Amirouche Moktefi

In this paper we argue that there were several currents, ideas and problems in 19th-century logic that motivated John Venn to develop his famous logic diagrams. To this end, we first examine the problem of uncertainty or over-specification in syllogistic that became obvious in Euler diagrams. In the 19th century, numerous logicians tried to solve this problem. The most famous was the attempt to introduce dashed circles into Euler diagrams. The solution that John Venn developed for this problem, however, came from a completely different area of logic: instead of orienting to syllogistic like Euler diagrams, Venn applied Boolean algebra to improve visual reasoning. Venn’s contribution to solving the problem of elimination also played an important role. The result of this development is still known today as the ‘Venn Diagram’.

The most familiar scheme of diagrams used in logic is known as Euler’s circles. It is named after the mathematician Leonhard Euler who popularized it in his Letters to a German Princess (1768). The idea is to use spaces to represent classes of individuals. Charles S. Peirce, who made significant contributions to the theory of diagrams, praised Euler’s circles for their ‘beauty’ which springs from their true iconicity. More than a century later, it is not rare to meet with such diagrams in semiotic literature. They are often offered as instances of icons and are said to represent logic relations as they naturally are. This paper discusses the iconicity of Euler’s circles in three phases: first, Euler’s circles are shown to be icons because their relations imitate the relations of the classes. Then, Euler’s circles themselves, independently of their relations to one another, are shown to be icons of classes. Finally, Euler’s circles are shown to be iconic in the highest degree because they have the relations that they are said to represent. The paper concludes with a note on the so-called naturalness of Euler’s circles.

The scaffolding metaphor is used in education sciences to express a temporary support for a learner to complete a task which, otherwise, could not be achieved. This metaphor travelled to cognitive sciences, where it refers to external supports that allow us to reach goals that are beyond us. Mathematical representations are often viewed as scaffolds of this sort, enabling us to conduct reasonings which, otherwise, could hardly be conducted. This view has been criticised on the grounds that scaffolds are inert, temporary and external, while notations allow manipulation, are not temporary and are an essential part of cognition. This paper addresses these objections.

It is commonly held that mathematical logic did not find supporters in France in its early years of development prior to Louis Couturat, who mainly exposed the ideas of others. However, in 1879 – the very year of Frege’s Begriffsschrift –, Léon Foucou produced a fascinating logic work, relevantly titled Aperçu d’une Nouvelle Logique (Overview of a New Logic). Our paper introduces to the man, and to his original system of logic, focusing on the relations between logic and mathematics, quite similar to Frege’s and later Russell’s logicist project. We argue that in a French context that did not welcome it, Foucou developed a work which truly belongs to the burgeoning tradition of mathematical logic. He viewed his ideas as the continuation of a long tradition of studies, from Aristotle to Boole, and within a movement that would eventually reshape the field of logic. This prophecy proved true, although the change took place without him.

Euler diagrams often require several figures to adequately represent propositions and syllogisms. Euler’s followers, notably Friedrich Ueberweg, endeavored to overcome this difficulty with the use of dotted lines to express uncertainty about the relation between the terms of a proposition. Subsequently, Venn regarded such attempts as ineffectual and went to construct his own celebrated scheme. In this paper, we argue that Ueberweg’s method could be expanded to meet Venn’s expectations, and hence, produce alternative Venn-like diagrams.

L’usage des diagrammes en logique est ancien. Aux débuts de la logique mathématique, ils servent notamment à résoudre le problème de l’élimination. Cela consiste à extraire la conclusion qui découle d’un ensemble de prémisses en éliminant les termes et les propositions indésirables ou superflus. À cette fin, les logiciens inventent une multitude de notations. Il convient dès lors de s’interroger sur la place des méthodes diagrammatiques dans ce programme de recherche ainsi que leurs interactions avec les autres méthodes de résolution, symboliques et mécaniques. Les diagrammes éliminent-ils vraiment?

In this paper we argue that there were several currents, ideas and problems in 19th-century logic that motivated John Venn to develop his famous logic diagrams. To this end, we first examine the problem of uncertainty or over-specification in syllogistic that became obvious in Euler diagrams. In the 19th century, numerous logicians tried to solve this problem. The most famous was the attempt to introduce dashed circles into Euler diagrams. The solution that John Venn developed for this problem, however, came from a completely different area of logic: instead of orienting to syllogistic like Euler diagrams, Venn applied Boolean algebra to improve visual reasoning. Venn’s contribution to solving the problem of elimination also played an important role. The result of this development is still known today as the ‘Venn Diagram’.

Lewis Carroll published a system of logic in the symbolic tradition that developed in his time. Carroll’s readers may be puzzled by his system. On the one hand, it introduced innovations, such as his logic notation, his diagrams and his method of trees, that secure Carroll’s place on the path that shaped modern logic. On the other hand, Carroll maintained the existential import of universal affirmative Propositions, a feature that is rather characteristic of traditional logic. The object of this paper is to untangle this dilemma by exploring Carroll’s guidelines in the design of his logic, and in particular his theory of existential import. It will be argued that Carroll’s view reflected his belief in the social utility of symbolic logic.

Extension and intension are two ways of indicating the fundamental meaning of a concept. The extent of a concept, C, is the set of objects which correspond to C whereas the intent of C is the collection of attributes that characterise it. Thus, intension denotes the set of objects corresponding to C without naming them individually. Mathematicians switch comfortably between these perspectives but the majority of logical diagrams deal exclusively in extension. Euler diagrams indicate sets using curves to depict their extent in a way that intuitively matches the relations between the sets. What happens when we use spatial diagrams to depict intension? What can we infer about the intension of a concept given its extension, and vice versa? We present the first steps towards addressing these questions by defining extensional and intensional Euler diagrams and translations between the two perspectives. We show that translation in either direction leads to a loss of information, yet preserves important semantic properties. To conclude, we explain how we expect further exploration of the relationship between the two perspectives could shed light on connections between diagrams, extension, intension, and well-matchedness.

Logic, the discipline that explores valid reasoning, does not need to be limited to a specific form of representation but should include any form as long as it allows us to draw sound conclusions from given information. The use of diagrams has a long but unequal history in logic: The golden age of diagrammatic logic of the 19th century thanks to Euler and Venn diagrams was followed by the early 20th century's symbolization of modern logic by Frege and Russell. Recently, we have been witnessing a revival of interest in diagrams from various disciplines - mathematics, logic, philosophy, cognitive science, and computer science. This book aims to provide a space for this newly debated topic - the logical status of diagrams - in order to advance the goal of universal logic by exploring common and/or unique features of visual reasoning.

It is well known that Charles L. Dodgson (alias Lewis Carroll, 1832–1898) worked on a logic treatise that would popularise the subject of symbolic logic. The first part appeared in 1896 but the next parts never appeared. It has been claimed that Carroll worked in isolation and did not read the main works of his time. The object of this paper is to inquire what Carroll’s private library teaches us on his readings. The content of this library is known thanks to the sale catalogues that were issued when the library was auctioned at Carroll’s death. This paper provides an overview of the logic books owned by Carroll. Then, it investigates the extent to which Carroll was acquainted with the main logic works of his time. Finally, the paper considers some methodological issues related to the use of ‘library arguments’ in intellectual history.

It is of common use in modern Venn diagrams to mark a compartment with a cross to express its non-emptiness. Modern scholars seem to derive this convention from Charles S. Peirce, with the assumption that it was unknown to John Venn. This paper demonstrates that Venn actually introduced several methods to represent existentials but felt uneasy with them. The resistance to formalize existentials was not limited to diagrammatic systems, as George Boole and his followers also failed to provide a satisfactory symbolic representation for them. This difficulty points out issues that are inherent to the very nature of existentials. This paper assesses the various methods designed for the representation of existential statements with Venn diagrams. First, Venn’s own attempts are discussed and compared with other solutions proposed by his contemporaries and successors, notably Lewis Carroll and Peirce. Since disjunctives hold an important role in an effective representation of existentials, their representation is also discussed. Finally, recent methods for the diagrammatic representation of existing individuals, rather than mere existence, are surveyed.

It is little known that Schopenhauer (1788–1860) made thorough use of Euler diagrams in his works. One specific diagram depicts a high number of concepts in relation to Good and Evil. It is, hence, uncharacteristic as logicians of that time seldom used diagrams for more than three terms (the number demanded by syllogisms). The objective of this paper is to make sense of this diagram by explaining its function and inquiring whether it could be viewed as an early serious attempt to construct complex diagrams.

In a letter to Bertrand Russell, dated 17 May 1905, Hugh MacColl tells how he abandoned the study of logic after 1884 for about thirteen years and how it was the reading of Lewis Carroll’s Symbolic Logic (1896) that ”rekindled the old fire which [he] thought extinct.” From then onwards, he published several papers containing some of his major logical innovations. The aim of this paper is to discuss MacColl’s acquaintance with and appreciation of Carroll’s work, and how that reading convinced him to come back into the realm of logic. There is no evidence that the two men ever met or exchanged any personal correspondence. However, MacColl reviewed in The Athenaeum several of Carroll’s books, mainly on geometry and logic. Often, Carroll replied to MacColl’s criticisms in the following editions of his works. A discussion of these sources provides interesting insights to MacColl’s (and Carroll’s) mathematical and logical investigations.