Model, Theory, and Evidence in the Discovery of the DNA Structure

In this paper, I discuss the discovery of the DNA structure by Francis Crick and James Watson, which has provoked a large historical literature but has yet not found entry into philosophical debates. I want to redress this imbalance. In contrast to the available historical literature, a strong emphasis will be placed upon analysing the roles played by theory, model, and evidence and the relationship between them. In particular, I am going to discuss not only Crick and Watson's well-known model and Franklin's x-ray diffraction pictures (the evidence) but also the less well known theory of helical diffraction, which was absolutely crucial to Crick and Watson's discovery. The insights into this groundbreaking historical episode will have consequences for the ‘new’ received view of scientific models and their function and relationship to theory and world. The received view, dominated by works by Cartwright and Morgan and Morrison ([1999]), rather than trying to put forth a ‘theory of models’, is interested in questions to do with (i) the function of models in scientific practice and (ii) the construction of models. In regard to (i), the received view locates the model (as an idealized, simplified version of the real system under investigation) between theory and the world and sees the model as allowing the application of the former to the latter. As to (ii) Cartwright has argued for a phenomenologically driven view and Morgan and Morrison ([1999]) for the ‘autonomy’ of models in the construction process: models are determined neither by theory nor by the world. The present case study of the discovery of the DNA structure strongly challenges both (i) and (ii). In contrast to claim (i) of the received view, it was not Crick and Watson's model but rather the helical diffraction theory which served a mediating purpose between the model and the x-ray diffraction pictures. In particular, Cartwright's take on (ii) is refuted by a comparison of Franklin's bottom-up approach with Crick and Watson's top-down approach in constructing the model. The former led to difficulties, which only a strong confidence in the structure incorporated in the model could circumvent. 1. How to Get to the Structure1.1. X-ray diffraction and its synthesis1.2. Model building and Pauling's panache1.3. The structure of proteins1.3. 1. A failed inference to the best explanation1.3. 2. The misleading 5.1 Å spot in proteins and how to get rid of it1.3. 3. Derived predictions from Pauling's alpha-helix of protein molecules2. The CCV Theory of Helical X-Ray Diffraction2.1. The role of the CCV theory in the discovery of the DNA structure3. Killing the Helix3.1. Appreciating all evidence—in vain4. Conclusion5. Epilogue: Chargaff's Ratios How to Get to the Structure X-ray diffraction and its synthesis Model building and Pauling's panache The structure of proteins A failed inference to the best explanation The misleading 5.1 Å spot in proteins and how to get rid of it Derived predictions from Pauling's alpha-helix of protein molecules The CCV Theory of Helical X-Ray Diffraction The role of the CCV theory in the discovery of the DNA structure Killing the Helix Appreciating all evidence—in vain Conclusion Epilogue: Chargaff's Ratios