It was 1790, and Jefferson was still enraptured with Francis Bacon and his Novum Organum Scientiarum published in 1620. If that work, which had deeply influenced the generation of Voltaire, had one dominant theme, it was the need to rid the mind of prejudices and prior conceptions. Jefferson, although still capable of growth, was nonetheless inclined to smirk at ideas that contradicted truths he held to be “self-evident.”1 At the time of his 1785 arrival in Paris his worldview was still grounded in the medieval cosmology of the Great Chain of Being, and his understanding of science was still rooted in deductions from “the laws of nature.” He could show respect for experimentalism, as late as his 80s, when he had a burst of excitement for vivisection as practiced by Jean-Pierre Flourens in the field of brain physiology.