Epistemology
Epistemology deals with the questions "What is knowledge?" and "How do we know this?".
The questions are as old as Philosophy itself, and the search for answers continues today.
Reason and observation are important to all knowledge, but are viewed differently from different vantage points.
Hellenistic views
Plato viewed knowledge as universal unchanging Ideas. One dialogue even shows how all knowledge is inherent in everyone, whereas another says that that which must be there already to be able to percieve the world around him.
Aristotle tried to divide the types of knowledge, that which can be said of knowledge, into ten Categories. He further attempts to classify knowledge in books, physics, metaphysics, poetry (including theater), biology and zoology, logic, rhetoric, politics, government, and ethics and these terms are still in use today.
Scientific revolution: reason and experiment
Emiricims
Empiricists, like John Locke later stresses the importance of observation and experiments to obtain knowledge, even to the point of assuming that a new born child has a tabula rasa.
Ratonalism
Rationalists like René Descartes stress reason to arrive at knowledge since one can not always trust the senses.
Modern
Immanuel Kant tries to resolve many issues, uses his twelve categories of knowledge and argues, as Plato did before, that there has to be something in man already there to let observation into the mind. That which is already there before is a priori and gives a rational basis for handling the empirical knowledge, which is a posteriori knowledge.