Rather than focusing on the process by which mechanism schemas are constructed, evaluated, and revised, Steele focuses on the question of how one extrapolates from a sample population or a model organism to the structure of a mechanism in the target. Will a treatment proven to suppress tumors in mice (a model organism) also suppress tumors in humans (the target population)? After developing a probabilistic account of mechanisms, Steele considers how researchers get around what he calls the extrapolator’s circle: determining 