 Something could be true even if nobody knew that it were so. We therefore need a concept of truth that does not require of every truth that someone knows it. For this concept Bolzano introduced the term ‘Wahrheit an sich’ (‘truth in itself’). If something is a truth and no human being knows that it is, then — for Bolzano — it must be a “truth in itself”. A “truth in itself” (Wahrheit an sich) is nothing but a true proposition, i.e., a proposition that has the property of being true. Bolzano takes the proof that there is at least one “truth in itself” to be fundamental not only for logic and for philosophy in general, but for any science. Bolzano offers therefore several arguments such as the following ones for the claim that there must be “truths in themselves”: (i) There are obviously truths that are unknown and therefore (so Bolzano) “truths in themselves”. E.g. one of the two propositions “There are winged snakes” or “There are no winged snakes” must be true, but we do not know which one (WL I, 108); and one of the propositions stating of a specific tree at a specific moment that it bears a certain number of blossoms must be true, even if nobody knows which one (WL I, 112). (ii) The Pythagorean theorem or the Copernican theory that the earth rotates around the sun have not become true by their discoveries but have always been true, i.e. (according to Bolzano), they are “truths in themselves” (Zimmermann 1847, 71 f., 136). (iii) If no thinking being existed, it were true that there exists no thinking being, but this (according to Bolzano) could only be a “truth in itself” (Bolzano 1839, 150). There is one proof for the existence of a “truth in itself” or a true proposition, however, which Bolzano takes to be decisive. It is an improved version of the traditional refutation of scepticism by self-application (RW I, 35, WL I, 145). Before we present and discuss this proof, a terminological remark seems to be in order: The word ‘truth’ (as well as the German word ‘Wahrheit’) is ambiguous insofar as it denotes the property of being true on the one hand and a bearer of this property (a true proposition, a true sentence, a true judgment etc.) on the other hand (WL I, 108 f.). Only the latter can be meant, however, as soon as the word ‘truth’ is preceded by the indefinite article (‘a truth’) or if it is put into the plural form (‘truths’). It is in such clear cases only that we will allow the word ‘truth(s)’ without epithet to be used in the sense of Bolzano’s ‘truth in itself’ or ‘truths in themselves’, but otherwise we will use ‘true proposition(s)’ instead to avoid confusion. In general we will therefore reserve the word ‘truth’ for the property of being true. That being said, we can now turn to Bolzano’s alleged proof for the existence of true propositions: