The modern era of Western philosophy is traditionally marked by a shift from the dogmatic authority of the Church and Scholasticism toward reason and scientific inquiry. Rene Descartes (1596–1650) is widely regarded as the "Father of Modern Philosophy" because he sought to establish a firm, indubitable foundation for all human knowledge.
Descartes did not use doubt for the sake of being a skeptic. Instead, he used it as a tool to find something that could not possibly be doubted. This is known as Universal or Radical Doubt.
"Cogito, Ergo Sum" – I think, therefore I am.
Even if a demon is deceiving him, he must exist to be deceived. The act of doubting is an act of thinking, and thinking requires a thinker. This is the first indubitable truth of his philosophy.
Having established his own existence, Descartes examines the contents of his mind, which he calls Ideas. He classifies ideas based on their origin:
| Type of Idea | Source | Example |
|---|---|---|
| Innate Ideas | Born with the mind; placed by God. | God, Perfection, Infinity, Identity. |
| Adventitious Ideas | From external sensory experience. | Heat of the sun, sound of music. |
| Factitious Ideas | Invented by the imagination. | Chimeras, Sirens, Unicorns. |
Descartes argues that his idea of a Perfect Being (God) could not be factitious (he is imperfect) or adventitious (God isn't sensed). Therefore, it must be innate. Since the cause must have at least as much reality as the effect, only a perfect God could have placed the idea of perfection in a human mind.
Descartes defines substance as "a thing which exists in such a way as to depend on no other thing for its existence." Strictly speaking, only God fits this definition. However, in a relative sense, he identifies two created substances.
Descartes proposes that reality consists of two distinct, independent substances:
Q: Why is Descartes called a Rationalist?
A: Because he believes the foundation of knowledge is reason (the Cogito and Innate Ideas) rather than sensory experience.
Q: What is the "Cartesian Circle"?
A: Critics argue Descartes uses God to prove the reliability of clear/distinct ideas, but uses clear/distinct ideas to prove God exists—this is circular reasoning.
To remember the three types of ideas, use "I.A.F.":