Rene Descartes (1595-1650)

Rene Descartes was a famous French mathematician, scientist and philosopher. He was arguably the first major philosopher in the modern era to make a serious effort to defeat skepticism. His views about knowledge and certainty, as well as his views about the relationship between mind and body have been very influential over the last three centuries.
Descartes understands doubt as the contrast of certainty. As my certainty increases, my doubt decreases; conversely, as my doubt increases, my certainty decreases. The requirement that knowledge is to be based in complete, or perfect certainty, amounts to requiring a complete absence of doubt — an indubitability, or inability to undermine one's conviction. Descartes' methodic emphasis on doubt, rather than on certainty, marks an epistemological innovation. This so-called ‘method of doubt’

Plato (427—347 BCE)

Plato is one of the world’s best known and most widely read and studied philosophers. He was the student of Socrates and the teacher ofAristotle, and he wrote in the middle of the fourth century B.C.E. in ancient Greece. Though influenced primarily by Socrates, to the extent that Socrates is usually the main character in many of Plato’s writings, he was also influenced by Heraclitus, Parmenides, and the Pythagoreans. 

Platonic epistemology holds that knowledge is innate, so that learning is the development of ideas buried deep in the soul, often under the midwife-like guidance of an interrogator. Plato believed that each soul existed before birth with "The Form of the Good" and a perfect knowledge of everything. Thus, when something is "learned" it is actually just "recalled."