Abstract

This article examines the origins and development of the so-called Schlussmaß (or Logometrum), a diagrammatic method for representing logical inferences developed by the mathematician and philosopher Erhard Weigel in the 17th century. The starting point is Weigel's critique of scholastic and Aristotelian logic, which he sought to reform by combining logic and geometry. The article shows how Weigel first formulated the idea of illustrating logical relationships by means of geometric diagrams and proportions in his 'Analysis Aristotelica ex Euclide restituta', and how his students, in particular Johann Christoph Sturm, further developed these approaches in the 'Universalia Euclidea'. Using examples, it reconstructs how the Schlussmaß was conceived as a geometric test procedure for the validity of inferences and subsequently influenced Leibniz, Euler and Kant. The historical arc ranges from the early modern period to the diagram tradition of the Enlightenment to the rediscovery of diagrammatic methods in modern logic and artificial intelligence. It is argued that Weigel's Schlussmaß can be regarded not only as a precursor to Euler's diagram, but also as a paradigmatic example of the long-term interaction between logical and geometric modes of thought – a connection that is gaining new relevance in the digital age.