 Bolzano’s logic was based upon a fundamental view that was the very opposite of the common view of his time. Whereas it was quite common at his time to mix logical with psychological investigations, Bolzano made every effort to separate them. For him logical concepts are concepts of their own, and their definition therefore must be kept free from any psychological admixture (WL I, 61–66). Bolzano’s approach to logic was — long before Frege and Husserl — unmistakably antipsychologistic, even if he did not yet use this term. In order to overcome psychologism and to achieve a strict separation of logic from psychology, Bolzano “opened up” for logic a realm or “world” of its own, different from the world of material objects as well as from the world of mental phenomena, a World 3 to speak with Karl Popper. Bolzano’s motive for postulating such a logical realm of its own obviously was the erroneous belief that logical properties (such as logical truth) and logical relations (such as logical consequence) need purely logical objects as their bearers in order to remain purely logical and free from any psychological admixture themselves. What is more, Bolzano had an unshakeable intuition that there are and must be such purely logical objects, namely “objective sentences” or “sentences in themselves” (Sätze an sich), and their parts, i.e., “objective ideas” or “ideas in themselves” (Vorstellungen an sich). In today’s terminology, Bolzano’s “sentences in themselves” are called ‘propositions’; this term (without any epithet) will be used for them in what follows. (The term ‘sentence’ without epithet, however, will be used in its linguistic sense for certain strings of words or symbols.) Moreover, following Bolzano’s practice, we will use the term ‘idea’ (‘Vorstellung’) without epithet for ‘idea in itself’ (‘Vorstellung an sich’). Propositions and ideas are the objects which can be “grasped” by mental phenomena (subjective propositions, in particular judgments, and subjective ideas) and expressed in language, but — despite their close connection to their mental and linguistic correlates — they must be rigorously distinguished from them.