Knowledge is important for us, human beings, for a variety of reasons, starting with trivial but necessary reasons to live your life (knowing not to cross the street when the traffic light is red, knowing that water quenches thirst, knowing where you left your bicycle last night, knowing how to buy a train ticket, knowing how to brush your teeth, etc. etc.). Western man also has a collective project that is constitutive of its culture: science; and the aim of science is to gather knowledge about the world in its broadest meaning: from the origin of a particular disease to the origin of man, life, planet Earth and the universe, from why the orbits move as they do to why a mass of people behaves differently from individuals, from why the sky is blue to why the sea is salty, from why the climate seems to change to why rainbows appear, etc. etc. The quest to understand what knowledge is, is as old as philosophy itself, and the philosophy of knowledge is called epistemology. The substantive ‘knowledge’ and the verb ‘to know’ are used in a variety of manners: we can know a person, we can know how to ride a bicycle and we can know that snow is white. The last-mentioned use, propositional knowledge (knowing that p, where p is proposition), is the most pervasive use and has been the main focus of philosophers. Already in the Meno, Plato raises a question how someone knowing that p differs from thinking that p is true when p is true. In this thesis I am also going to focus exclusively on propositional knowledge.