One oft the most fascinating abilities of humans is the ability to become conscious of the own physical and mental states. In this systematic investigation of self-consciousness, a representational theory is developed that is able to distinguish between different levels of self-consciousness. The most basic levels are already present in such simple animals as ants. From these basic forms, which are also relevant for adult human self-consciousness, high-level self-consciousness including self-knowledge can arise. Thereby, the theory is not only able to integrate developmental considerations but also to sharply distinguish different aspects of the complex phenomenon self-consciousness. Pathological breakdowns of these different aspects, as they can be found in schizophrenia, are explained by specific impairments on different levels of self-representation. In this way, the work shows that a naturalistic theory of self-consciousness is possible, if the analysis starts with very simple and basic mechanisms instead of starting on the 'top of the iceberg'.