Selections from the Principles of Philosophy (1644) Informative Summary


This eBook presents excerpts from René Descartes’ “Principles of Philosophy,” originally published in 1644. The selections focus on Descartes’ method of seeking truth, his arguments for the existence of God, and his understanding of the mind-body distinction. Descartes, considered the father of modern philosophy, introduces his renowned method of doubt, advocating for a systematic questioning of all pre-conceived notions and sensory experiences to arrive at foundational truths. He asserts that the only indubitable truth is “I think, therefore I am,” and builds his philosophy from this bedrock.

Descartes meticulously dissects the nature of substance, distinguishing between thinking substance (mind) and extended substance (body), and argues that both are creations of God, whose existence he strives to prove through several arguments. He emphasizes the clarity and distinctness of ideas as criteria for truth, urging readers to rely on their rational faculties rather than sensory experiences or preconceived opinions. Descartes also explores the nature of space, motion, and the material world, providing a mechanical explanation for natural phenomena.

Key Findings:

  • The first and most certain knowledge is “I think, therefore I am.”
  • God exists, and can be proven through the idea of his perfection.
  • The mind and body are distinct substances.
  • True knowledge comes from clear and distinct ideas perceived by the intellect.
  • Sensory perceptions can be misleading and should be scrutinized.


  • Methodical Doubt: The reader will learn about Descartes’ method of doubt, which encourages a systematic questioning of all beliefs until arriving at indubitable truths. This method forms the foundation of his philosophical system.
  • Mind-Body Dualism: The text elucidates Descartes’ concept of mind-body dualism, where mind and body are distinct substances. The mind, characterized by thought, is non-physical, while the body is characterized by extension. This idea has had a lasting impact on Western philosophy.
  • Proofs for God’s Existence: Descartes puts forth several arguments for the existence of God, including the argument from the idea of perfection and the argument from necessary existence. These arguments aim to establish a firm foundation for knowledge and certainty.
  • Nature of Material Things: The reader will learn about Descartes’ view of the material world as a mechanical system governed by laws of motion and figure. He proposes that natural phenomena can be explained through the interactions of material particles.

Historical Context:

Descartes’ “Principles of Philosophy” was written in a period marked by intellectual ferment and a questioning of traditional authorities. The Scientific Revolution was underway, challenging Aristotelian views of the natural world. Descartes’ work contributed to this intellectual shift by emphasizing the power of human reason and promoting a mechanistic understanding of the universe.


  1. Thinking is the essence of the mind: This is true because the mind, even when doubting everything else, cannot doubt its own existence as a thinking thing.
  2. God is not a deceiver: This is true because deception arises from imperfection, and God is perfect.
  3. The will is more extensive than the understanding: This is true because the will can extend to an infinite number of possibilities, while the understanding is limited by what it can clearly perceive.
  4. Error arises from judging what is not clearly perceived: This is true because when we have a clear and distinct understanding of something, we cannot be mistaken about it.
  5. There is a real distinction between mind and body: This is true because God has the power to separate them, even if they are intimately united.
  6. The nature of body consists in extension: This is true because all other properties of bodies, such as hardness, weight, and color, can be removed without the body ceasing to exist.
  7. Space is not distinct from body: This is true because space is simply the extension of body, conceived in a general way.
  8. There cannot be a vacuum in the philosophical sense: This is true because space is identical with extended substance, and there cannot be extension without something extended.
  9. Matter is infinitely divisible: This is true because any extended thing can be divided in thought, and what can be divided in thought is divisible in reality.
  10. There is only one kind of matter in the universe: This is true because the nature of matter consists simply in extension, and all imaginable spaces are already filled with extended substance.
  11. All the variety of matter depends on motion: This is true because the only way matter can be differentiated is through the movement and arrangement of its parts.
  12. Motion is the transporting of a body from one place to another: This is true because motion is defined by the change in a body’s position relative to other bodies.
  13. There are no atoms: This is true because any extended particle, no matter how small, can always be divided in thought, and God could always divide it in reality.
  14. The world is indefinitely extended: This is true because we can always imagine spaces beyond any given limit, and these spaces must contain extended substance.
  15. The earth and heavens are made of the same matter: This is true because there is only one kind of matter, which is extended substance.
  16. Sensible qualities exist as dispositions of bodies to move our nerves: This is true because we only know about external objects through the effects they have on our senses, and these effects are produced by local motion.
  17. The soul perceives only in the brain: This is true because damage to the brain can affect our senses, even if the sense organs themselves are intact.
  18. The soul is capable of perceiving a variety of sensations from bodily motions: This is true because we see that things like words and music can excite a wide range of thoughts and emotions in us.
  19. We know nothing of external objects beyond their figure, magnitude, and motion: This is true because these are the only properties of bodies that we can clearly and distinctly perceive.
  20. Sensible bodies are composed of insensible particles: This is true because we can observe bodies increasing and decreasing in size gradually, even though we cannot perceive the individual particles that are added or removed.


The text does not contain statistics in the conventional sense of numerical data points.


  1. Cogitatio (Thought): Any mental activity of which we are immediately aware, including understanding, willing, imagining, and sensing.
  2. Substance: A thing that exists in such a way that it needs nothing beyond itself in order to exist. God is the only truly independent substance.
  3. Attribute: A fundamental property of a substance that constitutes its essence and upon which all other properties depend.
  4. Mode: A modification or variation of a substance, a way in which a substance can be affected.
  5. Extension: The fundamental attribute of corporeal substance, consisting in length, breadth, and depth.
  6. Space (Internal Place): Identical with the extension of body, conceived in a general way.
  7. Vacuum: A space in which there is no substance. Descartes argues that a true vacuum is impossible.
  8. Motion (Local Motion): The transporting of a body from one place to another.
  9. Sensation (Sensus): A confused thought excited in the mind by the motion of the nerves.
  10. Moral Certainty: A level of certainty sufficient for practical purposes, but which does not exclude the possibility of error.


  1. The Wax Example: In the Meditations, Descartes uses the example of a piece of wax to demonstrate that the essence of body is not found in its sensible qualities, but in its extension.
  2. Phantom Limb Pain: The example of a girl experiencing pain in her amputated hand is used to demonstrate that the soul perceives only in the brain, not in individual body parts.
  3. The Ship at Sea: The example of a person sitting on a ship that is sailing is used to illustrate the relativity of motion.
  4. The Sponge: The example of a sponge expanding and contracting when soaked with water is used to explain the concept of rarefaction.
  5. Clocks with Different Mechanisms: Descartes uses the example of a clockmaker building two clocks with different mechanisms to demonstrate that God may have created the world using means that are unknown to us.
  6. Cipher Example: The example of deciphering a coded message by substituting letters alphabetically is used to illustrate the concept of moral certainty.
  7. Tree Growth: The example of a tree growing is used to demonstrate the existence of insensible particles that are too small to be perceived.
  8. The Sword Cut: The example of a sword cutting the skin is used to illustrate that bodily motions can cause sensations that are entirely different from the motions themselves.
  9. Sparks from a Blow to the Eye: The example of seeing sparks when struck in the eye is used to show that sensations can be caused by local motion alone.
  10. Humming Sound in the Ear: The example of hearing a humming sound when plugging one’s ear is used to illustrate that sound is caused by the motion of air.


Descartes’ “Principles of Philosophy” offers a groundbreaking philosophical system that emphasizes the power of human reason and provides a mechanistic understanding of the material world. His method of doubt, arguments for God’s existence, and the mind-body distinction remain influential concepts in Western thought. By carefully analyzing clear and distinct ideas, readers are encouraged to utilize their own rational faculties to discover truth and gain a deeper understanding of the world around them.

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