Understanding Hypotheses in Newton's Scientific Thought

In Isaac Newton’s major scientific work, the Principia Mathematica, he proposed a theory of gravity but refused to explain why gravity occurs. ‘Hypotheses non fingo,’ he wrote famously, or ‘I feign no hypotheses.’ For Newton, a ‘hypothesis’ was a conjectural causal explanation, and his refusal to give one for gravity gave his contemporaries the impression that his new science was neither conjectural nor philosophical. The accessibility of Newton’s unpublished and overlooked works on alchemy, theology, and natural philosophy around the mid-20th century, helped commentators form new, more accurate pictures of Newton. They examined what the “non fingo” phrase meant and whether it really was indicative of Newton’s general approach. Some, such as Cohen 1969, Dobbs 1991, Koyré 1965, Walsh 2014 concluded that it was not: Newton did use scientific hypotheses. While showing that Newton did use hypotheses, most scholars set aside the task of articulating the makeup of Newton’s hypotheses: that is, exploring what specific roles hypotheses had his thought and what criteria they needed in order to be justified and worthy of acceptance. The aim of my dissertation is to fill these two crucial gaps in the literature. I argue that there are six criteria Newton thought hypotheses needed to satisfy to be justified: (i) non-contradiction, (ii) parsimony, (iii) mechanism, (iv) divine conformity, (v) the analogy of nature, and (vi) experiment. Serving as tools to guide empirical research and as provisional underlying mechanisms for a scientific explanation were the two main roles justified hypotheses played in Newton’s scientific thought, particularly his physics and optics. I specifically focus on the corpuscularity of light and the aether in his thought, which many specialists believe were acceptable hypotheses for Newton. It is with this framework in mind that we are able to make sense of Newton’s method and remarks on hypotheses.