How Do We Know?

Platonic Idealism: All "true" knowledge is innate.  Our souls descend from The One, the True, and so elements of that truth exist within our souls.  Thru philosophical enquiry we can strip away the false to reveal these innate knowledges and truths.  What we see in the physical world is only a fallen, inaccurate representation of this higher One or Form or Truth.

John Locke tabula rasa:  There are no innate ideas.  We arrive with a "blank slate" of a mind and we gain knowledge thru sensation and reflection: we observe and we ponder.  So long as our knowledge is in accord with what we can observe of the real world, that knowledge is relatively valid and accurate.

Jean Jacques Rousseau: "I felt before I thought. This is the common lot of humanity" (Confessions 435).  Although there are no innate ideas, human emotion, especially compassion/love/pité, are instinctual and innate, and these color our perception or observation (Locke's "sensation") of the world, as well as our thoughts ("reflection").  Once man discovered ownership/property, we began sublimating our "true" and good (loving) selves in order to exploit each other...this will feed into both Freud and Marx.

Sigmund Freud and the Subconscious:  Our consciousness (Locke's "knowledge") has more than one level and is contaminated by the Id, that is, our instinctual drives conflict with our social lives, and our consciousness is filtered through this prism.  Conscious knowledge is likely false in comparison to repressed truths known by the subconscious.  Things are not what they seem.

Karl Marx and Ideology:  Our deepest held truths are in fact nothing more than "Ideology": myths that serve to maintain the Bourgeois hegemony over the Proletariat.  Knowledge is “existential”: created by material conditions and relationships; thus the essence of belief follows existence, historical reality or “facts on the ground”.  What we thought was accurate knowledge was just mythology used to keep the powerful in power etc.  (Note this is quite similar to Freud: what appears as truth is a myth developed because of social pressures.  Both draw heavily from Rousseau here: a pure, "natural" individual's knowledge is corrupted by a society, either for often sexual reasons or for property.)

Friedrich Nietzsche Beyond Good And Evil: Like Marx, Nietzsche argues that morality is nothing more than a socially constructed set of rules developed to control human behavior. Although Nietzsche doesn't use Marx's term "Ideology", their views of knowledge are very similar with this most important distinction: Marx believes Ideology functions to keep the powerful in power (hegemony), while Nietzsche argues the opposite:  Ideology is used by the weak to keep the strong from becoming too strong. Marx's view of Ideology drives much Left Wing political theory, while Nietzsche's drives Right Wing theory;  ironically, although Nietzsche was an atheist who's argument was aimed at Christianity, his view has since between widely adopted by the Christian Right.