Starting point of this study is the second edition of Schlick?s major work, Allgemeine Erkenntnislehre, which appeared in 1925. After a chapter dedicated to Verificationism, the interplay of ideas already contained in Erkenntnislehre with the incorporation of new influences (notably Wittgenstein) is reconstructed and the development of Schlick?s thought is retraced up to ?Form and Content?, produced in 1932. With this relatively extensive writing, which was intended to solve what he then considered to be the main fault of his Erkenntnislehre, namely the lacking of logico-semantical foundation, Schlick?s linguistic approach reached its peak, culminating in the radical thesis of the incommunicability of the content of our experience, the qualia. Schlick himself eventually decided not to publish this work, and ? as I try to prove ? the numerous publications during his prolific final two years of his lifetime show a departure from this radicalism. In this last period, in permanent opposition against other members of the Vienna Circle like Carnap and Neurath (in the so-called Protocol-Sentence-Debate), Schlick?s aim was to defend and to expand his basically empiricist convictions. Accordingly his theory of the ?Konstatierungen? marks the pivot of the second part of this study; as an infallible base they should assure the connection of the system of knowledge with the experienced reality. Notwithstanding the importance of the external relations to other philosophers ? which are pursued in detail ? the main focus here is on the autonomy and the permanent development of Schlick?s ideas. By intensive inclusion of his hitherto unpublished Nachlass (the literary remains) the goal of this study is both a historical reconstruction and (at least at central points) a systematic discussion of underlying philosophical problems.