Rationalism: Some Similarities Between Plato and Descartes

Plato

Descartes

Justification is by reason rather than by the senses. (We need to turn away from the world of the cave, which we find out about by sensory experience, and toward to world outside the cave, the world of Forms, which we discover by means of reason) Ditto. The skeptical arguments of the first meditation show that the senses cannot be trusted. Later meditations suggest that a scientific picture of the world will not appeal to sensory properties but to (primarily) mathematical ones.
The objects of knowledge, namely the Forms, are eternal, necessary, and unchanging (we want to find the permanent order that underlies the flux) We can have knowledge of the physical world. But the most basic objects of knowledge are general principles (e.g. the basic laws of physics), so the goal is still to penetrate behind the veil of appearance.
The most important and basic knowledge is a priori (that is, not based on sensory information): this is true of knowledge of mathematics, of goodness, of justice, etc. We can gain some knowledge by means of the senses, but only after establishing a priori that they are more or less trustworthy; the most basic knowledge is a priori.
Mathematics is a kind of model for the rest of knowledge. (The easiest way to think of the Forms is to think of such mathematical objects as the perfect circle.) Ditto: the metaphor of building knowledge up on firm foundations relies on a mathematical model (the foundations are like the axioms of a mathematical system; building up the rest of knowledge is like proving theorems)


Last update: March 17, 2001.
Curtis Brown  |  Introduction to Philosophy  |  Philosophy Department  |  Trinity University
cbrown@trinity.edu