René Descartes’ Meditations on First Philosophy (1641) is a treatise dedicated to the Theology Faculty of the University of Paris. Descartes says that the treatise has two objectives: (1) to show that the soul is immortal and that God exists, and (2) to show that these assertions are truths that are logically necessary and can be proved by reason.

In the synopsis at the beginning of the Meditations, Descartes explains that the First Meditation describes how it is possible to have an attitude of doubt about the nature of all things. The Second Meditation explains how the mind discovers that it must necessarily exist, and explains how the mind can have a concept of the soul. The Third Meditation provides a proof of the existence of God. The Fourth Meditation describes the difference between truth and error. The Fifth Meditation provides another proof for the existence of God, and the Sixth Meditation describes how knowledge of the human mind and how the mind can be guided by God leads to valid knowledge of material objects and the physical world.

In the First Meditation, Descartes says that there is nothing about which it is impossible to doubt. But he says that: ‘even if I doubt everything, I cannot doubt that I am doubting.’ Thus, ‘I cannot doubt that I exist.’

For Descartes, the essence of human existence is that ‘I am a thinking thing.’ To think is to exist. To be a human being is to be capable of thinking. Furthermore, ‘my consciousness of my own thinking means that I am conscious of my own existence.’

Descartes claims that the idea of something that exists must be caused by something that exists. Something that exists cannot be caused by something that does not exist. Something which is merely potentially existent cannot be a cause of the existence of anything. A thing or being must have some actual or formal existence in order to cause another thing or being to exist.

Descartes also claims that for any relation of cause and effect to occur between events or things, the cause must have at least as much reality as the effect. A cause cannot be less real than its effect, because the cause is what gives reality to the effect. Thus, there must be a First Cause from which all things derive their reality. The First Cause is that from which all things derive their reality.

According to Descartes, the more perfect something is, the more real it is. The cause of an effect must be at least as perfect as the effect. An effect may be less perfect than its cause, but a cause cannot be less perfect than its effect.

Descartes explains that the cause of an idea must be at least as perfect as the idea. An idea cannot be caused by something that is less perfect than itself. If the idea of a perfect being occurs in an imperfect being, then the idea must derive its reality from a perfect being.

Descartes also explains that the idea of perfect being is derived from God. God is perfect Being, from which all things derive their reality.

In the Third Meditation, Descartes offers a proof for the existence of God, arguing that the idea of God as Infinite Being could not occur in the finite mind of a human being unless God really existed. The idea of God as Infinite Being is an innate idea in the human mind, an idea that cannot be created by any finite being. This perfect idea can only be created by God.

Descartes says that in God perfection is actually existent, rather than potentially existent. The idea of God cannot be caused by something that is merely potentially existent, but only by an actually existing reality.

Descartes also says that God is Absolute Being. Nothingness is Non-Being. Reality depends for its being on God. Truth is the degree to which an idea corresponds to reality, and error is the degree to which an idea does not correspond to reality.

Descartes explains that in order to determine the truth of an idea, we must determine to what degree the idea corresponds to reality. Human susceptibility to error is caused by the fact that we, as human beings, do not have an unlimited ability to recognize the truth, and the fact that we are free to choose truth or falsehood.

According to Descartes, God is perfect and therefore is not the cause of any error. When we think of God, we find no cause of error or falsehood. The reason we doubt the truth is that we are incomplete in our ability to recognize the truth. We depend for our existence on God, who is complete and independent.

In the Fifth Meditation, Descartes gives another proof of the existence of God. Descartes argues that existence cannot be separated from the essence of God. ‘I cannot think of God as not actually existing...I cannot think of God other than as existing...I cannot say that God does not exist if I am thinking about God.’ God has all perfections, including the idea of every perfection. Perfection of existence is found in God. Thus, ‘I can be certain of the knowledge that God exists, because existence is a perfection that belongs to God.’

For Descartes, the essence of human existence is found in the activity of thinking. Thinking includes understanding, doubting, affirming, denying, perceiving, imagining, willing, refusing, and other acts of the intellect. The nature of the human mind is that it is a thinking thing.

Descartes says that ideas may be innate, adventitious, or factitious. Innate ideas are ideas that are not dependent on our perceptions or on our own will. Innate ideas are inherently present in the reasoning of the mind. Adventitious ideas are ideas derived from our experience of the world. Factitious ideas are ideas that may be illusory or invented by the imagination.

Descartes also says that all innate ideas are clear and distinct concepts of reality. Adventitious or factitious ideas, however, may be unclear and indistinct.

He explains that an idea may be clear and distinct insofar as it sufficiently and accurately represents reality. An idea may be clear without being distinct. However, any idea which is distinct is also clear.

In the Fifth Meditation, he also explains that the idea of God as an infinite, eternal, all-knowing, all-powerful Being is an innate idea, which has more objective reality than the ideas by which finite substances are represented in the mind.

In the Sixth Meditation, having explained that innate ideas or self-evident truths that can be known by reason, he describes how we can know that material things exist in the physical world. God can produce everything in the world exactly as we perceive it. When we have clear and distinct ideas about the world, we can know true reality.

Descartes says that if we agree that God exists and that all things depend on God, then we can no longer doubt that we can have true and certain knowledge of material things. Knowledge of the truth of things may depend on knowledge of God. The more we know God, the more we can know the truth of things.

According to Descartes, materiality is characterized by such qualities as quantity, continuity, extension, length, breadth, depth, size, shape, position, motion, diversity, and duration. Material objects can be thought of as having many parts.

Because the mind is a thinking and unextended thing, and because the body is an extended and unthinking thing, it is clear that the mind is distinct from the body. The mind is indivisible, but the body consists of many parts. And because the mind is distinct from the body, the destruction of the body does not necessarily mean the destruction of the mind. Thus, in Descartes' view, the mind or soul is immortal.

A criticism that may be made of his claim that there is an innate idea of God in the human mind is that it implies that all human beings have the same idea of God. This implication is false and does not explain why such a wide range of beliefs about God are held by human beings.

A criticism that may be made of how he explains that knowledge is possible of material things is that his assertion that the world may be created by God exactly as we perceive it does not prove that the world is in fact created exactly as we perceive it.

Descartes says that we can know true reality when we have clear and distinct ideas. Clear and distinct ideas prove the existence of God. But the existence of God proves the validity of clear and distinct ideas. This is a circular argument.

He also says that we cannot be deceived in our perceptions by God, but he admits that our own perceptual or intellectual limitations can cause us to be deceived by the appearances of material things.

Descartes’ subjectivism, method of systematic doubt, and position of metaphysical dualism regarding mind and body were an important influence on the development of philosophy.