Timm Lampert

Newton claimed to have proven the heterogeneity of sunlight by experiment. Olaf Müller argues that one could likewise prove the heterogeneity of darkness from inverting Newton’s experiments. Müller concludes that both, Newton’s proofs and his underlying methodology, are inconclusive and that Newton’s theory is underdetermined. This paper argues that Müller’s criticism is not justified because the conducted inverted experiments to which Müller refers do not replace Newton’s source of light with a source of darkness. Therefore, Müller does not refute Newton’s methodology, nor does he provide an example of underdetermined theories.

An essential feature of Wittgenstein's early philosophy of logic is the conjecture of a positive solution to the question of whether logical truth in first-order logic is decidable (Entscheidungsproblem). It is often argued that the proofs of a negative solution to the Entscheidungsproblem presented in the 1930s refuted Wittgenstein's early conception of logic. However, we argue Wittgenstein's philosophy of logic does not share the principles assumed by the proof strategies of those undecidability proofs. Thus, the undecidability proofs do not refute Wittgenstein's philosophy of logic based on its own convictions. Therefore, we provide a purely logical refutation of Wittgenstein's early philosophy of logic that does not depend on assumptions that he does not accept. Wittgenstein claimed that the logical properties of first-order logic are decidable based on patterns in a suitable notation for first-order logic. We explain why this conviction is mistaken.

Turing proved the unsolvability of the decision problem for first-order logic (Entscheidungsproblem) in his famous paper On Computable Numbers, with an Application to the Entscheidungsproblem. From this proof it follows that attempts to specify a solution for the Entscheidungsproblem through pattern detection in automated theorem proving (ATP) must fail. Turing’s proof, however, merely predicts the non-existence of such solutions; it does not construct concrete examples that explain why specific attempts to solve the decision problem by pattern detection fail. ATP-search often runs in infinite loops in the case of unprovable, i.e. not refutable, formulas and one can ask why finite patterns of repeated inference steps cannot serve as criteria for unprovability. We answer this question by constructing pairs of formulas (ϕ,ϕ′) such that ϕ is provable (refutable) and ϕ′ is unprovable (satisfiable in an infinite domain) but all but the last proof step of the ATP-search for ϕ is a proper part of the endless ATP-search for ϕ′. We generate such pairs of formulas by mimicking computable sequences for a certain kind of universal Turing machine, namely, splitting Turing machines (STMs), via sequences of inference steps in ATP. In contrast to Turing’s and the textbooks’ method to formalize Turing machines, our method does not rely on further axioms and allows us to transfer the straightforward insight that the halting problem cannot be solved through pattern detection to the case of the Entscheidungsproblem. Our method is a constructive alternative to general undecidability proofs that explains why a scientific problem, namely the Entscheidungsproblem, is unsolvable in a specific way. This explanation provides a better understanding of the failure of pattern detection, which is of interest to: (i) the programmer, who is concerned with prospects and limits of pattern detection, (ii) the logician, who is interested in identifying logical properties by properties of an ideal notation, and (iii) the philosopher, who is interested in proof methods.

In philosophical contexts, logical formalisms are often resorted to as a means to render the validity and invalidity of informal arguments formally transparent. Since Oliver and Massey , however, it has been recognized in the literature that identifying valid arguments is easier than identifying invalid ones. Still, any viable theory of adequate logical formalization should at least reliably identify valid arguments. This paper argues that accounts of logical formalization as developed by Blau and Brun do not meet that benchmark. The paper ends by suggesting different strategies to remedy the problem

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.

In two letters to Russell from 1913, Wittgenstein conjectured that first-order logic is decidable. His conjecture was based on his conviction that a decision procedure amounts to an equivalence transformation that converts initial formulas into ideal symbols of a proper notation that provides criteria for deciding the logical properties of the initial formulas. According to Wittgenstein, logical properties are formal properties that are decidable on the basis of pure manipulations of symbols. This understanding of logical properties (such as provability or logical truth/falsehood) is independent of and prior to any interpretation or application of logic. Wittgenstein’s conception of logic is incompatible with the undecidability proof of Church and Turing from 1936. Thus, Wittgenstein’s conjecture and his understanding of logic appear to be refuted. This paper argues that Wittgenstein did not drew this conclusion and it explains why he never withdrew his conjecture.

One of the central logical ideas in Wittgenstein’sTractatus logico-philosophicusis 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$a=b \wedge b=c \rightarrow a=c$. 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 asingleformula of Wittgenstein’s exclusive notation. Instead, there will often be a multiplicity of possible translations, revealing the original “inclusive” formulas to beambiguous. (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.

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.

Das Buch vermittelt die Grundlagen der Aussagen- und erweiterten Prädikatenlogik in 12 Lektionen. Neben Techniken zum überprüfen der Schlüssigkeit von Argumenten bilden die Kunst des Formalisierens wissenschaftlicher Argumente und metalogische Fragen den Inhalt des Buches. Das Buch eignet sich in Verbindung mit begleitenden interaktiven Übungseinheiten und Klausuren, die ber Internet zugänglich sind, sowohl zum Selbststudium als auch für Einführungskurse in die Logik. Die zweite berarbeitete Auflage erscheint in einem größeren und besser lesbaren Format.

Es wird anhand von Fallbeispielen aus der Geschichte der Farbenlehre inwissenschaftstheoretische Probleme eingeführt. Das Buch dient als Grundlagefür eine anwendungsbezogene Lehre und als Einführung in folgende ThemenbereicheNewton vs. Goethe; Theorie und Experiment, Colormetrie; Empfindungsmessung;Helmholtz vs. Hering; Theorienevaluation, Psychologische Farbenlehre; Phänomenologie,Farbausschluss; Beweistheorie, Farbdefinitionen; Theorien- und Begriffsbildung.Neben Aufgaben, Texten und Lösungsvorschlägen finden sich eine Bibliographiesowie Einleitungen zu den behandelten Fragen und Lösungsvorschlägen derjeweiligen Themen.

Es wird gezeigt, dass Wittgenstein in seiner Frühphilosophie ein nicht-axiomatisches Beweisverständnis entwickelt, für das sich das Problem der Begründung der Axiome nicht stellt. Nach Wittgensteins Beweisverständnis besteht der Beweis einer formalen Eigenschaft einer Formel – z.B. der logischen Wahrheit einer prädikatenlogischen Formel oder der Gleichheit zweier arithmetischer Ausdrücke – in der Transformation der Formel in eine andere Notation, an deren Eigenschaften sich entscheiden lässt, ob die zu beweisende formale Eigenschaft besteht oder nicht besteht. Dieses Verständnis grenzt Wittgenstein gegenüber einem axiomatischen Beweisverständnis ab. Sein Beweisverständnis bedingt ein Programm der Grundlegung der Mathematik, das eine Alternative zu den Ansätzen des Logizismus, Formalismus und Konstruktivismus darstellt. Wittgensteins Ansatz steht im Widerspruch zu den Ergebnissen der Metamathematik, da er die Möglichkeit der Formulierung von Entscheidungsverfahren in der Prädikatenlogik und Arithmetik voraussetzt. Um seinem Ansatz gegenüber der traditionellen Metamathematik Recht zu geben, müsste gezeigt werden, dass sein Beweisverständnis im Bereich der Logik und Arithmetik – der traditionellen Metamathematik zum Trotz – realisierbar ist.

In §8 of Remarks on the Foundations of Mathematics (RFM), Appendix 3 Wittgenstein imagines what conclusions would have to be drawn if the Gödel formula P or ¬P would be derivable in PM. In this case, he says, one has to conclude that the interpretation of P as “P is unprovable” must be given up. This “notorious paragraph” has heated up a debate on whether the point Wittgenstein has to make is one of “great philosophical interest” revealing “remarkable insight” in Gödel’s proof, as Floyd and Putnam suggest (Floyd (2000), Floyd (2001)), or whether this remark reveals Wittgenstein’s misunderstanding of Gödel’s proof as Rodych and Steiner argued for recently (Rodych (1999, 2002, 2003), Steiner (2001)). In the following the arguments of both interpretations will be sketched and some deficiencies will be identified. Afterwards a detailed reconstruction of Wittgenstein’s argument will be offered. It will be seen that Wittgenstein’s argumentation is meant to be a rejection of Gödel’s proof but that it cannot satisfy this pretension.

Anhand der genaueren Analyse von Newtons experimentum crucis und der Argumentation, die er auf dieses Experiment stützt, sowie Goethes Kritik hieran sollen im Folgenden zwei verbreitete Vorurteile revidiert werden: 1. Newton ist kein Dogmatiker, der methodische Ansprüche vertritt, die er nicht einlösen kann, sondern gründet seinen Anspruch, experimentelle Beweise führen zu können, auf einer vorbildlichen Methodologie kausaler Erklärungen, was seine Kritiker allerdings übersehen. 2. Goethe ist kein Antiwissenschaftler, der einen einzigartigen Kontrapunkt zur vorherrschenden wissenschaftlichen Tradition bildet, sondern steht inmitten traditioneller Auffassungen zur Farbenlehre, deren experimentelle und methodologische Grundlagen bezüglich eines Erklärungsanspruches denen Newtons unterlegen sind.

In vol. 2 of Grundlagen der Mathematik Hilbert and Bernays carry out their undecid- ability proof of predicate logic basing it on their undecidability proof of the arithmeti- cal systemZ00. In this paper, the latter proof is reconstructed and summarized within a formal derivation schema. Formalizing the proof makes the presumed use of a meta language explicit by employing formal predicates as propositional functions, with ex- pressions as their arguments. In the final section of the paper, the proof is analyzed critically by applying Wittgenstein’s view on meta language, which does not lead to a questioning of the assumptions on which the proof is based but of its presumed use of meta language. Finally, it will be argued that determining if it is the undecidability proof or Wittgenstein’s analysis of meta language that is right depends on whether a decision procedure for a predicate that undecidability proofs have seemingly proven undecidable can nonetheless be defined. Thus serious attempts to define a procedure for such a predicate should not be ruled out without studying them.