Who were the ancient founders of rationalism and a priori knowledge?

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According to the philosophy of epistemology (the study of knowledge), there are two basic types of knowledge: a priori and a posteriori. A priori knowledge is that gained by virtue of the use of reason alone, without any sensory perceptions. A posteriori knowledge depends on sensory experiences.

Rationalism is the view that the principal source of knowledge is derived from the operation of the intellect alone. Rationalists posit that human beings can achieve knowledge of the world through reason alone.

In the sixth century BCE, Pythagoras was the first thinker to express rationalist insight through mathematics. He reasoned that the world is static and governed by mathematical laws.

The first of the thinkers to develop a rationalist-type philosophy in the ancient world was Parmenides in the fifth century BCE. He begins his reasoning process by the examination of what is and what is not. Parmenides suggests that since reason is superior to the senses, the acquisition of knowledge must start with self-evident truths. He claims sensory perceptions are only illusions because the world is constantly changing.

Based in large part on the positions held by Parmenides, the seventeenth-century philosopher René Descartes, the father of modern philosophy, developed his own theory of rationalism. Descartes favors the belief that a priori knowledge is more reliable than sensory perceptions. His fundamental precept is cogito ergo sum, which translates to “I think, therefore I am.” This is an expansion of Parmenides’ examination of what is and what is not.

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