Based on participant-observation fieldwork, interviews with western Zen practitioners, public dharma talks and personal interviews given by two contemporary Sōtō Zen teachers (Hōgen Yamahata and Ekai Korematsu), this paper explores the challenges to 'everyday' dualistic thought structures that Zen practice poses to the questioning student and the ontological and epistemological significance of these challenges to the worldview of the experiencing student. First, the teaching styles and non-dual emphases of the two teachers in the context of teacher/student exchanges are examined; and, secondly, the experiential challenges and changes in worldview from the practitioner's point of view are phenomenologically explored. By teasing out the parallels and links between the phenomenology of Zen practice and the philosophical underpinnings of Zen practice instructions, foundational philosophical tenets can be shown 'in action' in the contemporary practice situation and a window is opened on the ontological and epistemological significance of the experiential impact of Zen teachings.