The existence of physical things which are extended in space outside, Descartes proves by appeal to our sensations which, according to him, ought to be caused by the presence of these things. Things or bodies are substances, and their existence is extra-mental, they are not dependent on our thoughts. Thus, Descartes posits three existences or substances: God, mind and physical things or bodies. The mind and the physical bodies are different from each other, known only through their functions and properties; but both these are dependent on the supreme substance, God. Bodies are moved by God, for they have no capacity for independent motion; they are passive, inert. And their motion obeys the laws of mechanics. But Descartes does not think that God can interfere with the mechanistic scheme of the world. God, when He created the world, endowed it with a certain amount of motion and rest, and He confines Himself to the operation of matter within the limit prescribed by Himself originally. There cannot be increase in the amount of motion, though God could have made the world otherwise, if He liked, at the time of creation. Motion and rest, which are properties of matter, do not increase or decrease.

Mind, according to Descartes, is without the extension characteristic of bodies. It is absolutely different from bodies. The mind and the bodies are not dependent on each other, though both are dependent on God. There is a dualism between the mind and the bodies, and the latter are determined by the laws of mechanics. The mind is not a part of the physical world which consists of extended bodies. Descartes has no teleology about matter and its laws. No purpose or final cause determines the ways of the world. The mechanism of physical science reigns supreme in the world of matter. Even the human body, though organic in nature, is mechanical in its functions, and is moved by the heat generated in the heart. The body works not purposively but automatically like a machine.

The most curious part of the philosophy of Descartes is his view that the mind and the body do not interact in order to produce changes, and that their apparently mutual interaction is really the agreement between their functions due to their running parallel to each other like two well-adjusted clocks which show the same time, though they do not influence each other in any way. Descartes is not inclined to admit any dependence of body on mind or vice versa. The parallelism in the workings of the mind and body is attributed to the will of God, Who has made them in that way.

There is a radical difference between Descartes and the Vedanta in regard to the relations subsisting among God, world and soul. Descartes confuses between the mind and the soul and he seems to think that the mind is the soul or the inner self of a person. To the Vedanta, the soul is consciousness. The Supreme Soul, or God, is the absolute consciousness existing as the background of the activities of the mind. The individual soul, however, is the very same consciousness manifest through the medium of the mind and thus partaking of the temporal and fluctuating characteristics of the mind. The mind as such is inert, has no consciousness in it; it is merely a vehicle of individuality, and its consciousness is borrowed from the supreme Soul. The mind, therefore, cannot be the soul.

God is not cut off from the world and the souls; it is God that appears as the world and the souls. If God, as Descartes thinks, is different from the world and the souls, there can be no relation between Him and these, so that He cannot even set them to work parallelly and independently. By holding that God is other than the mind, Descartes would be stultifying his own position that God has impressed His idea on the mind of man. Consequently it would also follow from the dualism between God and man that man cannot have knowledge of God, cannot have even any kind of relations with Him. Sometimes Descartes finds himself forced to establish a causal relation between God and the world of individuals, in order to answer to the objection that no interaction would be possible between them if strict dualism or pluralism of substances is admitted. Descartes does not remove the discrepancy given rise to by his contradictory views that God is the only real substance and also that God, world and mind are three real substances. He creates great gulfs without trying to bridge them.

The theory of parallelism propounded by Descartes is opposed to facts of experience. The feelings and passions experienced by individuals prove the interaction between the mind and the body. The complex emotions that arise in the mind and the different sensations of hunger, pain, colour, sound, etc., cannot be exclusively the functions of the mind alone or the body alone; these are results of a mutual interaction between the mind and the body. According to the Vedanta, man is neither pure Spirit alone, nor pure mind alone, nor pure body alone. Man is a blend, together, of spirit, mind and body. The spiritual Self, the mind and the senses together constitute an individual. We are an organic whole, not merely divided parts, as Descartes thinks. The highest organism, however, is Ishvara, in Whom the world and the individuals are merged to form a wholeness of being. Ishvara's will is the supreme mover, director and organiser of all things. To the Vedanta, the world and the individuals are not realities independent of Ishvara, but appearances of Ishvara Himself. Ishvara is the only reality, Brahman viewed from the empirical standpoint. The individual, that is man, is a part of Ishvara, Who is the Inner Controller of all beings, and in essence man is inseparable from Ishvara. This identity is realised when the independent and distorted functioning of the will of man ceases and he allows his higher intelligence to rise to the infinite that is Ishvara. Ishvara's relation to the world and the individuals is something like the relation that the waking individual has to the dream world and to the individuals seen in dream. The differences among God, world and the individuals are therefore mere makeshifts of the empirical consciousness, while in truth there is only one Being which is called God in relation to the empirical world and the individuals, and the Absolute, in itself. The trinity of substances in Descartes militates against reason and fails to accord with experience.

Descartes raises a point which is not in disagreement with the Vedanta when he says that God does not interfere with the workings of the world which He Himself has determined when creating the world. In the Vedanta, Ishvara does not so much create the world as make possible the manifestation of the unmanifested potencies of the unliberated individuals which lay dormant at the time of the dissolution of the world during the previous cycle. All that happens in this world is in perfect accordance with the will of Ishvara and hence the view that He does not change the present scheme of the world into something else is no denying of His omnipotence. He is all-powerful. He can make the world other than what it is; only there is no need for His doing so. There is no reason why He should interfere with the movements of the world when he Himself has willed them to be such.

But the explanation of the world by the laws of mechanics offered by Descartes is in no way tenable according to the Vedanta. The world appears to work in strict obedience to the laws of mechanics, because our ways of looking at it are limited to the operation of the space-time phenomenon, which makes us feel that causation is a straight-line process of the temporal precedence of the cause in the production of effects. But the truth is otherwise. The world of matter is not segregated from man or God. An organic unity cannot be explained by mechanical laws even as the functions of the human body cannot be subjected entirely to the mathematical laws of physics. A higher vision and understanding disclose the fact that there is a supreme end towards which the apparently mechanical operations of matter tend, that the movements of the world are purposive, and that the fulfilment of the phenomena of the world and the individuals is in the final realisation of the Absolute. Mechanics is what is seen from a surface-view; as its implication hidden behind sense-perception is the great truth that all change and motion is a yearning to unfold within oneself the reality that is immutable.