There were important post-medieval philosophers before Descartes—Renaissance thinkers and early Protestant Reformation scholars who played significant roles in shaping modern thought. However, Descartes, in the early 17th century, made such a definitive break with medieval philosophy and established patterns that have preoccupied philosophers ever since that he is widely regarded as the starting point of modern philosophy.
Descartes epitomized the 17th-century philosopher: a mathematician who made crucial contributions to analytic geometry, a scientist, and a philosopher, all at once. Educated by Jesuits and trained as an army engineer, he traveled extensively, particularly through Paris, the intellectual center of Europe at the time. There, he became one of the key figures among a generation of scientist-philosophers—including Gassendi, Hobbes, and Mersenne—who challenged Scholasticism in light of the new Copernican-Galilean science. Indeed, Descartes’ most famous book was published with commentary from several of these thinkers. Today, we will examine the arguments of that influential work, Meditations on First Philosophy. We will explore how he begins with radical doubt, discovers what he believes to be indubitable truth, and attempts to prove the existence of God and the external world. More importantly, we will see how his method permanently altered the course of philosophy.
The fundamental question Descartes seeks to answer is: What can we know with certainty? On one side, we have the Scholastic tradition, encompassing the thought of Aristotle and medieval Christian philosophers; on the other, we have the emerging Galilean-Copernican science. How can we determine which is correct? To address this, Descartes sets out to establish a new foundation for knowledge—what some call an Archimedean point. While the term was not his own, it refers to Archimedes' assertion that, given the right place to stand and a lever long enough, he could move the Earth. Similarly, philosophers have long sought an unshakable foundation for knowledge, and none pursued this idea more vigorously than Descartes. His goal is to identify a fundamental certainty upon which all further knowledge can be securely built.
To achieve this, he adopts a method of radical doubt: systematically questioning every belief to determine if any are beyond doubt. Sitting in his study, he categorizes his beliefs and scrutinizes each category for absolute certainty. His standard is exceptionally rigorous—he seeks knowledge that is entirely immune to doubt.
First, he examines sensory perception. He asks himself, Are my senses always reliable, or can they be deceived? The answer is immediate: our senses can sometimes mislead us. For instance, a stick partially submerged in water appears bent, though it is not. If our senses deceive us in some cases, how can we be certain they are not misleading us in others? Thus, Descartes concludes that all sensory-based knowledge is, in principle, dubitable. Importantly, he is not claiming that everything perceived through the senses is false, only that it cannot meet his criterion of absolute certainty.
Next, he considers general truths about the world, such as Fire is hot or Objects fall under gravity. Even if specific sensory perceptions are unreliable, surely these general facts must be certain. However, he then realizes an even deeper source of doubt: What if he is dreaming? After all, he has often had vivid dreams in which he believed he was experiencing reality. If he could be dreaming at any given moment, how can he trust any knowledge derived from experience?
Finally, Descartes turns to logic and mathematics, which have traditionally been considered immune to doubt. Statements such as All triangles have three sides and 2 + 2 = 4 seem necessarily true. Yet he introduces another disturbing possibility: What if an all-powerful evil genius is deceiving him, implanting false mathematical beliefs into his mind? In modern terms, we might ask: How do we know we are not in The Matrix? If such a deceiver existed, could we trust even the most fundamental logical truths?
At this point, Descartes finds himself doubting everything—mathematics, physics, history, memory, and sensory experience. But amid this radical skepticism, he discovers one truth that cannot be doubted: Cogito, ergo sum—I think, therefore I exist. In Meditations, he phrases it slightly differently: I think, I exist. His reasoning is that even if he is deceived about everything, the very act of doubting proves the existence of a thinking mind. If he doubts, he must be doubting. Therefore, he exists as a thinking entity.
From this foundational certainty, he derives further indubitable claims. First, he must exist as a res cogitans—a thinking, non-physical substance. This does not imply that his body or brain exists; only his mind is certain. Furthermore, while he can doubt the existence of the external world, he cannot doubt that he perceives certain ideas in his mind, even if they do not correspond to reality. For instance, while he may doubt that an actual lectern exists, he cannot doubt that he has the perception of a lectern.
Descartes then establishes a key epistemological principle: ideas that are clear and distinct must be true. This clarity and distinctness, he argues, is guaranteed by the natural light of reason—the same intuitive certainty by which we recognize that 2 = 2.
Moving forward, Descartes seeks to prove the existence of God. His argument hinges on the idea that the cause of an idea must be at least as great as its effect. He observes that he possesses the idea of a perfect, infinite being (God), yet nothing in his experience could have produced such an idea, since all his experiences are of finite, imperfect things. The only possible source of this idea is an actual infinite being—therefore, God must exist.
His second proof of God’s existence is a variation of the ontological argument from St. Anselm. He argues that God, by definition, is a being with all perfections, and existence is a perfection—therefore, God must exist.
Having established God’s existence, Descartes then proves the existence of the material world. He reasons that God, being perfect, would not deceive him about everything. Since he has a strong, innate inclination to believe in the existence of the physical world, and God is not a deceiver, matter must exist.
This leads to Descartes’ dualistic metaphysical framework. Reality consists of:
Mind and body, he asserts, are entirely distinct. The mind is a non-spatial thinking entity, whereas the body (including the brain) is purely physical. This distinction leads him to claim that the soul is immortal, as it has no physical parts and thus cannot decay. Furthermore, Descartes assigns science the task of studying physical reality, while ethics and religion pertain to the domain of the mind and soul.
However, Descartes’ system leaves unresolved questions. One major issue is mind-body interaction: If mind and body are fundamentally different substances, how can they influence each other? If mental events do not occupy space, how can they cause physical actions, and vice versa?
Despite these challenges, Descartes profoundly shaped modern philosophy. His foundationalist approach—seeking a bedrock certainty to refute skepticism—defined philosophical inquiry for centuries. His strict dualism influenced debates on consciousness, science, and religion, and his method of doubt laid the groundwork for rationalist and empiricist traditions alike. Even today, we grapple with the legacy of his bold attempt to reconcile science, psychology, ethics, and religion.
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