Subject of this book is an epistemological consideration - a consideration which could be characterised as a main theme - maybe the main theme - of that part of philosophy we all know as epistemology: the nature of knowledge. But other than the most essays on the subject of knowledge, here I am going to deal with a largely overlooked account to try to find an answer to the epistemological question of knowledge. This is the mental state account of knowledge (Price in his 'Belief' the formulation ``mental acts'' and Williamson talks about a ``state of mind''). Or to put it into the question I chose as title: is knowledge a mental state? We have to concede first that there is only a small group of philosophers who used to explain knowledge in terms of a mental state, particularly the `Oxford Realists'. And secondly, the acceptance of the MS thesis is low and negative. There is an interesting detail here: unlike the poor interest in an epistemic theory such as the MS thesis, philosophers like Prichard or Austin (and their philosophical thinking) are not really living in the shadows of philosophical consideration. Indeed their philosophical impact is high level, if we consider for instance Prichard's moral writings or Austin's theory of speech acts. I think we can conclude from this fact that the reason of the `negative' ignorance in respect of their epistemological point of view was not caused by a negative quality of their philosophy. Now, the question we are faced with (and that should be answered here) is: what is wrong with the MS thesis even though it is held by high class philosophers? Why is the epistemic thinking of Cook Wilson, Prichard and Austin afflicted with such ignorance? I will try to explain this later on with the notion of an unreflected Platonian heritage during 2000 years of epistemic thinking - a notion which is similar to a point Hetherington has called ``epistemic absolutism''. So, there are three main purposes which I am pursuing in this consideration: 1.To explain the reasons why there is such an ignorance towards an assertion of the MS thesis. I am going to pursue this through an analysis of knowledge which will demonstrate the inappropriateness of the JTB thesis as an adequate analysis of knowledge. 2.To describe that it is a mistake to ignore or at least underestimate the MS thesis in the discussion of an appropriate definition of knowledge and to maintain that the MS thesis is the key to a general theory of knowledge. 3.Conclusion: If the first two steps are correct, the JTB thesis is insufficient in order to give an account of the nature of knowledge in general. A consequence from this is: all the epistemic theories which are dealing with the JTB thesis are based on deficient assumptions. Hence their results - notably the well-known externalism/internalism debate - are insufficient, too. So, there is a need for a new theory of knowledge based on the MS thesis. In the course of my consideration I am going to justify the following three theses: i) The JTB thesis as a definition of knowledge in general is deficient, as the JTB thesis describes the propositional aspect of knowledge only. But the propositional knowledge - the so-called `knowledge that' - is merely one element among others that has to be recognized in search of a theory of knowledge. ii) The status of the `knowledge that' is derivative and not ultimate. It is derived from the non- propositional knowledge in order to make the non-propositional knowledge communicable to others. The mode of the `knowledge that' is indirect and thus can be stated in the third person point of view only. This ultimate kind of knowledge - the knowledge which the `knowledge that' is derived from - is the non-propositional knowledge. Its mode is direct and hence it is restricted to the first person point of view. Therefore the basis towards a theory of knowledge in general has to be this non-propositional aspect of knowledge. iii) Hence, taking the first two theses for granted, an appropriate theory of knowledge needs an account of the non-propositional knowledge. The MS thesis will accomplish this task.