Summary
Meditation I: Things that may be doubted
Descartes resolves to demolish opinions accepted without adequate foundations. Because the senses sometimes deceive, prudence advises against placing complete trust in them.
The dream argument challenges the certainty of present experience. Even if complex scenes are imagined, simpler elements such as extension, shape and number may remain. The deceiving-God and evil-genius hypotheses then extend doubt to mathematical reasoning and entrenched habit.
Meditation II: The human mind
Amid universal doubt, Descartes discovers that his own existence cannot be denied while he thinks: “I am, I exist” is necessarily true whenever it is entertained.
He cannot yet define himself through a body whose existence remains doubtful. He is immediately known as a thinking thing whose activities include doubting, understanding, willing, imagining and sensing as modes of consciousness.
The wax changes colour, smell, shape and texture when heated, yet is judged to remain wax. Its identity is understood through the mind’s judgement of an extended, flexible and changeable thing, not through one stable sensory image.
Meditation III: God exists
Descartes inventories ideas, volitions and judgements. Ideas considered merely as mental events are not false, but judgement may falsely represent them as resembling external things.
He distinguishes formal reality in things from objective reality represented in ideas. A cause must contain at least as much reality as its effect. The idea of an infinite and perfect being cannot be adequately caused by a finite, imperfect self.
God therefore exists as the source of the idea of infinity and as the sustaining cause of the meditator. Since deception indicates defect, a perfect God is not a deceiver.
Meditation IV: Truth and error
The non-deceiving God guarantees that a properly used cognitive faculty is oriented towards truth. Yet human error plainly occurs, so Descartes examines the relationship between understanding and will.
Understanding is limited: it presents only some matters clearly. Will can affirm or deny beyond those limits. Error arises when assent is given without clear perception.
Withholding judgement is therefore a positive use of freedom when evidence is inadequate. Error is a privation in the finite creature rather than a positive creation attributable to God.
Meditation V: Material essence and God
Before proving that bodies exist, Descartes considers what their essences would be. Mathematical properties of extension, shape, duration and motion appear clearly intelligible and possess necessary relations not invented arbitrarily by the thinker.
The ontological argument claims that necessary existence belongs inseparably to the essence of a supremely perfect being. God’s existence becomes certain when that essence is clearly and distinctly understood.
Knowledge of God secures the enduring reliability of clear and distinct demonstrations, including those not held continuously in attention but remembered as properly proved.
Meditation VI: Bodies and the mind-body distinction
Imagination requires an effort and presents images, whereas pure understanding can grasp figures without picturing every side. This difference suggests, without yet proving, that the mind is joined to a body.
Sensory ideas arrive without the will and seem to be caused externally. Because God is not a deceiver and supplies a strong natural inclination towards belief in bodies, material things exist, though they may not possess every quality exactly as sensation presents it.
Mind is essentially thinking and unextended; body is essentially extended and divisible. Since each can be clearly conceived without the other, they are really distinct.
Nevertheless, sensations such as hunger, thirst and pain show that mind and body form an intimate union. The mind is not merely a pilot observing a vessel but experiences bodily conditions directly.
Sensory error remains possible because bodily signals are designed for ordinary preservation rather than infallible theoretical knowledge. Descartes ends by distinguishing waking experience from dreams through coherent memory and causal connection.