Plato and RenT Descartes share a philosophical legacy that turns upon a notion of Rationalism. Each thinker, after his own fashion, assumes that human beings acquire knowledge of reality through the proper use of their faculties, aided by reason. It is in reasoning a priori that human beings can embark upon a process of ethical discovery; by beginning at sound first principles and subsequently proceeding, point-by-point, to logical conclusions, human beings will ultimately unearth objective truths about the universe. In this, these thinkers will disagree most profoundly with those that contend that there is no such thing as objective morality (such as the Sophists in Plato's time), and that all ethical conceptions of the Good are in fact largely arbitrary, and usually heavily contingent (modern day proponents of value pluralism rely on this observation). That Plato and Descartes are both rationalists, yet lived in eras separated by nearly two millennia is not altogether striking; the idea that the good lifeù and the components of proper human conductù can be discovered through reason is not an uncommon one. In fact, this is an idea that has persisted in the minds of many great thinkers throughout the ages, with mixed results.
It is useful, when examining Plato and Descartes, to liken Rationalism in philosophy with Rationalism in mathematics. If one happens upon an apple, one will see that it can be held and touched, and that it is, in a manner of speaking, representative of one unit "apple". This serves as a sound, rational starting point that lends itself to a series of other logical leaps. If, for example, one were then to happen upon another apple, one will likely use the same faculties to deduce that the original unit "apple" has now been augmented by an additional unit, in effect doubling one's quantity of apples. A mathematical truism now emerges: one-plus-one-equals-two. It is from these sound first principles...