The Chemical Category: How Nineteenth-Century Chemistry Prefigured the Logic of Structure

This report posits that the major conceptual challenges of nineteenth-century chemistry—the crisis of atomic weights, the classification of an exponentially growing chemical space, and the paradoxes of isomerism and allotropy—collectively constituted a demand for a new form of structural and relational reasoning. We argue that the solutions developed by chemists of the era, culminating in the periodic system, implicitly prioritized relationships (chemical similarities, reaction pathways) over intrinsic properties (atomic mass). This intellectual shift represents a profound, albeit unconscious, move towards the fundamental principles later formalized in the twentieth century as category theory. By analyzing key historical episodes through a categorical lens, we reveal how the science of chemistry, in its struggle for coherence, foreshadowed the categorical emphasis on morphisms over objects, demonstrating a deep structural continuity in scientific thought.