Book excerpt: Human actions unfold over time, in pursuit of ends that are not fully specified in advance. Rational Powers in Action locates these features of the human condition at the heart of a new theory of instrumental rationality. Where many theories of rational agency focus on instantaneous choices between sharply defined outcomes, treating the temporally extended and partially open-ended character of action as an afterthought, this book argues that the deep structure of instrumental rationality can only be understood if we see how it governs the pursuit of long-term, indeterminate ends. These are ends that cannot be realized through a single momentary action, and whose content leaves partly open what counts as realizing the end. Sergio Tenenbaum argues that we need to focus on temporal duration and the indeterminacy of ends in intentional action, even to explain the rational governance of relatively simple actions. Theories of moment-by-moment preference maximization, or indeed any understanding of instrumental rationality on the basis of momentary mental items, cannot capture the fundamental structure of our instrumentally rational capacities. Tenenbaum provides a new theory of instrumental rationality as rationality in action.