Book excerpt: All lineages of Tibetan Buddhism today claim allegiance to the philosophy of the Middle Way, the exposition of emptiness propounded by the second-century Indian master Nagarjuna. But not everyone interprets it the same way. A major faultline runs through Tibetan Buddhism around the interpretation of what are called the two truths--the deceptive truth of conventional appearances and the ultimate truth of emptiness. An understanding of this faultline illuminates the beliefs that separate the Gelug descendents of Tsongkhapa from contemporary Dzogchen and Mahamudra adherents. The Two Truths Debate digs into the debate of how the two truths are defined and how they are related by looking at two figures, one on either side of the faultline, and shows how their philosophical positions have dramatic implications for how one approaches Buddhist practice and how one understands enlightenment itself.