Rene Descartes is rightly called the father of modern philosophy. In him the modern tendency to free philosophy from the aesthetic interests of the ancient Greeks and the theological bent of the medieval scholastics, and to rest it mainly on scientific and rational foundations, took its origin. Descartes recognised that the principles of philosophy should be based on self-evident truths, which are certain for all time and free from doubt and dispute, even as the axioms of mathematics are, from which we can correctly deduce all other truths in a logical order, provided we do not go wrong in our calculation and reasoning. His problem was to find out such a self-evident principle on which to base all further discovery and research.

Descartes began with doubt. He found that we cannot trust sense-experience, for it often deceives us and it is hard to assure ourselves of the reality of things which appear to correspond to our sensations. We cannot even be sure of the reality of our own bodies; perhaps we are dreaming that we have bodies; perhaps we are dreaming that we are seeing objects outside. How can we know whether we are waking or dreaming? We may be entirely mistaken in believing what we see. Perhaps the world is only in the mind, in imagination. It may be just an illusion produced by thought. Everything may be doubtful, even mathematical truths. The only certainty seems to be that there is nothing certain!

Now comes the stroke of genius in Descartes. He discovered that though all things may be doubtful, the fact that we doubt is itself not doubtful. The basis of doubt cannot be doubted. There is doubt, thinking; this is certain. And so the existence of the doubter or the thinker, too, must be certain. 'Cogito, ergo sum' concluded Descartes. "I think, therefore, I am." From the fact of thinking it is to be concluded that the thinker is a spiritual being; I am, and I must be essentially spiritual in nature. This knowledge is the only certain one, and it does not come from sense-perception or imagination. Here is the self-evident rational basis for all deduction in philosophy. This is a universal and necessary proposition.

In the Vedanta we have a reversal of this process of deduction followed by Descartes. The former deduces the thought from the thinker and not the thinker from the thought. Instead of saying "I think, therefore I am", it would say "I am, therefore I think." The Self, to the Vedanta, is prior to the act of thinking. What is indubitable and self-evident is not the fact that we think, but that we are. The awareness of the existence of one's own self is not deduced from thinking or doubting. It is the only self-evident truth beyond all proofs, it being the source of all proofs. As the famous dictum of Shankara goes, "no one doubts his own Self", and this is not the result of a chain of reasoning or a deduction from a process of empirical functioning of thought. In short, to the Vedanta, the highest Consciousness is of the Self, and this Consciousness is identical with Existence. We cannot make a distinction between 'sat' and 'chit', Existence and its Consciousness. The experience of the world through the senses and the mind, the various processes of thinking and the different implications of this experience are all offshoots of the consciousness of the Self. Thought does not precede the thinker; the thinker precedes the thought and the consciousness of the thinker precedes the fact of his being a thinker.

Descartes makes his 'Cogito ergo sum' the starting point of his proof for the existence of God. 'I think, therefore, I am' is an undisputed truth. Thought must exist, for the thinker exists. Well; now, take the thought or the idea of God which arises in the mind. Naturally, as every effect has a cause, the idea of God must have a cause, a basis. It is also known that the cause cannot be less than the effect; if there is any value or reality in the effect, it must be present in the cause, also. For, nothing can come from nothing; this, too, is a self-evident truth. The effect cannot have, therefore, a greater reality than the cause. So the idea of God, which is of an infinite Being, cannot arise from me, a finite being. This idea of the infinite must therefore be due to the existence of an infinite cause thereof, which must have placed this idea in me. This infinite existence which is responsible for the rise of the idea of the infinite in me is God. Thus, the existence of God is proved.

Descartes could have as well argued this out better in the following manner: The idea of the infinite is in my mind; it has arisen from my mind. But I am a finite being; how, then, has this happened that a greater effect has arisen from a smaller cause? This cannot be, for the cause is always at least as great as the effect. But it is also true that the idea of the infinite has arisen in my mind, it does not come from some other's mind. And my idea is real, it exists, for I, who am its cause, exist as a reality,—my existence cannot be doubted, 'Cogito ergo sum'. Hence, if my idea of the infinite exists, and if it must have a cause, and if I am its cause, and also if the cause is not less than its effect, I must be an infinite being. Infinite Being must be God, for there cannot be two infinite beings. Thus, the proof for the existence of God would, at the same time, be proof of my identity with God. This reasoning, had he but followed it, would have taken Descartes nearer to the truth of the Advaita Vedanta, that the individual soul is essentially one with the Absolute. But this Descartes did not do; he left his self in its finite individual state. This self of Descartes, therefore, is different from the Atman of the Vedanta.

The argument is also justified by its moral aspect, which makes the infinite the goal of the moral urge within us, which ever makes us strive to reach it, to become perfect, and to discover the implication of its existence in the very feeling and acceptance of our being finite and imperfect. Descartes resorts to this aspect of the argument and says that a reference to the infinite and perfect being is necessarily included in the recognition of one's finitude and imperfection. I know that I am finite; therefore the infinite exists. The knowledge of limits points to what is beyond limits.

This conclusion of Descartes is quite acceptable to the Vedanta, for whom Mumukshutva, or the longing for liberation from finitude, which arises in the self, can have meaning only when the Infinite exists. But to it, the proof for the existence of the Infinite consists not in mere logical deduction, but in an inner persuasion and conviction independent of reasoning, an essentially moral urge, though the aspirant may later on, for his satisfaction and strength, seek to justify the validity of this inner call by resort to reasoning. What the Vedanta calls viveka, or the discrimination between the real and the unreal, which is the fundamental requisite of a spiritual aspirant, is a rational conviction of a higher order, in which the moral urge directing the aspirant to the Eternal is necessarily implied as an invariable concomitant.

Descartes comes to the conclusion, from the nature of the perfection that God is, that God is the ultimate causeless cause, by an argument akin to that of Aristotle. Only the admission of such a God can avoid an infinite regress in our search for an ultimate cause. Descartes holds that the innate idea of God that rises in the mind is sufficient proof of God's having made man in His own image. God's existence is the precondition of the existence of all other things, including the individual souls, and also of His idea in the human mind. There cannot be an idea of God without the existence of God. God is incorporeal, intelligence, all-knowing, good and just. He is omnipotent, eternal. He has no changes, no modes of attribute, no modifications. As a deeply religious man, Descartes regards reason as valid only when it does not conflict with authority. This is the position of the Vedanta, too, for which unaided reason is more a hindrance to success in spiritual pursuits than a safe guide. The value of reason rests on its conforming to sruti, or intuitional revelation.