 “I frame no hypotheses.” Thus had Newton famously written in the Principia. Hartley responds directly: “It is in vain,” he writes, “to bid an inquirer form no hypothesis” (OM 1, prop. 87). Remember that Hartley was a practicing physician; on the basis of ambiguous symptoms and fallible medical knowledge, he was every day “framing hypotheses,” making diagnoses. The question, for Hartley, concerns the degree of confidence we may place in our hypotheses. How do we measure how far our “first position” is from the truth, when we don’t know what the truth is? And how do we measure the degree to which the degree of error in our second and third positions has diminished?