Within the realm of science, the uncertainty principle speaks of the fundamental limits of knowledge and measurement vis- -vis the external world, and how the very act of seeing alters what is seen. Martin Herbert’s The Uncertainty Principle is a collection of essays that reveals layers of unknowing and open-endedness within a diversity of contemporary art practices since the 1970s. If a work of art is always completed by the viewer, as Marcel Duchamp put it, then the works considered here equate completion with construction. In navigating us through a succession of artists’ approaches, Herbert also discloses how constructed experiences of not knowing can lead to deep engagements with a range of specific issues and themes- from history to politics, from epistemology to mortality.Martin Herbert is a writer and critic living in Tunbridge Wells, UK, and Berlin. He is associate editor of ArtReview and a regular contributor to Artforum, frieze, and Art Monthly, and has lectured in art schools internationally. His monograph Mark Wallinger, a comprehensive study of the British artist’s career, was published in 2011.