart from the receptivity of our senses.

The Analogy of the Piece of Wax. In the second section of Descartes Meditations, he explains the nature of physical substances and how their nature and qualities are determined by using the analogy of a piece of wax.  This particular piece of wax is from the beehive and possesses certain qualities such as color, shape, size, hardness, coldness, shape, and an audible sound if you strike it. However, as the piece of wax is subjected to flame, the taste of honey, the smellthe color, the shape, the size, the audible sound, and the temperature are all gone. Moreover it liquefies as well.

Now if a question was asked whether this piece of wax subjected to the fire is the same piece of wax as the original, everybody would say yes. But how do we know for sure that this is the same piece of wax as the original Because of the properties of the wax that remain constant despite the change  the properties of being extended in space and capable of change, which are the only true characteristics of wax and of any material body, or res extensa. Descartes is pointing out here that it is these properties, the properties known by reason and intellect or res cogitans, that are the only properties by which res extensa is known. These properties include the size, shape, spatial extension and a capability of motion, but especially size and shape.

On the other hand, the sense qualities  color, taste, odor, sound, texture  are not qualities which are necessary to the existence of a physical thing. These are the very qualities of an object that are known empirically, or through the senses, and not through reason or the mind, or res cogitans. This is another one of Descartes numerous refutations against empiricism.

Once more, on the subject of distinct qualities, res extensa, or physical substances, in contrast with res cogitans, possess mass, density, location, tactile properties, duration, taste, smell, and other sense qualities. However, among these, only mass, density, location, tactile properties, duration as well as size and shape remain as the qualities by which res extensa is identified, hence these are the only permanent qualities. (

The two spheres of reality  res cogitans and res externa  do not have any contact with each other and are completely dissimilar. The proof of the separation of res cogitans and res extensa is the doubt that Descartes himself had on the reliability of the senses in determining the existence of physical objects. He mentions that sensation through the external senses does not necessarily mean that material bodies exist exactly as our senses show them to be and that the internal senses were also defective. In order to prove his claim in the preceding statement, Descartes mentioned that people with amputated limbs sometimes even feel sensations in them.

Although the aforementioned statements imply a problem of communication between res cogitans and res extensa, it is clear from Descartes evaluation of the senses that he was trying to refute empiricism, for Descartes was a rationalist.

The Pineal Gland.  The pineal gland is a tiny organ located in the center of the brain and was believed by Descartes to be the principal seat of the soul and the place in which allthoughts are formed (Lockhorst). It was also the pineal gland that Descartes regarded as the point where res cogitans interacted with the physical body. In his Treatise of man, Descartes underlined the role of the pineal gland as the specific part of the body involved in sensation, imagination, memory and the causation of bodily movements (Lockhorst).

Descartes was however mistaken at this concept because of the three errors he committed regarding this particular organ that the pineal gland is suspended in the middle of the ventricles, that many arteries surround it, and that the ventricles are filled with a certain very fine air or wind (Lockhorst). Most of Descartes proof for the choice of the pineal gland as the solution to the problem of communication between res cogitans and res extensa is based on his scientific and anatomical background. Yet, the greatest weakness of Descartes theory on the pineal gland is that it cannot logically answer this question how can the non-physical res cogitans interact with a physical pineal gland And even if the pineal gland were truly the solution to the problem, it does not sufficiently explain the interaction between res cogitans and the res extensa which is external to the human body, which means, for example, that the pineal gland theory cannot sufficiently explain how the mountains and seas formed. The only possible solution, therefore, according to Malebranche was res infinita.

Res Infinita
It was theorized by Descartes that all the world, and every body, is a machine but outside the world is God, or res infinita, and within the body is the spiritual soul. Descartes also said that every movement of every animal, and even of the human body, is a mechanical movement  for example the circulation of blood and that God governs all these mechanical clockwork.

Problem of Communication and Possible Solution. Both res cogitans and res extensa do not have any contact with each other and are completely dissimilar thus this is the root of the problem of communication between the two spheres of reality, of the problem of interactionism. This problem is further based on the following argument  that considering that both res cogitans and res extensa are completely dissimilar and that both have exclusively distinct properties, how can I come to know the world, or in short, how can res cogitans know res extensa And the even more significant question is, if my body and I are indeed two different realities without any possible interaction, how can I govern my body and make it move. Indeed, if ones body can move and given that the mind cannot possibly move it, then it must be God, or res infinita, that moves the body.

God is considered the ontological basis of the two finite substances res cogitans and res extensa and that it must be God who effects this impossible communication between the substances. This was, however, a theory not of Descartes but of one of his disciples, Nicholas Malebranche. Descartes solution to the problem of communication between res cogitans and res extensa was his aforementioned theory on the pineal gland, which had its own logical defects.

The principles behind Descartes res cogitans and res extensa lend themselves to several weaknesses, most especially in terms of logic.

Argument against res cogitans. The first is the proof of the cogito, which is also the proof of res cogitans.  Descartes proved the existence of the cogito by saying that the doubter is presumed to exist for if it does not exist, then the doubting cannot possibly happen. But actually, the doubter just because it can doubt everything, does not necessarily escape doubt for he can also be doubted at the same time that it doubts. Another thing is that there is no proof that it is the I that doubts, for no one can possibly ascertain that it is res infinita that doubts in behalf of the cogito or that causes him to do it.

Argument against res extensa. The rational proof of res extensa was ontological (see Ontological Proof of res extensa). This is a big problem, however, as even Descartes himself doubts the ability of res cogitans to determine the existence of res extensa. And so res extensa remains doubtful, what with Kant who theorized that the thing-in-itself cannot be known, and Berkeley who declared that all matter do not exist.

Argument against the Separation of res cogitans and res extensa. As Descartes himself could not find a clearly established link between res cogitans and res extensa, he leaves the gap gaping wide for others like Malebranche to say that res infinita fills it. But the problem here is the separation if res cogitans and res extensa are truly separate as what Descartes had declared, then why and how can the former perceive the latter and make conclusion of the ontological proof of the latters existence

Such were a few of the questions that had been and should still be raised concerning Cartesian psychophysical dualism.

Res cogitans is thinking substance and res extensa is physical substance. Both were ontologically proven by Descartes to be dissimilar and separate, which means they have exclusive qualities. Res cogitans is the mind, the reason, the emotion and any other aspect of reality which is  purely non-physical. On the other hand, res extensa is matter and has four essential qualities size, shape, spatial extension, and capability of motion. These four essential qualities survive change unlike the qualities perceived by the senses. Descartes res cogitans and res extensa, despite the ontological and empirical proofs he provided in the Meditations, lend themselves to weaknesses when it comes to the idea of their separation, the seemingly questionable ontological proofs, and the validity and reliability of res cogitans in proving the existence of res extensa.


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