The ArgumentIn this contribution, I argue for epistemological impurity as the key to the historical reconstruction of the proto-biological sciences of the eighteenth century.The traditional approaches to the more or less complex and more or less stratified past of science either focus on the ideal content of that which has in the meantime been recognized as standard biological knowledge or otherwise try to uncover the implicit cognitive principles at work in order to reveal their shortcomings.A closer look at the breakdown (...) of the classical models of mechanistic explanation and the detailed analysis of the new empirico-experimental research in the neurophysiology of the eighteenth century shows, however, that eclectic procedures of various kinds have dominated the field. This eclecticism supported, and was in turn supported by, what has recently become known as “thinking with one's hand.” The paper illustrates this specific kind of thinking with reference to the case of Nicolas Le Cat's microphysics of nervous activity. (shrink)