This is an open access title available under the terms of a CC BY-NC-ND 4.0 International licence. It is free to read on Oxford Academic and offered as a free PDF download from OUP and selected open access locations.Two Models of Biblical Purity: The Science of Ritual investigates the ancient Israelite and early Jewish purity systems, proposing a framework that distinguishes between two conceptual approaches to ritual pollution: the qualitative and quantitative models. In the qualitative model, forms of pollution differ fundamentally in type, like distinct illnesses with unique symptoms and treatments. The quantitative model, on the other hand, views pollution as varying in intensity or degree, similar to temperature, where different sources simply make one "more" or "less" impure. The book argues that the Hebrew Bible primarily reflects a qualitative model, where impurities are categorized by type rather than severity. Through careful philological analysis, the study develops "litmus tests" to detect these models within biblical and late Second Temple texts, showing a gradual shift toward quantitative thought in later Jewish sources. Each chapter applies these methods to pivotal texts, including the Priestly literature, the Temple Scroll, and other late Second Temple and rabbinic writings, revealing the complex evolution of purity laws. This approach provides insights into the inner logic and diachronic development of ritual systems, offering a foundational perspective for comparative studies across diverse ritual traditions. Finally, it examines the historical contexts that may have instigated the shift and considers the advantages of the "fundamental science" approach to the study of biblical purity.