Bolzano's presentation of logic is embedded in the vast body of the Theory of Science (henceforth TS). His logic is based on the abstract concepts of proposition in itself (an sich) and idea in itself, which are both independent of thought and language. His logic of ideas contains a new treatment of their content and extension and, among other things, yields an analysis of ideas without objects. A purely logical definition of intuitions as simple singular ideas allowed Bolzano to distinguish them from concepts and to complete the traditional epistemological distinction between a priori and a posteriori by the logical distinction between conceptual and empirical propositions (and sciences).

Jan Sebestik, Bolzano's Logic, The Stanford Encyclopedia of Philosophy.