Resume The aim of this project is to explain and to solve the problem of Cartesian skepticism. This problem occurs in contemporary literature as the following argument: If I have knowledge about the external world, then I know that the skeptical hypothesis is false. I do not know that the skeptical hypothesis is false. Therefore, I do not have knowledge about the external world. This argument confronts us with a puzzle: Each of the premises seems highly plausible, but the conclusion seems unacceptable. Therefore, a satisfying solution to the skeptical problem has first to show how we can have knowledge about the external world, and second to explain the striking plausibility of the skeptical scenario. Explaining and solving this skeptical problem is still one of the central epistemological challenges. In contemporary epistemology, there are various competing anti-skeptical strategies discussed, attacked or defended. These anti-skeptical strategies are unsatisfactory for different reasons. Some cannot provide a convincing argument that we can have knowledge about the external world, some cannot explain the plausibility of the skeptical scenario and some fail for both reasons. In this respect, the skeptical problem has still to be regarded as unsolved. In this research project, I will defend the following view: Persons can take two distinct perspectives in reflecting on themselves, an ordinary perspective and a 3rd-person-perspective. Moorean reasoning which means to have knowledge about the external world and to know through inference from this knowledge that the skeptical hypothesis is false, leads to 1st-person-knowledge, which is not one from a 3rd-person-perspective. In the context of Cartesian doubting, we take a 3rd-person-perspetive towards our own mental states and only regard 1st-person-knowledge from a 3rd-person-perspective as adequate. Our capacity of taking 3rd-person-perspectives towards ourselves explains the two intuitions that Cartesian skepticism is strikingly plausible and that Moorean reasoning is inappropriate. However, in the process of ordinary knowledge acquisition we need not take a 3rd-person-perspective. Therefore, we can gain empirical knowledge about the external world and knowledge that the skeptical hypothesis is false through Moorean reasoning in processes of ordinary knowledge acquisition. The skeptical puzzle rests on a confusion of two perspectives of 1st-person-knowledge. Thus I can achieve the two goals of this project: First, to explain how we can have external world knowledge and second to explain the plausibility of the skeptical argument by taking a view which can be labelled as “perspectivism”.