Breaking down Descartes’ argument for the existence of God (and finding out if an evil demon is deceiving us)

René Descartes (1596–1650) was a French philosopher and mathematician best known for the phrase “I think, therefore I am,” a famous proclamation in modern Western thought. His philosophy, known as Cartesianism, has shaped intellectual discourse beyond those who share his conclusions. The Cartesian revolution of the 17th century remains influential today, and Descartes is regarded as one of the three great rationalists — alongside Gottfried Wilhelm Leibniz and Baruch Spinoza — emphasizing reason as the foundation of knowledge. This commitment to rationality is evident in his Meditations on First Philosophy, where he seeks to establish the existence of God through methodical doubt and logical proof.

Descartes’ Meditations on First Philosophy is divided into six meditations, each written after a day of contemplation. The structure invites the reader to follow along, reading one meditation per day over the course of six days. Each meditation builds on the last, so we can trace how Descartes’ thought develops step by step — from radical doubt to the proof of God’s existence. Substance dualism, the view that the mind and body are fundamentally distinct substances, will be the focus here.

This blog post will analyze each of Descartes’ six meditations, explaining the core arguments. I will use select passages from each meditation to highlight each argument.

First Meditation

I will therefore suppose that, not God, who is perfectly good and the source of truth, but some evil spirit, supremely powerful and cunning, has devoted all his efforts to deceiving me. (First Meditation)

In the First Meditation, Descartes begins by questioning the reliability of the senses. His starting move is the dream argument: in dreams we often experience things as vividly as if they were real, and there is no certain test to distinguish dreaming from waking. Think of nightmares that feel eerily real, or the times we confuse something we dreamed with something that actually happened. If our senses can deceive us in this way, then they cannot serve as an indubitable foundation for knowledge.

This line of thought culminates in what’s known as the evil demon hypothesis. Descartes asks us to imagine a powerful deceiver who tricks us into believing that everything we see, hear, and feel is real, when in fact it might all be an illusion. If such deception is possible, then all of our beliefs about the world must be treated with doubt.

Second Meditation

So that, having weighed all these considerations sufficiently and more than sufficiently, I can finally decide that this proposition, ‘I am, I exist’, whenever it is uttered by me, or conceived in the mind, is necessarily true. (Second Meditation)

In the Second Meditation, Descartes builds directly on the doubts raised in the first. Since the senses cannot be trusted, he asks: what, if anything, is beyond doubt? He finds that the act of doubting must presuppose something to exist in order to doubt. This leads to his most famous conclusion: “I think, therefore I am.” The “I” here is not the body or related to the senses. It is a thinking thing.

To test what this “I” really is, Descartes turns to the example of wax. When wax melts, its color, texture, and smell all change. Yet we still recognize it as the same piece of wax. That recognition does not come from the senses — which only report shifting qualities — but from the mind’s judgment. Think of it this way: even if a brain were sitting in a vat, wired up with electrical impulses to produce the illusion of reality, there would still be a brain in that vat — a self that exists.

From this, Descartes concludes that the mind should be treated with more philosophical seriousness than the body. The body, as an extended thing, could still be an illusion, but the mind as a thinking thing cannot be doubted.

Third Meditation

By the name ‘God’ I understand an infinite, independent, supremely intelligent, supremely powerful substance, by which I myself and whatever else exists (if anything else does exist) was created. But certainly, all these properties are such that, the more carefully I consider them, the less it seems possible that they can be derived from me alone. And so I must conclude that it necessarily follows from all that has been said up to now that God exists. (Third Meditation)

In the Third Meditation, Descartes considers different kinds of thoughts; as we saw earlier, there were thoughts of himself as a thinking thing, thoughts of wax and color, and so on. But these ideas could all be fabricated, like hallucinations or illusions created by an evil demon, so they cannot serve as a secure foundation.

The idea of God, however, is unique. God is conceived as perfect, infinite, and all-powerful. Descartes recognizes that he himself is limited, finite, and prone to doubt — so how could the idea of perfection and infinity come from him? Since Descartes lacks those qualities, he argues that the idea of God cannot have originated in his own imperfect mind.

From this, Descartes concludes that the only possible source of the idea of God is God himself. Only something infinite and perfect could be the cause of Descartes’ finite and imperfect mind thinking of something infinite and perfect. Therefore, God must exist. And because deception would imply a flaw, Descartes reasons that God cannot be a deceiver.

Fourth Meditation

So what is the origin of my errors? It can only be this: that, since the range of the will is greater than that of the intellect, I do not confine it within the same limits, but extend it even to matters I do not understand; and since it is indifferent to these, it easily falls away from the true and the good, and this is both how I come to be deceived and how I come to sin. (Fourth Meditation)

In the Fourth Meditation, Descartes questions where error comes from. He argues that God gave him two particular powers. The first is intellect which is the ability to understand and conceptualize things. The second is will which is the ability to choose and judge things. However, the intellect is fundamentally limited because it is finite. Descartes recognizes that he cannot know everything. Yet, will itself is unlimited because he can choose to believe in anything — including things he does not fully understand.

If error cannot come from God — because God is all perfect — then error must come from Descartes himself. Error must result from when Descartes makes judgments about things that he does not fully understand. To solve for this problem, Descartes argues that self-discipline is necessary: he should only make judgments about things that his intellect presents clearly. Here, there is no evil demon deceiving Descartes because God himself is without flaw, so the existence of an evil demon would be erroneous because it would be incapable of undermining God’s truthfulness. Thus, there is no external demon that deceives Descartes — it is Descartes himself.

Fifth Meditation

For instance, when I imagine a triangle, even if perhaps such a figure does not exist, and has never existed, anywhere at all outside my thought, it nonetheless certainly has a determinate nature, or essence, or form, that is immutable and eternal, which was not invented by me, and does not depend on my mind. (Fifth Meditation)

In the Fifth Meditation, Descartes begins by examining the properties of a triangle. Even if no triangle were to exist in the physical world, its essential properties remain true: the angles always add up to 180 degrees, the hypotenuse is opposite the largest angle, and so forth. These truths are not inventions of his own mind, but necessary and eternal features of the triangle’s essence. In this way, Descartes shows that certain mathematical truths are undeniable and hold independently of human perception.

When applying this logic to the existence of God, Descartes writes:

And I clearly and distinctly understand that eternal existence belongs to his nature — just as clearly and distinctly as I understand that the properties I can demonstrate of some shape or number belong in fact to the nature of that shape or number. (Fifth Meditation)

Just as the essence of a triangle necessarily includes certain properties, Descartes argues that the essence of God necessarily includes existence. If God is perfect, then existence itself must belong to perfection; a being lacking existence would not be supremely perfect. This is Descartes’ ontological argument: existence cannot be separated from God’s essence. From this, he concludes that because God is perfect, he cannot be a deceiver. Therefore, whatever we clearly and distinctly perceive must be true, since God ensures that our rational faculties align with reality. Error arises only when we misuse our will by affirming things that are not distinctly understood. On this basis, Descartes secures the certainty of mathematics, geometry, and other rational sciences.

Sixth Meditation

And although perhaps (or rather certainly, as I shall shortly claim) I have a body, which is very closely conjoined to me, yet because, on the one hand, I have a clear and distinct idea of myself, in so far as I am a thinking and not an extended thing, and, on the other, a distinct idea of the body, in so far as it is only an extended and not a thinking thing, it is certain that I am really distinct from my body, and can exist without it. (Sixth Meditation)

In the Sixth Meditation, Descartes reaffirms that the cogito — the thinking thing — can be conceived independently of the body. Even if the biophysical body does not exist, the cogito would still endure as a thinking substance. Yet in this final meditation, Descartes also concludes that the body necessarily exists. This proof of embodiment relies on his analysis of God: since God is infinite, perfect, and incapable of deception, the bodily sensations we experience cannot be illusions without cause; they must arise from something real. Thus, Descartes recognizes a genuine union between mind and body. At the same time, he continues to maintain they are two distinct substances, joined but not identical.

Concluding Remarks

Cartesian thought has left an enduring mark on the Western canon. Yet the central question remains: was Descartes right in his analysis of substance dualism? Substance monist thinkers like Spinoza and Parmenides have offered competing accounts. The question of which view is correct continues to animate philosophy.