“The philosophy of classification” is not a phrase one frequently comes across in either philosophy or biology. Yet the philosophy of classification addresses foundational issues in both disciplines. Consider its two main questions: How should we classify the world's entities? What is the relationship between classification and the world itself? Philosophers interested in ontology, epistemology, and representation wrestle with both questions. Biologists, particularly biological taxonomists, confront the same questions when considering how to construct classifications of the organic world.

This chapter provides an introduction to the philosophy of classification by starting with the first of the above questions: How should we classify the world's entities? The focus of this book is classification in science, so we need to ask how scientific classifications should be constructed. The history and philosophy of science offer numerous answers. To tackle the wealth of suggested approaches, three general philosophical schools will be presented: essentialism, cluster analysis, and historical classification. Essentialism sorts entities according to their essential natures. Cluster analysis divides entities into groups whose members share a cluster of similar traits, though none of those traits are essential. The historical approach classifies entities according to their causal relations rather than their intrinsic qualitative features. The job of the first three sections of this chapter is to distinguish these major approaches to classification. Section 1.1 introduces essentialism, Section 1.2. outlines various cluster approaches, and Section 1.3 is an extended discussion of the historical approach.