John Locke was a serious student of medicine, chemistry, physiology, and botany. Over the long course of his reading life, he worked his way through a great swathe of scientific literature, took careful notes on much of what he read, and even published reviews of some scientific works, including Newton’s Principia. He visited and corresponded with scientific men at home and abroad. He was a member of the Royal Society, an intimate of Boyle and Newton. He collected and distributed seeds and plant parts, kept meteorological records for decades, and once (on Boyle’s instructions) attempted to descend into a mine to make measurements with a barometer. Though Locke’s engagement with natural philosophy was profound, his contributions to it were modest. He was as he famously observes in the Essay’s ‘‘Epistle’’, but an ‘‘Under-Labourer’’ to the ‘‘Master-Builders’’ of the scientific revolution. He is remembered not for his contributions to natural philosophy, but for his reflections about its foundations—for his account of the nature and limitations of scientific knowledge, his distinction between the primary and secondary qualities of bodies, and his conventionalist treatment of species concepts.