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Von Gerechtigkeitstheorien wird gemeinhin erwartet, dass diese handlungsleitend sein können. Gleichzeitig dominierten in den vergangenen Jahrzehnten sogenannte ideale Theorien den Diskurs um Gerechtigkeit, die auf kontrafaktischen Annahmen basieren, welche nicht die Realität abbilden. Gemäß der Mainstream-Auffassung konfligiert dieser ideale Charakter nicht mit dem Desideratum der Handlungsleitung: Ideale Theorien zeichnen die (vollkommen) gerechte Gesellschaft als ein Ziel, dem es entgegenzustreben gilt. Um dieses Ziel zu erreichen, brauche es darüber hinaus eine nicht-ideale Theorie, die mehr an der empirischen Wirklichkeit ausgerichtet ist und es als Ziel hat, bestehende Ungerechtigkeiten durch entsprechende Handlungsanweisungen zu beseitigen. Dieser Auffassung nach ist ideale Theorie somit zwar nicht selbst handlungsleitend, wohl aber eine notwendige Bedingung für Handlungsleitung. Dass ideale Theorie dieser Funktion gerecht werden kann, stößt hierbei zunehmend auf Widerspruch. Dabei lassen sich im Wesentlichen drei Gruppen von Fundamentalkritiken unterscheiden, die alle drei in der Konklusion münden, dass ideale Theorie nicht zur Handlungsleitung unter Realbedingungen beitragen kann: Erstens, Kritiken, denen entsprechend ideale Theorie nicht zur Genese von Handlungsanweisungen fähig und nötig ist; zweitens, Kritiken, denen entsprechend ideale Theorie nicht anwendbar auf das Hier-und-Jetzt ist; drittens, Kritiken, entsprechend denen idealer Theorie der Bezug zur Gerechtigkeit fehlt. In der vorliegenden Arbeit verteidige ich ideale Theorie gegen jede dieser drei Kritiken. Dafür (re-)konstruiere ich als erstes die Unterscheidung in ideale und nicht-ideale Theorie, stelle dann die Kritiken dar und formuliere schließlich entsprechende Entgegnungen. Dabei akzeptiere ich das Desideratum der Handlungsleitung und argumentiere dafür, dass alle drei Kritiken scheitern., Theories of justice are commonly expected to be able to guide actions. At the same time, the debate led on justice during the last decades was dominated by so called ideal theories. These theories are based on contrafactual assumptions, which do not portray the reality. According to the mainstream-view, this ideal character is not detrimental for the desideratum of action-guidingness: ideal theories describe the (fully) just society as an aim one ought to strive for. To accomplish this goal, however, a non-ideal theory is needed, which is rooted in the empirical reality and seeks to eliminate injustices through recommendations of actions. On this account, ideal theory is not action-guiding itself, albeit a necessary condition for action-guidingness. However, the idea that ideal theory can fulfil this function is increasingly disagreed with. Three main groups of fundamental critiques can be distinguished, which all three lead to the conclusion that ideal theory cannot contribute to action-guidingness under real-life conditions: Firstly, critiques, according to which ideal theory is neither able nor necessary to create recommendations for actions; secondly, critiques, according to which ideal theory is not applicable to the here-and-now; thirdly, critiques, according to which ideal theory does not connect to justice. In this thesis I defend ideal theory against each one of these three critiques. To do so, I, firstly, (re-)construct the distinction between ideal and non-ideal theory. Thereafter, I present the three critiques and formulate replies to them. In doing so, I accept the desideratum of action-guidingness and argue that all three critiques fail.

This article identifies problems with regard to providing criteria that regulate the matching of logical formulae and natural language. We then take on to solve these problems by defining a necessary and sufficient criterion of adequate formalization. On the basis of this criterion we argue that logic should not be seen as an ars iudicandi capable of evaluating the validity or invalidity of informal arguments, but as an ars explicandi that renders transparent the formal structure of informal reasoning

This paper reveals two fallacies in Turing's undecidability proof of first-order logic (FOL), namely, (i) an 'extensional fallacy': from the fact that a sentence is an instance of a provable FOL formula, it is inferred that a meaningful sentence is proven, and (ii) a 'fallacy of substitution': from the fact that a sentence is an instance of a provable FOL formula, it is inferred that a true sentence is proven. The first fallacy erroneously suggests that Turing's proof of the non-existence of a circle-free machine that decides whether an arbitrary machine is circular proves a significant proposition. The second fallacy suggests that FOL is undecidable.

One of the central logical ideas in Wittgenstein's Tractatus logico-philosophicus is the elimination of the identity sign in favor of the so-called "exclusive interpretation" of names and quantifiers requiring different names to refer to different objects and (roughly) different variables to take different values. In this paper, we examine a recent development of these ideas in papers by Kai Wehmeier. We diagnose two main problems of Wehmeier's account, the first concerning the treatment of individual constants, the second concerning so-called "pseudo-propositions" ("Scheinsätze") of classical logic such as a=a or a=b v b=c -> a=c. We argue that overcoming these problems requires two fairly drastic departures from Wehmeier's account: (1) Not every formula of classical first-order logic will be translatable into a single formula of Wittgenstein's exclusive notation. Instead, there will often be a multiplicity of possible translations, revealing the original "inclusive" formulas to be ambiguous. (2) Certain formulas of first-order logic such as a=a will not be translatable into Wittgenstein's notation at all, being thereby revealed as nonsensical pseudo-propositions which should be excluded from a "correct" conceptual notation. We provide translation procedures from inclusive quantifier-free logic into the exclusive notation that take these modifications into account and define a notion of logical equivalence suitable for assessing these translations.