Preprint published in British Journal for the History of Philosophy, 18/5, 2010, 797-819. PLEASE QUOTE FROM THE PUBLISHED VERSION Leibniz and Sensible Qualities Christian Leduc Université de Montréal This paper discusses the problem of sensible qualities, an important, but underestimated topic in Leibniz’s epistemology. In the first section, the confused character of sensible ideas is considered. Produced by the sensation alone, ideas of sensible qualities cannot be part of distinct descriptions of bodies. This is why Leibniz proposes to resolve sensible qualities by means of primary or mechanical qualities, a thesis which is analysed in the second section. Here, I discuss his conception of nominal definitions as distinct empirical representations. The provisional and modifiable status of nominal definitions is then explained in the third section. Since nominal descriptions always contain sensible determinations, Leibniz claims that empirical knowledge is indefinitely changeable according to progress in sciences. In the final section, I address the criterion of coherence that enables us to approve of hypotheses that are based on both sensible and empirical properties. Like many of his contemporaries, Leibniz was interested in the theoretical implications of sensible qualities. Several problems arise with the introduction of the distinction between primary and secondary, or sensible, qualities. Within this discussion, particular emphasis was placed upon two problems: the ontological status of sensible qualities and the epistemological function of sensible notions in scientific explanations. Leibniz addresses both. First, he is in partial agreement with the mechanist view, especially maintained by the Cartesians, which had consisted mainly in denying any corporeal reality to sensible qualities: sensory qualities such as heat, cold and colour do not exist in bodies, but are rather only the sensible effect, on the mind, of corporeal primary qualities, such as size,