We demonstrate that, in itself and in the absence of extra premises, the following argument scheme is fallacious: The sentence A says about itself that it has a certain property F, and A does in fact have the property F; therefore A is true. We then examine an argument of this form in Gödel's introduction to his classic paper on incompleteness and examine some auxiliary premises which might have been at work in that context. Philosophically significant as it may be, that particular informal argument plays no rôle in Gödel's technical results. Going deeper into the issue and investigating truth conditions of Gödelian sentences (i.e. those sentences which are provably equivalent to their own unprovability) will provide us with insights regarding the philosophical debate on the truth of Gödelian sentences of systems – a debate which goes back at least to Dummett in the 1960s.

Usually, a sharp break in the history of philosophy is assumed, separating continental or post-Kantian philosophy from analytic philosophy and the new logic from the old. In research, this division is typically dated to the year 1879, when Gottlob Frege's Begriffsschrift was published. In recent decades, however, research has repeatedly sought out Frege's predecessors and speculated on the path by which the mathematician Frege acquired his philosophical knowledge. The following paper presents a new source: Gottlob Frege's father Alexander studied with a philosopher from the broad circle of German idealism and tried in several writings to apply and continue his philosophy himself. This new source not only elucidates Gottlob Frege's familiarity with post-Kantian philosophy, but also has the potential to explain logical influences, such as that of Leibniz.

This paper offers a comprehensive reconstruction of the only example of contraposition of a negative particular judgement that Kant provides in his writings on logic, in Reflexion 3187. The interpretation focuses on the two main consequences of applying contraposition to a judgement for Kant: the generation of negative terms and the shift of the logical modality of a judgement from assertoric to apodictic. The paper provides a reconstruction that excludes the presence of infinite judgements and makes sense of the entire course of reasoning in Reflexion 3187 from the perspective of Kant’s pure general logic. It shows on what grounds the course of reasoning in said note on logic contains an instance of contraposition of a negative particular judgement for Kant. This is the case even if the sequence of judgements does not fully respect the modal constraints set out by Kant, since the conclusion can still be validly derived within Kant’s pure general logic.

In the Prior Analytics, book 2, chapter 25, Aristotle presents a strange type of argument called apagoge. Some, such as Ross, consider the situation in this chapter problematic, and some, such as Peirce, do not. Ross believes that apagoge is a semi-demonstrative, semi-dialectical syllogism, in the form of the first figure, with a probable conclusion that is obtained from a more probable minor premise with an apodictic major premise. Peirce says that apagoge is the very abduction or −in a more recent term−inference to the best explanation. Al-Fārābī, however, without explicitly discussing apagoge, replaces it with the Arabic translation of epagoge, i.e. ‘'istiqrā’ (induction)’, which inspires the hypothesis that apagoge is a miswriting of epagoge. Inspired by al-Fārābī’s words, we formulate another abduction, based on which the strange and problematic situation of Chapter 25 is explained by accepting the hypothesis that apagoge is a miswriting of epagoge. However, this reading is neither economical nor consistent.

In his recent paper, Priest proposes a formal model of dialectical logic (Priest [2023]. ‘The logical structure of dialectic’, History and Philosophy of Logic, 44 (2), 200–208.). This paper critically examines Priest’s formal model from two key perspectives: First, it assesses the model’s accuracy in describing the dialectical progression; second, it evaluates the model’s ability to explain this progression. The analysis reveals that Priest’s formal model falls short in both respects, as it neither accurately represents the dialectical progression nor provides a satisfactory explanation of it.

The treatise on logical consequence attributed to John of Holland and composed around 1370 is preserved in two currently known copies, namely Kraków, Biblioteka Jagiellońska, ms. 2660, fols. 24r–36r and Wien, Österreichische Staatsbibliothek, ms. 4698, fols. 138v–145v. While neither copy is complete, the missing parts do not overlap, and thus the content of the treatise can be reconstructed. The treatise presents an account of validity based on the containment of conclusions in premises, also incorporating the substitutional account of validity, and modal intuitions regarding truth-preservation as a necessary condition of validity. From the medieval-literature perspective, it can be viewed as a collection of sophisms relating to various rules of inference, including paradoxical contexts such as semantic paradoxes and ex contradictione quodlibet.

In celebration of the 90th anniversary of the publication of Kurt Gödel’s incompleteness theorems, this special issue brings together articles exploring Gödelian themes from historical and philosophical points of view. It is one of two special issues comprising articles by invited speakers of the conference ‘Celebrating 90 Years of Gödel’s Incompleteness Theorems’, held in Nürtingen (Germany) in July 2021.

In this paper I discuss the intricate relationship between logic and tradition and its crucial role in al-Farabi's political philosophy. An illustrative historical example of this relationship is Mattā–al-Sirāfi debate, particularly its focus on the conflict between logic and grammar and the problem of intelligibles. These issues are pivotal in my exposition of al-Farabi's philosophy of logic. The relation of logic to grammar possesses characteristics independent of specific traditions. To explain this, I introduce the concept of ‘meta-tradition’ where intelligibles and the path to attaining them, are situated. This path is depicted in the philosophies of Plato and Aristotle and, according to al-Farabi's view, both philosophers shared the same understanding of happiness, which is only attainable through the widespread adoption of philosophy. This requires an education system that al-Farabi's approach to logic and his articulation of it, provide the framework for such a system, enabling the propagation of philosophical thought throughout the virtuous city.

This paper offers a commented edition of a late manuscript by Boole, which he likely put together in late 1863 as a response to William Stanley Jevons's criticisms of his system, in the hope of publishing his own views before Jevons's Pure Logic came out in early 1864. The manuscript, entitled ‘On the Nature of Thought’, is different in character from those that have been published to date. Boole does not attempt to rephrase his logic without algebraic symbolism. Instead, he amends the general problem-solving method presented in his Investigation of the Laws of Thought so that it obeys ‘the express condition that no forms are to be employed which are not interpretable’, without, however, making any change to his underlying logical calculus (in particular, Boole does not adopt Jevons's inclusive reading of +). Though quite terse in places, the manuscript is largely successful with respect to its stated goals. Moreover, it sheds light on Boole's thinking about interpretability, highlighting a tension in Boole's work between an indirect notion of interpretation based on the method of development, and a compositional notion of interpretation that, as the manuscript shows, Boole ended up emphasizing at the end of his life.