 Green is concerned with a form of empiricism that he finds in commonsense, as well as philosophical, thought (PE §20). Though Green believes that Locke’s Essay was largely responsible for making empiricism philosophically influential, he thinks that only Hume adheres to empiricist principles consistently (Works I 1–5, 132) and that, as a result, the full metaphysical and epistemological difficulties with empiricism become clear only in Hume’s work, especially the Treatise. The empiricist program on which Green focuses sees all knowledge as resting on the deliverances of the senses. On this view, knowledge is built up from a foundation of simple discrete sensory experiences. In this sort of experience, the mind is supposed to play a purely passive role. The mind can then play an active role by performing various operations on these simple experiences, such as combining simple experiences to form complex ideas, comparing simple experiences for similarity and dissimilarity, and abstracting the common elements out of distinct ideas. In this tradition, reality is associated with simple ideas delivered to the senses and contrasted with the workmanship of the understanding and relations, which are in some sense conventional or mind-dependent.