Self-knowledge is knowledge of one’s own states (or processes) in an indexical mode of presentation. The philosophical debate is concentrating on mental states (or processes). If we characterize self-knowledge by natural language sentences, the most adequate utterance has a structure like “I know that I am in mental state M”. This common sense characterization has to be developed into an adequate description. In this investigation we will tackle two questions: (i) What precisely is the phenomenon referred to by “self-knowledge” and how can we adequately describe a form of self-knowledge which we might realistically enjoy? (ii) Can we have self-knowledge given the fact that the meaning of some words which we utter depends on the environment or the speech community? The theory we defend argues that we have to distinguish the public meaning of utterances, on the one hand, and the mental representations which are constituting a mental state of an individual, on the other. Self-knowledge should be characterized on the level of mental representations while the semantics of utterances self-attributing mental states should be treated separately. Externalism is only true for the public meaning of utterances but not for beliefs and other mental states including self-knowledge.