The most significant development and defense of a panpsychist philosophy in the twentieth century was undoubtedly that of Alfred North Whitehead (1861–1947).[5] Exploration of the details of Whitehead’s philosophy would require an article of its own, and would be fraught with interpretive difficulties in any case since Whitehead’s own presentation is forbiddingly complex. But roughly speaking Whitehead proposed a radical reform of our conception of the fundamental nature of the world, placing events (or items that are more event-like than thing-like) and the ongoing processes of their creation and extinction as the core feature of the world, rather than the traditional triad of matter, space and time. His panpsychism arises from the idea that the elementary events that make up the world (which he called occasions) partake of mentality in some—often extremely attenuated—sense, metaphorically expressed in terms of the mentalistic notions of creativity, spontaneity and perception. The echoes of Leibniz are not accidental here, and Whitehead also has a form of Leibniz’s distinction between unities and mere aggregates, which he explains in these terms: