By sharing their world, humans and other animals sustain each other. Their world gets determined over time as generations of animals act in it. Current approaches to psychological science, by contrast, start from the assumption that the world is already determined before an animal’s activity. These approaches seem more concerned with uncertainty about the world than with the practical indeterminacies of the world humans and nonhuman animals experience. As human activity is making life increasingly hard for other animals, this preoccupation becomes difficult to accept. This article introduces an ecological approach to psychology to develop a view that centralizes the indeterminacies of a shared world. Specifically, it develops an open-ended notion of “affordances,” the possibilities for action offered by the environment. Affordances are processes in which (a) the material world invites individual animals to participate, while (b) participation concurrently continues the material world in a particular way. From this point of view, species codetermine the world together. Several empirical and methodological implications of this view on affordances are explored. The article ends with an explanation of how an ecological perspective brings responsibility for the shared world to the heart of psychological science.